Hydrology: - WordPress.com · Web viewFig descriptive representative of hydrological cycle Precipitation:Snow and ice are basic form of precipitation. i.Some part of PPt evaporates

Class Notes on Forest Hydrology by M. K.Balla,Professor, IOF,Pokhara Campus

Hydrology: (Hydro – water and Logy or logus – science) It is the science of water in all its forms i.e. solid, liquid and gas on, in and above land surfaces. According to Ad. Hoc Panel on hydrology of the federal council for science and technology (USA), Hydrology is the science that treats water of the earth, their occurrence, circulation and distribution, their chemical and physical properties and their reaction with their environment including their relation to living things.Forest hydrology: It is the hydrology of forestland. It deals with the effects of forest and wild land vegetation on hydrologic cycle including the effects on erosion, water quality and microclimate.Hydrogramics: It describes the physics of water in its liquid state. Hydrometry: It describes the measurement of water.Hydrography: It describes the physical features and conditions of all waters of the earth surface.Hydrogeology: It describes the interrelationship of geologic materials and processes with water. Potamology:

It is the study of surface streams. Limnology: It deals with lakes. Cryology:

It deals with snow and ice. Glaciology:

It deals with glaciers. Oceanology: It deals with oceans.

Distribution of water On a broad sense, the distribution of saline water in land, lakes and seas is 27.208% and fresh water resources is 2.792%

Approximate distribution of fresh water resources:

Hydrologic storage component PercentagePolar ice and glaciers 77.35% Ground water (deep) >800 11.03% Shallow ground water (<800) 11.04% Lakes and streams 0.34% Soil water 0.18%Atmosphere 0.03%

Hydrologic cycle: It is the cycling of water from ocean to land and back again including all the pathways and processes associated with the storage and movement of water in all its state (i.e. solid, liquid and gas.)

Compiled and Prepared by Krishna Ram Dhital, Cell No: 9841181926 1

Fig descriptive representative of hydrological cycle

Precipitation:Snow and ice are basic form of precipitation. i.Some part of PPt evaporates back to atmosphere while falling.ii.Some PPt is intercepted by above ground part of vegetation, which can evaporate back to atmosphere.iii.Some PPt falls on the ground surface, which can either infiltrate or run over the land as over land run off or surface runoff.iv.The water that infiltrates the soil can be evapotranspiration back to atmosphere or percolate to become ground water. The ground water may appear on the ground surface in the form of springs and base flow in the streams. v.Overland flow (surface flow) accumulates into streams or rivers, which flow to the ocean or inland lakes/ seas.vi.The precipitation in the form of snow can evaporates back to atmosphere (sublimation) vii.Snow can also melt to become snowmelt run off. (Surface run off) Infiltration:1.The water that infiltrates down to the soil is evapotranspiration to form clouds.2.Soil infiltrated water remain in the form of soil moisture.3.The infiltration when happens to be percolation, it became ground water.4.Some part of the ground water is evapotranspiration.5.Some part of ground water maintains dry weather stream flow (base flow).6.Some ground water maintains ground water movement.


Compiled and Prepared by Krishna Ram Dhital, Cell No: 9841181926

Transpiration

EvaporationTranspiration

Rain Cloud

Infiltration Percolation

EE

Snow

Snowmelt Accumulation

Sream

Ocean

Surface run off

Rock formation

Deep percolation Ground

water

Cloud

2


Fig. Flow Diagram of deposition of surface runoff.


InfiltrationEvaporation

Soil moisture

Ground water reservoir

Evapotranspiration

Maintain dry weather Stream flow

Ground water movement

Depression storage Evapoptation

Infiltration

Surface run off Stream flow

Evaporation

Infiltration

Flow back to ocean

3

-1

+1 +76 -77+9 +8 -17

+2 +4 -6

-84 +77

+8 - 9+2

-10 +17

- 2 +2

-6 +6

Hydrosphere

Humid zoneArid zone

Fig. Quantitative representation of hydrologic cycle.

Note: 100relative unit =85.75g/cm2/yr =857 mm/cm2/yr


Man’s influence on hydrologic cycle1. The influence of man on hydrologic cycle may be intentional or unintentional 2. The influence of man on hydrologic cycle if happened planed way, is termed as intentional

influence. Man started agriculture >600 year ago. Before that man has managed or mismanaged or tried to control water since the dawn of man’s civilization. The change in vegetation or manipulation of vegetation by man changes the water balance or surface flow in an area.

A. Stages in hydrologic cycles that are influenced by change or manipulation of vegetation;3. Interception or precipitation.4. Transpiration of soil moisture.5. Infiltration6. Surface flow and stream flow.7. Tapping and shading of snow. (In area where snowfall occurs).

Some examples

8. In Coweta hydrologic lab (North Carolina state), when all vegetation (trees +shrubs) was removed from a watershed, the run off increased by 65% in the first year of cutting.

9. In Denmark a stand of beech trees was removed from a flat land. It resulted in the rise of ground water table by 6ft.



B. Induced precipitation Cloud seeding with silver iodide (AGI) crystals increased precipitation in the west coast of the USA by 15%C. Induced melting of snow and ice. Black colored materials (powdery) is spread on the ice or snow to induce melting. The effect is that the black powder increases albedo and increases the absorption of solar energy. In area where heavy snow and ice occurs, this process is done to facilitate navigation and to produce hydroelectricity.D. Urbanization It refers to the construction of buildings, pavements and roads in forest areas, farmlands etc. this process influence on precipitation, infiltration, evaporation, surface run off etc. in some areas the urbanization increases the precipitation. E. Evaporation control

10. Shelterbelts: the growing shelterbelts has been found to reduce the rate of evaporation by 5-15 % at a distance to leeward of 4 times the height of shelterbelts.

11. Use of chemicals: When the chemicals like hex-decanol and octadecanol are sprayed on water surface, the reduction in evaporation achieved was 10%.

F. Transpiration control Two groups of chemicals are used to control transpiration. They are called anti transpirant.

12. First group: This group includes transparent plastids, silicon oils, waxes etc and they control CO2 and O2 to pass through but not H2O.

13. Second group: The mode of control of transpiration by this group includes the partial close of stomata. The chemical used is phenyl mercuric acetate.

14. Acronize: 20 parts per millions of acronize sprayed on maize plants reduced transpiration by 40-50% because of stomata.

G. Interbasin water transference. Transfer of water through drainage and canal. e.g. Kulekhani hydroelecricity project, Melamchi drinking water project etc. other forms of construction includes the irrigation, pumping of ground water, reservoirs etc.

Importance of hydrologic knowledge in natural resource planning

15. Mostly land, water and life are concerned and air and solar radiation are taken into less account though they are equally important in the study of hydrology.

Uses of water:


Natural resource

LandwaterlifeAir Solar radiation

5


a. Life support: No life is possible without water. One individual need 3-litre water per day to sustain life.

b. Domestic use: Use of water for household clean, waste disposal, use in kitchen garden etc. In the USA the water used per day per person is about 1200 liters. In Katmandu, the demand for water is 18-million liter/ day in dry season and in wet season it is 16 million liter per day.

c. Transportation or navigation: The cheapest way of transportation is by water.d. Water for manipulating processes and industrial use: Huge amount of water is

used as a coolant in steel manufacture, Electrical power generation using coal and diesel, smelting industries like pulp and paper, fabric dying etc.

e. Water for agricultural and silvicultural raw materials: Agro-crops or vegetation requires water to grow.

f. Water for recreation: swimming, boating, rafting etc.g. Water as an aquatic habitat: water is the habitat for all aquatic fauna, the

relationship of which is directly or indirectly linked with the survival of human beings.

h. Water for hydropower and power for running turbines: water is the potential source for the generation of hydropowers and running many turbines in the rural areas.

In natural resource planning in river valleys, the hydrologic knowledge on flooding and inundation of land is important because of flood hazards (loss of life, land, vegetation, structures etc.)The knowledge of hydrology is the central to many planning problems. Most of the ecology, geology, geography and land uses in the region depend on the function of hydrologic cycle.Precipitation is the major component controlling the hydrologic cycle and therefore, it provides for constraints and opportunities in land and water management.Relative amount of precipitation, seasonal variation, size and intense of individual rain streams affect the runoff.Runoff forecasting is used for the planning of hydroelectricity generation irrigation and choice of crops. Knowledge of interception, though it is a very small component of hydrologic cycle is necessary because the change in vegetation results in a change in water balance. Interception has role in water balance. The knowledge of evaporation (loss of water) from water surface is important for water resource planning because of increase in surface area of water due to the continuation of more ponds/reservoirs.

