Hydrologic Equation

Hydrologic Equation

• Inflow = outflow +/- Changes in storage

• Equation is simple statement of mass conservation

Hydrologic inputs into area

• Precipitation

• Surface water inflow (streamflow + overland flow)

• Ground water inflow from outside area

• Artificial import (pipes + canals)

Hydrologic outputs into area

• Evapotranspiration from land areas

• Evaporation from surface water

• Runoff of surface water

• Groundwater outflow

• Artificial export of water through pipes and canals

Changes in storage

• Changes in volume of:

-- surface water in streams, rivers, lakes, and ponds.

-- soil moisture in vadose zone

-- ice and snow at surface

-- temperature depression storage

-- water on plant surfaces

-- ground water below water table

Mono Lake

• Inputs: precipitation; streams; ground water.• Outputs: evaporation; artificial streams.

Humidity

• Absolute Humidity number of grams of water per cubic meter

[ML-3]• Saturation Humidity maximum amount of water air can hold

[ML-3]• Relative Humidity = % ratio of absolute

humidity to saturation humidity.

Condensation

• Condensation occurs when air mass can no longer hold all of its humidity.

• Temperature drops => saturation humidity drops.

• If absolute humidity remains constant => relative humidity rises.

• Relative humidity reaches 100% => condensation => Dew point temperature.

Factors affecting Evaporation

• Water temperature.• Air temperature above water layer.• Absolute humidity of air above water surface.• Wind – keeps absolute humidity low.

- may increase the molecular diffusion.• Solar radiation – Langley = 1 cal./cm2; SI =>

joule/m2 = 4.18 x 104 Langleys.

Transpiration

• Plants pump water from ground to atmosphere; accounts for most vapor losses in land-dominated drainage basin.

• A function of plant density plant size limited by soil water.• Wilting point – surface tension of soil water

interface > Osmotic pressure.

Evapotranspiration

• Evapotranspiration = total water loss due to 1) free water evaporation, 2) plant transpiration, 3) soil moisture evaporation.

• Potential evapotranspiration – the water loss, which occur if at no time there is a deficiency of water in the soil for the use of vegetation.

• Actual evapotranspiration.

Cool, moist Cool, moistWarm, dry

Limited soil-moisture storage

Ample soil-moisture storage

Formation of Precipitation

• Humid air mass cooled to dew point temperature.

• Condensation or freezing nuclei (clay minerals, salt, combustion products …).

• Droplets coelesce to form raindrops.

• Raindrops must be large enough such that they do not evaporate.

Precipitation – cont.

• Adiabatic expansion

P decreases => V increases =>T decreases

• Dry lapse rate – Rising dry air 1OC/100m.

• Wet lapse rate – Rising moist air 0.5OC/100m.

Influences on rising air mass

• Convective processes.

• Movement of weather fronts.

• Orographic effects.

Effective uniform depth (EUD) of precipitation

• Arithmetic mean method – the rain gauge network is of uniform density.

EUD = an arithmetic average of the point-rainfall data.

Effective uniform depth (EUD) of precipitation

• Arithmetic mean method – the rain gauge network is of uniform density.

• Isohyetal line method:

- draw isohyets.

- EUD = the weighted average of each isohyetal area.

Effective uniform depth (EUD) of precipitation

• Arithmetic mean method – the rain gauge network is of uniform density.

• Isohyetal line method.

• Thiessen method.

- construct polygons

- weighted by polygon areas