NWS-COMET Hydrometeorology Course 13 - 28 October 1999 Hydrology Primer

NWS-COMET Hydrometeorology Course

13 - 28 October 1999

Hydrology Primer

Russell J. Qualls, Asst. Professor

University of Colorado at Boulder

Civil, Env. and Arch.Engineering

303-492-5968 (Tel)

303-492-7317 (fax)

qualls@colorado.edu (email)

http://spot.colorado.edu/~qualls/

AcknowledgmentsDennis L. Johnson, Asst. Professor

Juniata CollegeEnvironmental Science & Studies

1700 Moore StreetHuntingdon, PA 16652Phone: (814) 641-5335Fax: (814) 641-3685johnson@juniata.edu

http://www.juniata.edu/~johnson

Purpose of the Hydrometeorology CoursePurpose of the Hydrometeorology Course

• Increase the participants knowledge and understanding of the interaction between meteorology and hydrology in watersheds:

• Increase participants understanding of the functional aspects of watersheds;

• Enhance the participants knowledge of the capabilities, limitations, and applications of new hydrometeorological observing systems;

• Improve the participants ability to identify significant mesoscale meteorological events and to produce Quantitative Precipitation Forecasts;

• Increase participants understanding of the effectiveness of the NWS forecast and warning methodologies and plan future enhancements; and

• Build awareness of the need for close ties between RFC's and WFO's.

Purpose of the PRIMERPurpose of the PRIMER

• Provide an introduction between participants & establish backgrounds.

• Introduce participants to basic terminology and concepts of hydrologic forecasting that will be used throughout the hydrology portion of the COMET Hydromet course. The primer introduces these concepts and specific detail will be provided in week 3.

• Establish the course objectives as per the expectations of the participants.

• Establish hydrologic concerns in the various participants' regions.

In the end, it is intended that participants will understand the hydrologic forecast process, the assumptions in the process, and the responsibilities associated with interpreting and issuing the forecast.

Mission of NOAA's NWSMission of NOAA's NWSHydrologic Services ProgramHydrologic Services Program

To provide river and flood forecasts and warnings for protection of life and property

Provide basic hydrologic forecast information for the nation's economic and environmental well being.

Modernized NWSModernized NWS

• “It is essential to emphasize the complementary aspects of operational hydrology and meteorology in the modernized NWS, while recognizing the uniqueness of RFC and WFO operations. “

New or Improved ProductsNew or Improved Products

• ...the production of a variety of hydrologic forecast products for an increased number of river locations across the country, including ESP-based products

What is ESP?What is ESP?

• Ensemble Streamflow Production (ESP)

• Inputs the current moisture level of soil and the precipitation from previous years into a model which produces the diagram seen above.

• For example, the moisture content of today would be inputted, along with the precipitation that occurred over the next week, but 50 years ago.

• This would then be repeated for 49 years ago, 48, etc., and then an average discharge based on history can be determined.

HydrologyHydrology

… an earth science. It encompasses the occurrence, distribution, movement, and

properties of the waters of the earth and their environmental relationships." (Viessman,

Knapp, Lewis, & Harbaugh, 1977 - Introduction to Hydrology, Harper & Row

Publishers, New York)

HydrometeorologyHydrometeorology

… an interdisciplinary science involving the study and analysis of the interrelationships between the atmospheric and land phases of

water as it moves through the hydrologic cycle." (Hydrometeorological Service Operations for

the 1990's, Office of Hydrology, National Weather Service, NOAA, 1996).

Hydrometeorology - LinksHydrometeorology - LinksHydrology

Engineering/FluidMechanics

In-depth hydrologicanalysis

Execution of complexhydrologic models.Adjustment ofmodel parameters, andthe derivation ofhydrologic forecasts forall time scales

Applied hydrologicresearch

Development andcalibration ofhydrologic models

Development ofhydrologic applicationsprocedures.

MeteorologyThermodynamics/atmospheric

physics orientation

In-depth meteorologicalanalysis

Weather forecast andwarning operations

Climatological forecasting Applied meteorological

and climatologicalresearch.

Development and calibrationof meteorological models

Development ofmeteorological applicationsand procedures.

HydrometeorologyInterdisciplinary

Orientation

Assimilation/use ofWSR-88D basedprecip. estimates

Production and/oruse of QPF's andother hydromet.forecasts

Use of RFC guidance(e.g. flash flood) inhydrologic warningoperations

Use of soil moisturestates fromhydrologic model inatmospheric model

Appliedhydrometeorologicalresearch.

Units & Properties of WaterUnits & Properties of Water

Property Symbol Value CommentsDensity

(mass/volume)

~1.94 slugs/ft3

~ 1.0 g/cm3Slug = lb*s2/ft

Specific Weight(weight/volume)

62.4 Lbs/ft3

9.81 kN/m3

Specific Volume

Specific Gravity s.g. 1.0 for water@ 32.9o F

s.g.fluid = fluid/water

Vapor Pressure ~0.4 psi Vapor pressure ofthe fluid - not the

atmosphere

Common Common Unit ConversionsUnit Conversions

Area Volume Runoff Volume Discharge Power

AreaArea

• 1 acre = 43,560 ft2

• 1 mi2 = 640 acres

• 1 hectare = 100m x 100m = 2.471 acres = 10,000 m2

• 1 km2 = 0.386 mi2

AreaArea Volume Runoff Volume Discharge Power

VolumeVolume

• 1 acre-foot = 1 ac-ft = 1 acre of water x 1 foot deep = 43,560 x 1 = 43,560 ft3

• 1 ac-inch = 1 acre x 1 inch deep = 43,560 x 1/12 = 3,630 ft3

• 1 ft3 = 7.48 gallons

• 1 gallon H2O ~ 8.34 lbs.