Ground Water: The main source of water supply in arid/semiarid areas is the groundwater. In many areas, ground water is used in irrigation.

Evapotranspiration: Evapotranspiration is correlated with the primary production of vegetation and therefore the value of agriculture land.



UNIT- 2

Metrological factors affecting hydrological cycle

Atmosphere- Greek word (Atmos-vapour or breath & sphere – sphere or ball). So atmosphere is defined as the envelop of gases surrounding the earth and held by gravity.

Function of Atmosphere

i. It sustain both plant and animal life on the earthj. Its presence provides an indispensable shield from the harmful radiation from the sun.

For e.g. the ozone in atmosphere absorbs most of the ultraviolet rays.

Composition of Atmosphere: It is a mechanical mixture of gases and not a chemical compound. It consists of dry gases, water vapour and impurities.

Dry gases:Nitrogen Oxygen constitutes 99.98% by volume.Carbon dioxideArgonN2- 78.085%O2- 20.94%Ar- 0.93%CO2- 0.03%Neon- 0.0018%Helium- 0.0005%Ozone- 0.00006%Hydrogen- 0.00005%Krypton, Xenon, Methane- TracesWater Vapours.

It is variable in time and space. The variation may be temporal or spatial.Impurities.

Dust particles (esp. silicates), sea salts, organic matter and smoke. The discussion of atmosphere pertains to the lowermost layer of atmosphere called troposphere.Stratification of atmosphere.

ThermosphereMesosphere - 50Stratosphere- 16-50 km

Troposphere - 8-10 km thick. It contains all most all the water vapours and all most all the impurities and therefore all most all the clouds.



Weather; It is the condition of the atmosphere as a given moment or for short period of time.Climate; It refers to the average weather conditions at a particular place considered for a long time.e.g. Months or seasons of the year, in addition to the extremes as well.Meteorology; It is the study of physics of atmosphere or weather phenomena. Climatology; It is the study of climate. It concerned with average condition representative of long-term state of the atmosphere and variability.

Solar radiation and heat balance. Heat of the atmosphere and the earth comes from the sun through a form of transfer of energy called solar radiation. Insolation; Solar radiation received at the earth’s surface is called insolation.Only 50% of the solar radiation reaches to the earth.Solar radiation consists of X-rays, -rays, ultraviolet rays, visible light rays and infrared rays.The solar radiation at the outer surface of the atmosphere is almost constant (2calories/cm2/min). As it passes through atmosphere, about 32% is lost to the space by reflection from atmosphere and ground and by scattering. About 18% is absorbed by water vapour and dust in the atmosphere.Finally about 50% reaches the earth surface mainly in the form of visible light rays and infrared rays.Global radiation Budget

Reflection to space by scattering 5%Reflection from cloud to space 21%Reflection from earth’s surface 6%Total reflection loss to space 32%

Absorbed by dust, water vapours, clouds, CO2 –18% Absorbed by earth surface -50% Total absorbed by earth & atmosphere 68% Outgoing long wave radiation

Reradiation net loss from surface to atmosphere 13% Loss by latent heat transfer 20% Loss by mechanical heat transfer 9% Radiated directly to space from surface 8%Total radiation loss 50%Radiation to space from atmosphere 18%Grand total of the radiation 68%

Function of solar radiation;1. It maintains warmth of the atmosphere.


Artic circle (will have 24 hr day)Solar point

Artic circle(Will have 24 hr night)

Tropic of cancer.

Artic circle(Will have 24 hr night)

Artic circle (will have 24 hr day)

Tropic of capricon.Solar point


2. It evaporates ocean water to vapor which recycles back to earth in the form of precipitation by warming different regions unequally resulting in different wind system regional or global (planetary) scale.

3. It provides energy for photosynthesis plants. Insolation over the globe.

The amount of insolation varies due to the following factors.1.Age of strike of sun’s rays.

Intensity of insolation depends on the angle at which the sun’s rays strike the earth surface. When the angle of strike is low, the rays get scattered over large area and the amount of solar radiation per unit area is less. If the angle of strike is high, with the sun high in the sky, the amount of solar radiation received per unit area will be high.

2.Length of time of exposure. Inclination of the earth about its axis produces seasonal difference in the amount of insolation at particular latitude.

Solctic condition:

3.Atmospheric condition


Dec- 22, winter solctic.

June-21 ,Summer soltic

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Average absorption by the atmosphere due to the presence of water vapour, dust CO2 is about 18%. But there may be wide variation in absorption. The variation may be due to difference latitude, seasons and the conditions of the atmosphere.e.g. Cloudy and humid day absorbs more solar radiation than clear and dry day.

Reradiation:Principle of physics states that any substance that posses heat radiation radiate energy from its surface, similarly land and water surfaces after being heated due to the absorption of solar energy radiates energy from the surface. This is called reradiation.The radiated energy cannot pass through the humid air because it is long infrared rays. Instead , these rays are absorbed by humid air and CO2, thus transferring the energy to the absorbing substance causing temperature rise. Humid air and CO2 act as an insulating blanket that keep temperature dropping too much during winter.

Absorption and reradiation by land and water surface. Land and water surface behave differently in absorption and reradiation of energy. The

general law may be stated as;Land surface are rapidly and intensely heated due to the absorption of solar radiation where as

water surface are only slowly and moderately heated. Conversely, land surface cool off more rapidly and reach much lower temperature than water surfaces when the energy is cut off. Reasons;

The suns rays can penetrate to several meters in water and thus it is distributed to greater depth. In case of ground surface, the suns rays can’t penetrate beyond the surface and thus energy is concentratedThe water can have currents that help to distribute the energy. Where as the land does not have such currents. The water surface cool off by evaporation. If we apply a given amount of energy to the land and water , the land will attain higher temperature than water. In other words, the water will require much more (heat) energy than the land. About 5 times more energy id required raising the temperature in water than in land.

Atmospheric circulation (wind movement).

The main cause of air movement is the development of horizontal pressure gradients and the fact that the gradient persists is due to the earth’s rotation in creating Coriolis force.Pressure gradient force. In northern hemisphere, the vast landmasses of N. America and Asia separated by N. Atlantic and N. Pacific Oceans exerts strong control over the pressure (Atmospheric pressure) condition. During winter, the cold temperature causes air to become denser over landmasses resulting in high pressure. It is higher than the surrounding water bodies. During summer, the high temperature causes low-pressure areas over the landmasses and these are lower than over the water. In a climatic map of the world, there will be high and low pressure areas at a given time. The difference in pressure between adjoining high and low pressure centers give rise to the pressure gradient force. This force tends to blow the pressure centers. The greater the difference between high and low pressure, the stronger the wind and vice versa.

Coriolis force: If the earth did not rotate about its axis, the air would have moved according to pressure

gradient force. But the earth’s rotation about its axis produces a force called coriolis force. This force Compiled and Prepared by Krishna Ram Dhital, Cell No: 9841181926 10

High

HLow

Your back

Wind


tends to deflect moving objects towards the right directions. It is a weak force and its effect is only seen in moving air or water.Ferrell’s law:

a. The direction is right.The effect of this force on moving object in the Northern hemisphere may be summarized as; “Stand with your back towards the wind and the low pressure centers will be on your left. This is called Ballot’s law.

3. Frictional Force This force is exerted by the ground surface. It tends to hold back air and prevent it from blowing where it would have been according to the pressure gradient and coriolis Force. This force is the greatest near the ground surface. It disappears above 500mto 100m elevations. It affects the speed and direction of air movement.Local Wind Local conditions frequently exert more control over air movement than the forces we have discussed.Diurnal Effect In a normal day, the air movement is likely to be lowest at dawn because of less thermal mixing. The lower air is not influenced by more freely moving upper air. In the mid-afternoon, there is more thermal mixing due to terrestrial heating and the lower air is much influenced by the upper air.

Terrain Irregularities:



Terrain irregularities like mountains and valleys their own effect on air movement.

During the day, laterally restricted and vertically expanding warm air tends to blow up the valley. During the night, the cold dense air from higher elevation drains down towards the valleys. Mountains ranges act as the barrier to air movement forcing the air to blow across the mountains creating circumstances for the precipitation to occur.

Meteorological Phenomenon Jet streams:

These are narrow currents of air blowing at high speed (100km/hr-500km/hr) at high altitude (8km-16km). These are the result of air masses being moved by strong pressure gradient force and Coriolis force over Asia, the jet streams are strong in winter.

.

2. Cyclones and anticyclones:


Both summer and winter.

Tibetan plateau.Only winter.

Easterly Jet streams

Westerly jet streams.

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Old front Air mass:ts Warm fronts

Occludede fronts


Cyclones are more or less circular are of low pressure in which winds blow anticlockwise direction towards the center (inspiral) in northern Hemisphere.

Anticyclones:These are the more or less circular areas of high pressure in which winds blow clockwise and out spiral.

High

Fronts: Air mass:

An extensive body of or atmosphere having uniform physical [propertied of temperature and humidity.e.g. cold dry air mass from polar region, warm moist air mass from tropical region.

Fronts

Frontal surface: It is the boundary between two different types of air masses. e.g. cold dry air masses (polar),

warm humid air mass (tropical ).

Cold front: Cold air mass moves into the region of warm air. Cold air being denser remains in contact with the ground forcing the sir rise. Cold fronts are usually associated with heavy thunderstorms.


Low

Fig. Hurricanes or typhoons developed in tropical regions

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Warm fronts:If the warm air mass invades the cold air region, in this case also warm air rises over the cold

air mass. This front generally represents a stable condition but sometimes may cause thunderstorms.Occluded fronts:

If cold and warm / not so cold air masses occur in the neighborhood, cold air mass overtakes the warm not so cold air mass and cause the warm not so cold air to rise.

ELEMENTS OF WEATHER OR CLIMATE

Weather elements:

These elements are physical conditions prevailing in the atmosphere at a given time and a place.i. Atmospheric temperature ii. Atmospheric pressure iii. Atmospheric moisture (humidity, clouds, fog, ppt. etc)iv. Wind

Layout of a weather station1.Weather station is situated in a level ground with low vegetative cover in a large opening.2. It should be situated away from obstruction.3. It should be situated away from sources of dust and moisture.4.It should be located at a place that receives maximum possible hours of sunlight per day.5.If we have to locate the station on a slope south or west, exposure is considered.6.Area covered by the station should be at least 7.3 m X 7.3 m (25’ x 25’).7.Fencing is required; it should be not more than 1.22m(4’) in length.

Rules that govern location of a weather station.4. Locate the weather station at site that is representative of the area we are

concerned about. (e.g. vegetation cover type, topographic features, elevation, climate etc.)

5. Select the site that will provide the long-term operation with relative unchanged exposure.

6. Development plans e.g. roads, buildings, growing vegetation, availability of observer etc should be considered.

7. Consider the exposure requirements for each instrument prevailing winds, movements of sun, topography and vegetation cover, reflective surface and sources of moisture obstructions.

Instruments shelters.

Figure:Compiled and Prepared by Krishna Ram Dhital, Cell No: 9841181926 14


It is a special type of enclosure designed in such a way that it reduces (minimizes) radiant heat but allows free movements of air to pass the instruments placed inside. The temperature readings by thermometers placed inside represents the temperature of outside surrounding air. It is constructed of wood and painted white in both sides (i.e. inside and outside). It has double roof. It has louvered sides with slotted opening on the floors. It is mounted on open type of frame 4inch above ground.Weather elements:8. Air temperature: it is measured by thermometer. The thermometers may be two types

a. Minimum thermometer: This type of thermometer is alcohol filled thermometer. It has a glass index in the bore and has dumb bell shape. On decrease of temperature, the alcohol column retreats towards the bulb of the thermometer dragging the glass index with it. On rise of temperature, alcohol column moves towards the top leaving the index. Index indicates the lowest temperature reached.

b. Maximum temperature: It uses mercury, has small construction in the bore above the bulb. The construction allows the mercury to rise on expansion of mercury due to increase in temperature but does not allow retreat of Hg when the temperature lowers.

9. Mean daily temperature: Average of maximum and minimum temperature for the day. It measures the mean daily temperature for a particular calendar day, month annual average temperature.

10. Thermograph: It is an automatic thermometer that records temperature continuously on a clock driven chart.

It is of two types:11. Bimetallic stripII. Burdens tube filled with organic liquid

2. Atmospheric pressure:Air is compressible. At sea level the atmospheric pressure is about 1.03 kg/cm2 of surface

area. It is equal to the weight of air column one cm2 cross-section area extending upward to the outer elements of the atmosphere. Increase in elevation causes decrease in pressure and density of the air and vice-versa.

To measure the atmospheric pressure, two types of barometers are used:Mercuric barometerAneroid barometer

1. Mercuric barometer:



It is made up of glass tube about 1m in length. It is filled with mercury and inverted into the mercury and inverted into the mercury filled dish

Air pressure exerts a force on the Hg surface in the dish that balances the Hg column in the tube. Unit of atmospheric pressure is cm or inch of Hg. Another unit is mill bar, which is the force of 100 N/m2. (1cm of Hg=13.33mb.)

2. Aneroid barometer:

Figure: In this barometer a flexible diaphragm covers the airtight box. Variation in the pressure causes the diaphragm to move and the movement is transmitted by the recording mechanism and is indicated by an indicator needle in cm of Hg.

Vertical variation in the atmospheric pressure with elevation is not much of importance in meteorology, of importance is the small variation in atmospheric pressure among different places at a given time or variation at a particular place at different times. To reveal small differences in atmospheric pressure among different places, the atmospheric pressures measured are adjusted to a common basis of sea-level equivalents by eliminating the effect of elevations. After adjustment, the atmospheric pressure will vary from 980 mb to 1010 mb.


1m

Isobar line (line of equal atmospheric pressure)

L

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WIND In the measurement of wind we primarily concern the speed and direction of the wind. The

speed of wind is expressed in km/hr; m/sec; miles/hr; knots (1knot=1.852 km/hr).Wind speed is measured by anemometer. Widely used anemometer is Cup anemometer.

Cup Atmometer:

It is mounted at 6.1m (20ft) from the ground. Direction of the wind:

It refers to the direction from which the wind is blowing. e.g. SW,NE etc. it is measured with an instrument called wind vane. The formula to estimate the wind speed at different elevation is given:

V elevation = V anemometer (elevation anemometer) 1/7


Wind

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IceFreezing

Release of heatMelting

Absorption of heat (80 cal/gm of water.) i.e latent heat of fusion.


Atmospheric moisture.0.03% of the total fresh water resources are depends upon the atmospheric moisture. This will

equivalent to 13000 km3 of water in term of volume. This water will cover Nepal a depth of 90m.

States of water; Solid (snow and ice)liquid (water) Gas (vapour)When the water changes from one stages to other, there will be change in amount of heat that the water contains, the heat thus obtained is called latent heat.

The three stages of water can be presented in a triangular diagram as shown in the figure below.

580 cal /gm of water, heat is released.

Sublimation(580 +80) cal/gm of water) Sublimation (Released)


Condensation

Vaporization580 cal heats is absorbed

Water vapour

18

30

20

10

85

70

55

6 A,M Noon 6PM

40


Humidity: It is the degree to which the water vapour is present in the atmosphere.

Saturation point:It is the limit to which water vapour can be held in the air. This upper limit is called the

saturation point. At air saturation relative humidity is 100%.Dew point:

It is the critical temperature at which the air is fully saturated at 100% relative humidity.Relative humidity:

It is the ratio of the amount of actual water vapour present in the air as compared to the maximum amount necessary for saturation.

Relative humidity = Amount of actual water vapour

Maximum amount of water vapour that can be held in air.

Changing the tempereture of air can change relative humidity of air. Warmer air is capable of

holding more water vapour than air at lower temperature. When temperature is lowered, the water

vapour in the air is lowered and hence relative humidity of air is increased.

Fig. Effect of time and temp on R.H.

Measurement of Relative Humidity.By hydrometer (Hair hygrometer)Psycrometer.

Hygrometer:Hygrometer uses a small bundle of human hair.Psycrometer: It is a set of dry and wet bulb thermometer. Wet bulb thermometer has its bulb covered with wick or Muslim cloth that is kept moist (wet). Since evaporation causes cooling of the wet bulb temperature


R.H.Temperature.

19


reading will be lowered and dry bulb if R.H is <100%. Greater the difference between dry and wet bulb temperature readings, lower will be the relative humidity.

R.H. tables or tsychrometric table is used. Measurement of R.H. value is considered least accurate instrumental procedure because R.H value will differ by several percentages if the dry and wet bulb temperature readings are slightly misread. Wet bulb may be read before reaches the lowest temperature due to improper ventilation. Thick and dirty cloth around the wet bulb and use of dirty water may cause the temperature readings to vary.

Unit- 3Precipitation:

Precipitation is a broad term and component of hydrological cycle. Primary forms of precipitation are rain, snow, hail etc. Other forms of precipitation are frost, dew, drizzle (dropping of 0.5m m dia.) and sleet (frozen rain). There are the following requirements for the precipitation to occur.

Pressure of huge amount of water vapour in the atmosphere. Condensation or freezing nuclei.Mechanism by which moist air is lifted to higher elevation.Mechanism of growth of water droplets or ice crystals.

Pressure of huge amount of water vapour in the atmosphere.The most measurable process by which the water vapours present in the atmosphere is by

evaporation and transpiration. 75% of the earth surface is covered with ocean and hence evaporation is abundant to balance the water vapour in the atmosphere.Condensation of freezing nuclei: These are very minute and are of the size of 1 micron (). Some are of the size of 5. These are very minute particles of dust, ocean salt, combustion product of smoke, oxides of nitrogen and Sulfur dioxide. When the moist air is close to dew point (or saturation), the formation of droplets or ice crystals require the presence of these nuclei on which condensation occurs.Mechanism of which moist air is lifted to higher elevation;

Based on these mechanism the rain may be classified as Convective rain: This is simply an updraft of warm air. Differential heating of different surfaces cause the air to become warm over the warmer surfaces. The warm air expands and being lighter than surrounding air rises likes smoke. Warm air has the capacity to hold more water vapour and so increasing amount of vapours are taken up. If the rising warm moist air-cools to dew point temperature at higher elevation, the water droplets or clouds are formed by condensation. The latent heat of vaporization released due to the formation of water droplets is absorbed by air that causes the heated air to rise even higher. This causes further cooling and condensation to form water droplets. If the water droplets grow in size, eventually precipitation will occur.Orographic precipitation: it occurs when hills and mountains ranges act as the barrier to the moist air forcing it to rise over the hills and mountains. Its effect depends on the size of hills and mountains and their alignments with respect to the moving moist air. Orographic effects causes heavy precipitation to occur in windward side of the mountains and development of rain



shadow areas on the leeward. So south of Himalayas areas receive much precipitation than north of Himalayas. e.g. Tibetan plateau and Manang areas of Nepal.Cyclonic rain: It is of two types:

a. Frontal type cyclonic rain: cold fronts invade the warm air mass. The cold front is usually associated with heavy thunderstorms.

b. Cyclone type cyclonic rain: air move from higher-pressure region to lower pressure region.

L

The hurricanes and typhoons are associated with heavy thunderstorms.Mechanism to grow the ice crystals or water droplets:

Once the water droplets or ice crystals are formed, they must grow in size for precipitation to occur. These are supported in the uprising air since they fall at lower speed than the sped of rising air. When they grow in size, they become heavier and fall to earth. There are two theories about the growth of droplets.

Bergeron theory; According to this theory the ice crystals grow in size because of transport of water vapour from droplets to ice cryatalsCoalescence theory: according to this theory, water drop collide and grow in size.

Measurement of precipitation:Various instruments designed measure the precipitation. The precipitation is measured

in cm, mm and inch. The intensity of precipitation is the depth per unit time.Non recording rain gauge:

It consists of receiver or collector, measuring cylinder, overflow can and measuring stick. The rain gauge is of various sizes. In rain gauge, the cross section area of collector or receiver is 10 tomes more than the area of cross section of the measuring cylinder to ensure accuracy. The American standard rain gauge is of 8 inch in diameter and 31 inch height. The Indian siphons gauge is of 5 inch in diameter and 12 inch in height. In Nepal, the commonly used rain gauge is of 203 mm in diameter of which the measuring cylinder of 63.4 mm diameter is provided.

Recording rain gauge: This type of rain gauge are of three types;c. Weighing type: in this type of rain gauge a bucket to collect precipitation is set on a platform

of a spring or lever balance. The weight of water collected in the bucket is transmitted to the recording system, which moves the pen to make a trace of the depth precipitation on clock driven chart.


Hurricanes and typhoons

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d. Tipping type: it consists of a pair of small containers. In US standard, the size of container is 0.01 inch. When one of the containers receives the given amount of rainfall, it tips emptying the water collected and the position to receive the next rainfall. Each tipping actuates an electric circuit, which is recorded in terms of depth on a chart.

e. Float type: in this type a floats on the water collected and the increase in depth of water collected is transmitted to the recording mechanism which records the depth of the precipitation on a chart. Based on the mechanism it is of two types.

i. A siphon type: it automatically implies the water collected when certain amount of water is collected.

ii. Other type: in other type the water collected is manually emptied. Weighting type is preferred in areas, which receive high intensity rainfall because the flood and tipping bucket cannot keep up with the high intensity rainstormsHigh intensity rainstorms =12 cm /year.Storage gauge; it is used in remote areas where ppt, is recorded every 2 to 6 months.

Measurement of snowThree methods of measuring snow;

Rain gauge: Snow sampler or core samplerSnow stakes.

Rain gauge: it is used on light snowfall areas, receiver and measuring cylinder are removed and the overflow can is used to collect the snow. The snow is melted and the water is measured with the measuring stick. Snow sampler or snow core sampler: using a snow sampler snow is extending the depth of snow is taken. The depth of snow in the sampler can be read or the sampler is weighed in a specially designed spring balance, which shows the snow accumulation in terms of depth of water equivalent.Snow stakes: These stakes are usually wooden stakes of 13/4 sq. inch, supported by iron supports are used to measure the depth of snow accumulated at a place. The snow depth is converted to equivalent water depth using a ratio of 12:1 (snow depth: equivalent water depth.)

Rain gauge site: Wind can cause the precipitation catch to be less than it is actually should be. This is due to

the rain gauge acting as obstacle to the wind movement. This result in wind eddies and turbulences and air is speeded up, which cause the precipitation catch to be less. Therefore, to minimize the effect of wind, these recommendations are to be followed in locating a proper rain gauge site.

Avoid site with individual trees, buildings, poles or other objects near by: these can cause wind eddies. Select a large opening in an orchard or groves of trees forest.Distance between an object and the rain gauge most be at least twice the height of the

Object. It must not be less than 30m.


2h


If 2h<30, the distance of 30m, must be maintain. Locate the rain gauge in a level ground and try to avoid locating at a slope or top of the hill.In the hilly region or mountains where it is difficult to locate the gauge place at a place which is best shielded from high winds and where wind eddies are expected to occur less.Distance between the fence and the rain gauge must be more than one meter and the height of the fence above the top of the rain gauge must not exceed one feet. i.e.30cm

Rain gauge networking density: Network is the no of rain gauges in a unit area. Israel has the highest network density in the

world. It has more than 23 rain gauges in 500km2. It is due to the great significance of local variation of precipitation in economy of the country. India has about one rain gauge in 500 km2. Nepal has about 273 stations, which gives the network density of 1 rain gauge in 535km2. Katmandu valley has the highest rain gauge networking density in Nepal.Factors that are to be considered:

Type of rainstorm: To gather information on large general type of rainstorm a, low density is required. To study the rainfall pattern in individual rainstorm. e.g. thunderstorm, a high density is required.The use of data: To estimate the monthly or annual average precipitation a low density is required. But for storm pattern analysis or study the storm patterm of individual storms a high density is required.Installation and the maintenance cost: It is the most important factor to be considered in developing countries. Availability of skilled manpower to operate and maintain in the rain gauge (especially the recording type). Taking into the consideration of these factors, the rule of thumb for recommendation of network density in India are as follows:

Watershed area (Km2) No of rain gauges

Up to 75 175 - 150 2150-300 3300-550 4550-800 5800-1200 6

1. For general hydrological purpose the minimum network recommended for flat topography (area) is 1 rain gauge in 600 to 900km2 and for hilly mountainous areas 1 rain gauge in 100- 250 km2.

Average precipitation over an area (equivalent uniform depth of precipitation (EUD)Compiled and Prepared by Krishna Ram Dhital, Cell No: 9841181926

h

23


Precipitation measured at a station is called point precipitation, which may be converted to aerial average by the following methods.

2. Arithmetic average3. Theissen polygon4. Isohyetal methods

1.Arithmetic average: it is the average of precipitation recorded at stations in an area. This method gives satisfactory result if the stations are uniformly distributed and the area is fairly flat.2.Theissen polygon: In this method, polygons are drawn around each rain gauge stations. Then the weighted average precipitation for each polygon area around the station is calculated and then summed up to estimate the EUD or average precipitation over area. In other word, this method permits the arbitrary weighting of each rain gauge catch by the area nearest to it. (By the area enclosed by the polygon). This method is widely used. Once the polygons are drawn on the map, the polygon network is fixed. Polygons are to be drawn if there is change in location of any station. In this method, the knowledge of topography and storm pattern is not taken into account.3.Isohyetal methods: in this method, isohyets are drawn in the maps. Isohyets are the lines joining the points of equal precipitation. Draw the isohyets on the map from the knowledge of the topography and storm pattern and the amount of precipitation measured at different stations calculate the weighted average precipitation for areas enclosed between two isohyets. Sum of the all the weighted average precipitation gives EUD. This method is the most accurate but less frequently used methods. It is mostly used in research work or special storm analysis.

a. Measure the length of each straight line and locate the mid point.b. Draw a perpendicular bisector from the mid points.c. The side of perpendicular determines the watershed boundary of the station, which is enclosed,

by the bisector boundary.Estimation of missing precipitation:

5. Normal ratio methods: to estimate the missing precipitation from this method, the following formula is used.

PX= 1/n [NX/N1*P1 + NX/N2 *P2 +………. NX/Nn *Pn]Where,

PX = Missing precipitationn = No of near by stationNX, N1, N2 ……….. Nn = Normal annual ppt. at station x, 1,2,…………n.P1, P2 ………….Pn = precipitation measured at stations 1,2,3……….n

Rainfall intensity: it is the quantity or amount of rainfall per unit time. (Mm/hr)


10 yr

5yr3yr2yr1yr

Duration (min)

Ppt.IntensityPer hour


The knowledge of rainfall intensity can be used to estimate the storm runoff or to estimate the susceptibility of a watershed to flooding which in turn can be used to design of culverts, bridges, storm water drains and other flood control stations.It also enables us to assess the impact of land use changes and also used to estimate the soil loss.

Intensity, duration and frequency Analysis. Duration: how long the intensity lasted.Frequency: how often it is likely to occur. (Return period)

6. Experience of the analysis of long-term rainfall data indicates that very intense rainstorms last for short duration and are rare. (Less frequently). Rainstorms that last for a larger period tends to be less intense. They supply huge amount of rainfall and are also rare. So the analysis of the above-mentioned parameters is useful to analyze the inverse relationship between intensity, duration and frequency.

Data required: largest rainstorms intensity for a different duration each year.5min, 10min, 15min, 30min, 60min2hr, 4hr, 6hr, 12 hr, 24hr etc.

Set of largest intensity data is called maximum annual series.


5 10 15 20 25 30

25


UNIT-4

Evaporation and evapotranspirationEvaporation is the process by which water is transformed the land (i.e. soil, leaf surface,

falling raindrops ) and water masses to the atmosphere.

Pan evaporation in Nepal:

Evaporation measured in pan evaporimeter is called pan evaporation. In Nepal, it is 140 cm to 190 cm per year. In Gorkha, the runoff in cultivated watershed was 40% of annual rainfall where as from forest watershed it was 35% of the annual rainfall. In the USA, lake evaporation ranges from 50 cm to 218 cm from cold region to hot region about 30% of the annual rainfall.

Factor Affecting Evaporation:

Metrological factors: Solar radiation, water vapor pressure, temperature, wind, atmospheric pressure, and precipitation.

Over factors: Availability of water, water quality and irrigation.

a) Solar radiation and other sources of heat provide energy. The energy excites the water molecules break loose and enter the atmosphere as water vapor. Solar radiation depends upon the latitude, season, time of the day and atmosphere.

b) The water vapor pressure in the atmosphere air and at the water surface. It is the partial pressure or the measure of amount of water vapor in the air.

c) Evaporation is directly proportion to the water vapors pressures difference between water vapors in the air and at the water surface. At relative humidity of 100%, saturation vapors pressure exists in the air. The saturation vapor pressure also exists at the water surface. The greater the water vapors pressure difference (gradient) between the air and water surface, higher will be the rate of evaporation and vice versa.

Temperature:The water vapors pressure at the water surface depends on the temperature of the water body.

Increase in the temperature of the water body increases the water vapors pressure at the water surface because of the increase in kinetic energy of water molecules.

Wind:Wind or turbulence of wind causes the air capable of holding more water vapors to move

towards the water surface and water vapors away from the water surface causing the increase of evaporation rate.



Atmospheric pressure:The lower atmospheric pressure tends to increase evaporation because of less no. of

molecules above the evaporating surface and therefore, less inference.

Precipitation:When ppt. occurs a part of it evaporates while falling a part of it is intercepted by vegetation,

which can evaporate. When the available energy is used up to evaporate the ppt.(falling and intercepted), it is not available for evaporation from the water body. Also the water vapor pressure difference between atmosphere and the water surface decreases during precipitation causing reduction in evaporation.

Availability of water for evaporation

Available water:It is the difference between field capacity and wilting point. When the ground water is near

the surface, evaporation may be high as from a water body. But, if the ground water is at a greater depth, evaporation may be negligible because capillary action may not be able to draw the ground water to the ground water to the ground surface.

Water Quality:For every 1% increase in salinity the evaporation will decrease by 1% because of decrease in water vapor pressure in case of saline water than in fresh water.

Irrigation:If water is applied in the agriculture field by flooding (wasteful and inefficient method) methods,

evaporation loss will be high.

Drip irrigation method:

Measurement or Estimation of Evaporation.

Water Budget method.It is on the basis of continuity equation (simple form of equation).

Inflow = Outflow + Change in storage. Isurf +Isub +p = O surf +Osub + E + (V1-V2)Where,Isurf + Isub = Volume of water (inflows) from and ground flows.P = Volume of pptOsub + Osurf = Volume of outflows to the stream and ground water.E = volume of evaporation.V1 & V2 = Volume of lake water at the beginning and end of the period.

Energy Budget Method.Compiled and Prepared by Krishna Ram Dhital, Cell No: 9841181926 27

122CM

Wooden Frame

22.5cm


I T is based on the continuity equation.Qn – Qh – Qe = Qs- Qv

Where,Qn = Net radiation absorbed by the water body (can be measured by the use of different radiometer)Qh = Sensible heat transfer by conduction to the atmosphere (Qh – replaced by Bown ratio)Qe = Energy used for evaporation.Qs = Increase in energy storage in body of water.Qv = Adverted energy into the body of water.Qs-Qv = replaced by equation of water balance and temperature of respective water volumes.Qe = Converted to depth of evaporation.

Evaporation Pan.-Widely used.-It is class a pan U.S weather bureau.-Made of galvanized iron sheet-122 cm in diameter, depth of 25.5 cm with the depth of water about 20 cm.-Pan is mounted on wooden frame 15 cm from the ground surface.-The change in depth of water in the pan is measured with a micrometer hook gauge mounted on a stilling well.

Pan coefficient = 0.7e.g for 8 cm of evaporation, actual evaporation = 8*0.7 = 5.6cm

Objective:To create evaporation condition that is approximately same as for the natural are not exactly same, pan coefficient is to be applied.

4. Empirical Formula.


Micrometer hook gauge

28


i. General form of the formula;

Eo = CF (U) (es-ea)Where,

Eo= evaporationC= a constant called a mass transfer constant.f(U) = function of wind velocity ‘u’.e(s)= saturation vapour pressure at the water surface.e(a)= vapour pressure in the air.

b) Meyer’s formula

Eo = C (1+V/10) (es-ea)Where,

Eo= evaporation in inch.C =coefficient (value= 0.36)V = velocity of wind in miles/hr.e(s)= saturation vapor pressure at the water surface in inches of Hg.( based on the

temperature on the surface.) e(a)= vapor pressure in the air in inches of Hg.

5.Livingston Atmometer.It consists of hollow porous porcelain sphere of 5cm in diameter and it is 3mm thick. It is connected with a narrow cylindrical neck to a supply of distilled water in the jar. The most moist porcelain sphere assumes a constant and uniform exposes in all dimensions except the lower portion. The evaporation is measured by refilling the jar with distilled water up to the mark from burette.

Evapotranspiration.It is the combination of two processes i.e. evaporation and transpiration. Evaporation is the

consumptive use of water .so, evapotranspiration = evaporation + transpiration. Transpiration;

It is the process by which plants give off moisture to the atmosphere not necessary and impossible to distinguish between evaporation from the soil surface and transpiration by plants.

b. Soil water is absorbed by plants root hair and is used to build up the leaves to the atmosphere.c.Water must be absorbed for evaporation to occur.

Absorption:It is the uptake of water by plants. Root hairs absorb moisture from the soil.

Translocation:It is the movement of water inside the plant body. The absorbed water is translocated through the

network of conducting cells and vessels called xylem.Transpiration:

It is the loss of water in the form of vapor from the plant body. The water is transpired through the stomata, lenticels and cuticle present in the leaf of the plant.

Importance of knowledge of evapotranspiration in natural resource management.It is fundamental to understand the growth and distribution of plants.



It can be used as a basis for choosing crops in the area of unreliable rainfall.Estimation of rainfall is used to determine the irrigation water requirement, which is used in the design, and planning the irrigation projects and day to day control and management of water supply in farmer’s fields.In the areas where evapotranspiration rate is high and water supply are limited trees and other plants may be removed in the watershed and along the water convergence areas to conserve and increase the water supply.

Factors affecting evapotranspiration.All the factors that affect evaporation also affect the evapotranspiration in the same manner.

Metrological factors: Solar radiation, water vapor pressure, temperature, wind, atmospheric pressure, and precipitation.

Over factors: Availability of water, water quality and irrigation.

a) Solar radiation and other sources of heat provide energy. The energy excites the water molecules break loose and enter the atmosphere as water vapor. Solar radiation depends upon the latitude, season, time of the day and atmosphere. as it has influence in evaporation , ot ultimately affects the evapotranspiration.

b) The water vapor pressure in the atmosphere air and at the water surface. It is the partial pressure or the measure of amount of water vapor in the air. It has great influence in evaporation and in turn in evapotranspiration.

c) Evaporation is directly proportion to the water vapors pressures difference between water vapors in the air and at the water surface. At relative humidity of 100%, saturation vapors pressure exists in the air. The saturation vapor pressure also exists at the water surface. The greater the water vapors pressure difference (gradient) between the air and water surface, higher will be the rate of evapotranspiration and vice versa.

Temperature:The water vapors pressure at the water surface depends on the temperature of the water body.

Increase in the temperature of the water body increases the water vapors pressure at the water surface because of the increase in kinetic energy of water molecules. This increases the evaporation and ultimately to the evapotranspiration.

Wind:Wind or turbulence of wind causes the air capable of holding more water vapors to move

towards the water surface and water vapors away from the water surface causing the increase of evaporation rate. This influences the process of evapotranspiration.

Atmospheric pressure:The lower atmospheric pressure tends to increase evaporation because of less no of molecules

above the evaporating surface and therefore, less inference. This influences the process of evapotranspiration.

Precipitation:When ppt. occurs a part of it evaporates while falling a part of it is intercepted by vegetation,

which can evaporate. When the available energy is used up to evaporate the ppt. (falling and


Ground surface.

Outflow or water drained off.


intercepted), it is not available for evaporation from the water body. Also the water vapor pressure difference between atmosphere and the water surface decreases during precipitation causing reduction in evaporation. This ultimately affects the evapotranspiration.

Availability of water for evaporation

Available water:It is the difference between field capacity and wilting point. When the ground water is near

the surface, evaporation may be high as from a water body. But, if the ground water is at a greater depth, evaporation may be negligible because capillary action may not be able to draw the ground water to the ground surface.

Water Quality:For every 1% increase in salinity the evaporation will decrease by 1% because of decrease in water vapor pressure in case of saline water than in fresh water. Due to the decrease in evaporation, the evapotranspiration is affected.

Irrigation:If water is applied in the agriculture field by flooding (wasteful and inefficient method) methods, evaporation loss will be high with increasing the evapotranspiration.Vegetation type:

There is small variation in evapotranspiration among different types. It may be of greater significance under the conditions of limited resources.

Soil factors: These factors also have minor control on evapotranspiration but can be of significance under the conditions of limited moisture.

Measurement of evapotranspiration.d. Lysimeter: it is a container set in the field of growing plants. Thus soil and growing plants

constitute a sample of the field. Its size may range from 2.5 to 10 ft in diameter and 6 ft deep. Evapotranspiration is estimated by calculating the water balance in the lysimeter. There is some arrangement made to weight the lysimeter and the loss in the weight corresponds the evapotranspiration.

Inflow = Outflow + Change in storage.



P + I = Et + O + sO = Out flowP = PrecipitationI = Irrigation

s = Change in storageDisadvantages:-It is costly to build and requires complicated weighting instrument.-Reliability of Et estimation depends on nearness of reproduction of natural conditions.-Artificial condition may be caused by limitation of soil size and water regeneration system.

2. Soil moisture depletion method:This method involves the intensive the intensive soil moisture study. It is useful in the areas where the soil is fairly uniform and depth to the ground water must be greater enough such that it does not influence the soil moisture content in the root zone.The soil moisture is determined before and after each irrigation or precipitation with some measurement in between two measurements. Usually soil moisture is determined 2-4 days after irrigation and again 7 to 15 days after or just before the next irrigation.The soil moisture is determined before and after each irrigation or perception with some measurements in between two measurements. Usually soil moisture is determined 2-4 days after irrigation and again 7-15 days after or just before the next irrigation.Rates of Et per day per each period are determined then the Etcurve can be drawn against time. then the Et per day when summed up give the seasonal Et, using this method.The Et rate for different crops can be determined.

3.Water Budget method.4.Energy Budget method. 5.Climatic condition as an index of Et.

a) Thorn Waite Formulab) Blaney criddle formula

The above formula is used to estimate potential Et

Potential Et:It is Et that would occur under the condition of adequate moisture supply all the time.

Infiltration:Flow or entry of water through the soil or ground surface in to the soil is called infiltrationInfiltration capacity:There is the maximum rate at which water can infiltrate the soil. This upper limit is called the infiltration capacity.Unit –cm/hr.e.g.Rainfall intensity = 4 cm/hrInfiltration capacity = 6 cm/hrIt means there is no flow as run off.If, rainfall intensity = 6 cm /hr Infiltration capacity = 4 cm/hrIt means there will be excess rain and will flow as run off.

Importance of infiltration in water resource managements.



If infiltration rate is high, ground water is recharged/stored and surface is reduced, thus reducing the flood hazard and damage due to soil erosion.High rate of infiltration will reduce the desirable streams/rivers or canal flow thereby reducing the water supply downstream.`

Process of infiltration: Soil constitute solid particles of sand, silt and clay interspersed between organic matter, pores or channels of different sizes like shrinkage cracks, wormholes, root holes, spaces between lumps of soil and particles are present in the soilWhen rain falls on the surface, water enters these pores due to the force of gravity and capillarility force. Gravity moves the water vertically downward where as the capillary force moves the water in any direction.If the soil pores are small, then there is resistance to the flow of water by gravity. If the soil is big, there is high speed of flow of water by gravity.

There are three different processes; any of these can itself restrict the infiltration.e. Entry through the soil surface.f. Storage within the soil.g. Transmission through the soil.

During the early part of rain stream, the infiltration rate reduces rapidly. After 1 to 2 hours of rainfall infiltration rte will be almost constant. Reasonns are as follows.-Falling of small pores with water will reduce the capillary force.-Clay particles swell on wetting thereby reducing the size of pores.

Factors affecting infiltration.Basically the factors affecting infiltration can be grouped into six groups.

a) Rainfall characteristics.b) Soil properties.c) Vegetation cover.d) Land use.e) Human a activities.f) Condition of soil mass.

1. Rainfall characteristics:Intense rainstorms compacts the soil surface, disperses the fine soil particles and causes them to plug the soil pores. This results to decrease in infiltration. On the other hand, the long duration rainstorm tends to be less intense and may fully saturate the soil and that may cause the swelling of clay particles. This activity may gradually decrease infiltration.

2. Soil particles: -Texture, aggregation/structure, depth.

Texture:The coarse texture soil like sand will have large pore spaces and therefore will have infiltration.

Aggregation:



Well aggregated soil due to the presence of high amount of organic matter or small amount of clay particles will have loose friable structure. This will increase the infiltration.

Depth:Deep, well drained, coarse textured soil with high organic matter increase infiltration and shallow clayey soil decrease infiltration. Infiltration capacity for two types of soil:

Variation in infiltration with soil types:Minimum infiltration (mm/hr)

Soil characteristics

8-12 Deep sandy, deep coarse, aggregated soil. 4-8 Shallow lose and sandy soil. 1-4 Soil with low moisture content and high percentage of clay. 0-1 Soil of high swelling percentage, heavy clay and certain saline soil.

c) Vegetation cover:Vegetation protects the soil from the impact or being action of raindrops. Organic matter helps in

binding the soil particles together or helps in soil aggregation. Soil fauna like worms, insects that live on organic matter helps in soil aggregation by mixing the soil particles and organic matters. Root creates holes and decaying roots leaves root holes for the water to infiltrate. d) Land use:

Leaving land under natural vegetation with native trees and undergrowth provides the favorable condition for high infiltration. Urbanization, involving the replacement of vegetation by asphalt or concrete roads or pavement drastically reduces infiltration.

Relative influence of land use on infiltration ratesGood meadows forest layer highest infiltrationFair pastureGood forestPoor pastureFair small grain pastureRow crops with good rotationRow crops with poor rotation lowest infiltration

e) Human activity:Human activities that influence the infiltration like cultivation of land disturbs the soil

aggregation and destroy the holes made by burrowing animals, worms, insects and roots. Excessive ploughing of soil when wet destroys the soil aggregation. Contour cultivation and terracing increase infiltration because they allow more time for water to percolate.

f) Condition of soil masses:


intensityRunoff


Soil moisture content influences the infiltration rate. The greater the soil moisture, lower will be the infiltration rate. If the soil is frozen, soil mass will be almost impermeable.

MEASUREMENT OF INFILTRATIONInfiltration is measured by:-Making use of water ponded on the soil surface (ring cylinder infiltrometer)-Making use of artificial rainfall events (sprinkler infiltrometer) -Making use of natural rainfall events (hydrograph analysis)

1.Ring Cylinder Infiltration:A tube or ring of 10 to 30 cm in diameter and 60 cm height is used in this method. The ring is inserted into the ground with 5 to 15 cm above ground. Surface water is ponded 1 to 2 cm deep in the ring. A constant depth of water is maintained by supplying water from a graduated container/reservoir.The supply of water requires the maintenance of the depth of water, which is measured, and this corresponds to the water unfiltered

Advantages:-Cheap-Easy to install and use.-Easy to carry.Disadvantages:-Disturb the soil structure while driving the ring into the ground.-Raindrop impact is not stimulated.If a buffer ring is used around the infiltrometer, it is called double ring cylinder infiltrometer and its advantage is that it eliminates the lateral transmission or simply lateral movement of water.

2.Sprinkling InfiltrometerThis method uses the artificial rainfall events. The artificial rainfall is generated by rainfall simulators at a constant rates on plots, Plots vary in size from 1m to100m. Rates of runoff are measured. Infiltration is the difference between the rainfall intensity and the rate of runoff from the plots it requires pumping equipments and water supplies, which cannot be easily carried.

3.Hydrograph Analysis. This is similar to that of sprinkling infiltrometer i.e. principle is same. In this method, infiltration rate is estimated from the response of the plot or small drainage area to rainfall in producing the runoff. Though it is similar to sprinkling infiltrometer, it is somewhat difference in the sense that it requires regular monitoring and complex storm with several burst of rain is chosen. Volume of rainfall for each burst is calculated. Volume of runoff for each burst of rain is estimated from the hydrograph of runoff from the plot or area. Difference between the volume of runoff and the volume of rainfall is the volume of rainfall infiltrated. Time duration over this occurred is the duration of the burst of rain. Average infiltration rate is then calculated for each burst of rain and the composite infiltration capacity curve is then drawn from the average infiltration rates for the different burst of the rain.



Fig:

Unit-6Soil Moisture-Soil moisture is the water in the zone of aeration.-0.18% of the total fresh water on earth is in the form of soil moisture. Though it is quantitatively less, it has significance importance because vegetation growth depends on it and we people depend on it for our food. Physiological stress placed on plants may form a major determinant of vegetation distribution, animal distribution and land use.

1. Porosity:V = Total Volume soil =Vs + Va + VwVs = Volume of solid = Vs + VvVa = Volume of waterVv = Volume of voids = Va + VwTherefore, porosity is the percentage of soil volume occupied by pores or voids.Porosity = Vv/v*100%, depending upon the soil, the porosity varies from 30 to50%.Clay soil has greater porosity than sandy soil due to the large number of pores on it.2.Moisture Content:It is the measure of water in the soil. The moisture content is of two types:a) Volumetric Moisture Content.b) Gravimetric Moisture Content. Wet weight of soil sample = Ww Dry weight of soil sample = Wd Weight of water = Ww-Wd Moisture on dry weight basis = Ww-Wd

WdMoisture content on volume basis = Ww-Wd

VDegree of saturation: It is the percentage of pore volume (volume of voids) occupied by water.Ds = Vw/Vv*100At saturation, the degree of saturation is 100% i.e. Vw = Vv.

1. Field capacity: It is the moisture in the soil held against the force of gravity. Storage potential of capillary water is sometimes called field capacity.2. Permanent wilting point: It is the lower limit of soil moisture, which the plants can utilize. Below this the plants are not able to withdraw the soil moisture and thus the plants start to wilt or dry up.3. Available Water: It is the water in between the field capacity and wilting point. Or it is the difference between the field capacity and wilting point.


Infiltration

36

Unavailabe water.

Field capacity

Available water

Permanent wilting point

Gravitational waterRelativeDistribution of Water content

Airdry

Saturation

Gravitational water

Capillary water

Wilting point

Hygroscopic water


Fig. Relative distribution of moisture in different soil textural classes.

Classes and availability of water:Hygroscopic water: It is the thin film of water bound around the soil particles by molecular forces. Not available to plants.Capillary water: It is the water held in the pore spaces of soil when the excess water has drained down due to gravitational force. It is held by capillary force/surface tension force. It is available water for plant.Gravitational Water: The water in the excess of hydroscopic water and capillary water, which drains down due to the gravitational force is called gravitational water.

Ground water: It is the water in the zone of saturation below the zone of aeration.

Ground water balance:Zone of aeration – Moisture decrease for short timeZone of saturation – Moisture last for long period of timeInflow = Outflow + Change in storageRecharge (Qr) = Discharge + Change in storage (Qd)If recharge is more than the discharge from the ground water reservoir, storage will increase i.e. water table will rise. If the discharge exceeds the recharge into the ground water reservoirs, storage will decrease i.e. water level lowers.



Water table: It is the top of the zone of saturation or top of the ground water reservoir. Depending upon the relative difference between recharge and discharge, the water table rises or falls. Since zone of aeration is above the water table, it is in atmospheric pressure.

Recharge:Sources:1.Deep percolation of precipitation. (Major source of recharge)2.Influent flow:

Figure:If the water moves from the surface water body like ponds, lakes, streams/rivers etc to the ground water reservoirs, it is called influent flow.3.Underflow of water from a near by connected ground water reservoirs.4.Artificial Recharge: It is the recharge by mans activity. It may be planned or unplanned. Generally it may be done in a planned way through spreading water over the land surface through building special infiltration ponds and recharge wells.5.Other Ways: the other ways, by which recharge takes places are by irrigation, wastewater treatment ponds, septic system, soak pits or safety tank etc.

Discharge:1.Evapotranspiration: It is the process by which the ground water moves to atmosphere. In low lying areas, if the water table is near the surface, the capillary action draw the water to the root zone soil from where it is lost by evapotranspiration.2.Effluent flow:Flow of water out of ground water to the surface water body. Or if water flows from the ground water reservoirs to the surface water bodies, it is called affluent flow. It is the major form of discharge3.Springs or seeps:Where the water table intercepts the land surface or where the confined aquifer outlets to the surface, springs or seeps occurs.

4. Artificial Discharge-Pumping water, drawing water from the wells etc.

Capillary Fringe: It is the zone in which water has risen from the ground water table due to capillary action. Thickness of capillary fringe depends on the type of soil. e.g. clay - 3000 mm capillary fringe,Coarse sand < 100 mm capillary fringe.


Spring or seeps

38


Aquifer: Water bearing geologic bed or stratum like soil gravel or porous rocks. Good aquifer are coarse texture, unconsolidated, granular sedimentary stratum or bed which overlie the consolidated or aquiclude.A good aquifer must have:

1. A large volume in storage.2. Relatively high porosity.3. High permeability, allow easy movement of water (to move through easily)

Aquiclude: Geologic bed or stratum of very low or zero permeability. Water moves very slowly or cannot pass through clay and silt and rocks. Permeability of aquiclude is zero.Permeability: It is the ability of the water to pass through a geologic bed or stratum.Volume/time/area = m3/day/ m2

= m/dayClean gravel: It possesses high permeability i.e. > 4000m/dayUnweathered clay - <0.0004m/day.

Unconfined aquifer: An aquifer, which does not have confining layers of low or zero permeability. Top of unconfining aquifer is at atmospheric pressure.Confined Aquifer (Artesian aquifer): It has confining layer above it. The top of the confined aquifer is at high pressure than atmospheric pressure.Artesian Well (Free flowing well): If a pipe is inserted into the confined aquifer and the water flows out over the ground, that type of well is called artesian well. The water rises in a pipe inserted in to a confined aquifer because of hydrostatic pressure or weight of water above the point.Purched aquifer: An aquiclude present in an unconfined aquifer will intercept the percolating water and create a purched aquifer.Piezometric Surface: It is the height to which water will rise in a pipe or well inserted into a confined aquifer. If the piezometric surface is above ground an artesian aquifer occurs.Transmissivity: Permeability times the thickness of an aquifer is transmissivity.If, K = the permeability B = thicknessTransmissivity = (K + b)(K*b)

6.2 Energy of water in the soil Soil water moves in accordance with the forces acting upon it. Force is the capacity to do work. It is understood by considering the energy of water. So force is the ability of water to do work. Work = potential or ability to give motion to the water and move it from one place to another. It is the total potential energy of a mass of water.6.2.1 kinetic potential energy: it is energy due to motion. (Ignored)6.2.2 pressure potential energy: energy due to the pressure of a mass of water.



6.32.3 gravity potential energy: it is the energy due to the position (height) of a mass of water above some datum.6.2.4 Osmotic potential energy: it is the energy due to differ rent concentration of solution from place to place.6.2.5 Pressure potential: it is the energy potential to do work.i.e. To expend a force over a distance. P = force * Length = mg*L Gravity potential, Z = mgL Total potential energy = P + ZPotential energy components

4. Energy per unit weight of water ‘5. Length.

Pressure potential energy expressed by ‘H ‘6. i.e. the height above or below the water table.

Pressure of the soil water at the water table is atmospheric pressure.

Water pressure

Water pressure above the water table is positive. Water pressure below the water table is negative.

6.4 soil water movementTotal potential energy: is the hydraulic head (H)

Water moves from higher ‘H’ to lower ‘ H’. Greater the difference in ‘ H’ faster the flow and vice versa. Hence, the flow rate is inversely proportional to the change in hydraulic head (H)

The flow rate inversely proportional to the

1L

Hydraulic gradient (S)= change in hydraulic head

H ¿L ¿¿

¿

Darcy’s equation; Water velocity = permeability * Driving force

V = K change in hydraulic head

H ¿L ¿¿

¿¿¿¿ = K.S

Volumetric flow rate (Q) = AV = AKS = AK change in hydraulic head

H ¿L ¿¿

¿

6.6 Groundwater in Nepal:

Over geologic time, flowing water have carried much eroded material. In the steep mountainous river valleys, the water runs fast and carries coarse sand, gravel and even rocks.



In the Terai region, it slows down. Not able to carry large particles. Sand and gravels ceases moving and settle to the bottom causing the formation of large deposits of coarse material called alluvial fans.7. Along hills and mountains where rivers emerges and flow out onto the plains.8. Deposits vary in thickness when farther away from mountains9. More finer grained materials interspersed between layers of coarser materials10. Coarse-grained sedimentary deposits aquifer.11. Finer grained material (clay) = aquiclude.

Near the mountains, aquifer are unconfined and farther away into the plains, aquifer12. Dip down into the earth13. Are overlain by aquiclude.\

The areas having good sources of groundwater is called artesian aquifer.1. kailali and Kanchanpur districts.2. Kapilvastu, Rupandehi and nawalparasi districts.3. Eastern and central terai with the west end of the aquifer near Simra.

Groundwater movement.

Principle of ground water movement.1. To determine the maximum pumping rate.2. To determine the rate of ground water.

UNIT 7 RUNOFF

The portion of precipitation which ends up flowing over the land in various sizes pf channels , from large rivers to the small ditches between rows pf crops to sheet overflow on comp rating flat ground is called runoff. Lakes and streams: 0.34 % of total fresh water.Its knowledge is important because it is useful in many ways. It can cause destruction under certain conditions. Terminology: Runoff is the flow collected from a drainage basin and watershed and it appears at an outlet of the basin. Run off consist of;Surface runoffGroundwater runoff etc.3. Channel precipitation (cp): Precipitation that falls directly on the water surface of a flowing stream.Relatively small amount.Treated as a part of surface runoff.4. Overland flow (Rs) Part of the surface runoff that flow over the land surface toward stream channel without infiltrating into the soil.

5. Surface runoff: = it is the sum total of the channel precipitation and overland flows. I.e. (cp + Rs)

6. Sub surface flow or interflow (Ri): portion of precipitation, which infiltrates the soil but quickly runs laterally through the soil to the stream during the time of the storm.



7. Direct runoff or storm flow (Dr): part of runoff that enters the stream promptly after the precipitation or snow melting.

Dr= Cp +Rs +Ri 8. Base flow: it is the ground water flowing at a relatively steady long-term rate into stream.9. Stream flow (Q): it is the flow of water past any point above the bottom and sides of the channel.Q=Cp +Rs +Ri +Rg10. Underflow (U): it is the water flowing beneath the surface of a stream in the sediment and valley

deposits.11. Water Wy= Pt- Et-Δs yield (Wy): it is the watershed’s total yield of water during some period of

time. 10 Effective precipitation: it is the part of precipitation that contributes entirely to the direct runoff if only rainfall is involved.

Types of streams1. Stream or perennial stream: It carries water almost year round (90% of the time or

more.) in a well-defined channel.2. Wet weather or intermittent stream: it focuses only during the wet stream. (A few

month a year and they dry up.)3. Dry wash or ephemeral stream: it flows during and for short periods following

rainfall. Channels are after not well defined.

7.2 Variable source are concept.Watersheds are typically heterogeneous mixtures of soils, vegetative cover and land use. There may be a wide range if rainfall runoff responses.

At one extreme forested watershed with deep permeable soils may exhibit12. High infiltration capacity13. Predominantly sub surface flow

Where as rangelands with shallow soils may have low infiltration capacities and exhibit a quick stream flow response that is dominated by surface runoff.In a watershed;

14. Some areas produce surface runoff for any rainfall event (rocks road etc).15. Some seldom produce surface runoff, this response characterize a variable source are concept.

The concept suggests two mechanisms primarily responsible for quick flow response.1. An expanding saturated area that contributes flow and2. A rapid subsurface flow response from upland to lowland.

Runoff process At the start of a rainstorm, the precipitation first accumulated as Interception storage: precipitation held on the above ground surface of vegetation.Detention storage: precipitation detained only temporarily on the surface, the stored water then surfaces as depression storage.



As the rainstorm continuous infiltration rate decreases, depression storage is filled water starts overland. It then begins collectively into small streamlets, these gather together into larger streams. Interflow (subsurface flow also starts contributing to stream flow.)

7.3 Factor affecting runoffMeteorological factors

b. Precipitationc.Rainstorm movement (atmosphere)d. Temperature

Watershed characteristicsi. Shape and sizeii. Topographyiii. Surfaceiv. Storage characteristics

Precipitation-Rainfall amount, duration intensity and aerial distribution affect rate and volume of runoff.

v. For a given intensity, total runoff is related to the duration of the storm.vi. Short duration may produce no runoff. Long duration results in runoff.vii. Intense rainstorm exceeds infiltration capacity and produce great runoff.viii. Distribution of rainfall also contributes to size and intensity of runoff.ix. Maximum runoff is entire watershed contributes.

Atmosphere (storm) movement Runoff tends to reach at the same time. Higher peak runoff. Runoff of lower part is reduced before peak contribution from headwaters arrives at the outlet. Temperature Effect is indirect. It effects: type of vegetation and amount of ground water. Type of soil formation and organic matter. The Higher the temperature the higher will be the infiltration. It affects the viscosity of water. If temperature is low and the ground is frozen the infiltration is decreased.

Storm movementWatershed characteristics:

1. Shape and size: High intensity rainfalls are generally distributed over is relatively smaller area; small watershed is likely to produce greater discharge/ unit area.

Peak flow from all tributaries (approx) same size is likely to reach at the same time2.topography; It affects in velocity and amount of runoff.

Features Runoff characteristicsSlope gradient On steep slope, water will flow quickly

allowing fewer losses due to infiltration.Slope shape Increase runoff.Slope aspect Pm the windward side of the mountainous

watershed the intensity of runoff is more, causing increase in runoff.



Geological characteristics.

Geology or soil material also influences runoff. Infiltration and percolation increase if the soil (subsurface) is pervious but runoff decrease. In case of rocky surface without fissures and cracks runoff increase. Surface characters Patterns or arrangement of the natural stream channels increase drainage network. In well drained watershed;1. Length of overland flow is short.2. Surface runoff concentrates quickly.3. Peak flows are high.

Storage characters

a. Vegetation, agriculture and forestry practices influence infiltration.b. Structure like dams, levees, culverts cause to decrease run off rates.c. Vegetation retards overland flow and increase surface detention to reduce run off rates.

Measurement of runoff Various equations are used to measure the runoff. Some of the important equations are given below.

1. Manning equation.

V= 1/n R-2/3S1/2

Where, V= velocity (ms-1) N= roughness coefficientR= hydraulic radius (m) = A/P A= area of cross section

P= wetted perimeterS = watershed surface slopeQ= A *V

Rational methodQ = CIA/ 360Where,

Q= peak discharge (m3/sec) C= runoff coefficient

I= rainfall intensity (mm/ hr) A= Area in Ha.Intensity and duration



I= a/ t+b Where,

I=intensityt= duration of rainstorm


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