Area VolumeVolume Runoff Volume Discharge Power

Runoff VolumeRunoff Volume

• 1-inch of runoff over 1 square mile :

• 1/12 feet x 1 mi2 x 640 acres/mi2 x 43,560 ft2/acre = 2,323,200 ft3

Area Volume Runoff VolumeRunoff Volume Discharge Power

DischargeDischarge

• 1 cfs = 1 cubic foot per second

• 1 cfs x 7.48 gal/ft3 x 3600 sec/hr x 24 hrs/day = 646,272 gpd = 0.646 MGD

• 1 cfs x 3600 sec/hr x 24 hrs/day = 86,400 cfs/day

• 86,400 cfs/day x 1 ac-ft/43,560 ft3 = 1.983 ac-ft/day (~ 2 ac-ft/day)

• 1.983 ac-ft/day x 12 inches/ft x 1 day/24 hrs = 0.992 ac-in/hr

• 1 ac-in/hr x 43,560 ft3/ac-ft x 1 hr/3600 sec x 1 ft/12 inches = 1.008 cfs

Area Volume Runoff Volume DischargeDischarge Power

PowerPower

• 1 hp = 550 ft*lb/sec = 0.7547 kilowatts

Area Volume Runoff Volume Discharge PowerPower

Hydrologic CycleHydrologic CycleTopicsPrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

PrecipitationPrecipitation• ... primary "input" for the hydrologic cycle (or

hydrologic budget).

• … The patterns of the precipitation are affected by large scale global patterns, mesoscale patterns, "regional" patterns, and micro-climates.

• … Knowing and understanding the general, regional, and local precipitation patterns greatly aids forecasters in determining QPF values.

• … In addition to the quantity of precipitation, the spatial and temporal distributions of the precipitation have considerable effects on the hydrologic response.

PrecipitationPrecipitation -SnowEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

SnowSnow• ... nature of the modeling efforts that are required.

• … response mechanisms of snow are at a much slower time scale than for most of the other forms of precipitation.

• … The melt takes place and the runoff is "lagged" due to the physical travel processes.

• … Items to consider in the snowmelt process are the current "state" of the pack and the snow water equivalent of the snow pack., as well as the melt potential of the current climate conditions.

• … A rain-on-snow event may produce very high runoff rates and is often a difficult situation to predict due to the integral nature of the runoff and melt processes. The timing of these events is often very difficult to predict due to the inherent "lag" in the responses.

PrecipitationPrecipitation -Snow-SnowEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

EvaporationEvaporation• … Evaporation is a process that allows water to change

from its liquid phase to a vapor. • … Hydrologists are mostly interested in the evaporation

from the free water surface of open water or subsurface water exposed via the capillary action; however, precipitation that is intercepted by the vegetative canopy may also be evaporated and may be a significant amount in terms of the overall hydrologic budget.

• … Factors that affect evaporation are temperature, humidity and vapor pressure, radiation, and wind speed.

• … A number of equations are used to estimate evaporation. There are also a number of published tables and maps providing regional estimates of annual evaporation.

PrecipitationEvaporationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

TranspirationTranspiration

• … Water may also pass to the atmosphere by being "taken up" by plants and passed on through the plant surfaces.

• … Transpiration varies greatly between plants or crops, climates, and seasons.

• … Evaporation and transpiration are often combined in a term - evapotranspiration.

• … In many areas of the country and during certain seasons evapotranspiration is a major component of the hydrologic budget and a major concern in water supply and yield estimates.

PrecipitationEvaporationTranspirationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

Storage - SurfaceStorage - Surface

• ... Storage - Surface is used to describe the precipitation that reaches the ground surface; however, is not available for runoff or infiltration.

• … It is instead, held in small quantities on the surface in areas, such as the leafy matter and small depressions.

• … In general, surface storage is small and only temporary in terms of the overall hydrologic budget; however, it may have an effect on a storm response as it is effectively "filled" early on a storm event.

PrecipitationEvaporationTranspirationStorage-surfaceStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

InfiltrationInfiltration

• … Soils, depending on current conditions, have a capacity or ability to infiltrate precipitation, allowing water to move from the surface to the subsurface.

• ... "physically based” -> soil porosity, depth of soil column, saturation levels, and soil moisture.

• … The infiltration capacity of the soil column is usually expressed in terms of length per time (i.e. inches per hour).

• … As more water infiltrates, the infiltration generally decreases, thus the amount of water that can be infiltrated during the latter stages of a precipitation event is less than that at the beginning of the event.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationInfiltration -SubsurfaceStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

Infiltration cont.Infiltration cont.• … Storms that have high intensity levels may

also cause excess precipitation because the intensity (inches per hour) may exceed the current infiltration capacity (inches per hour).

• … periods of low rainfall or no rainfall will allow the soil to "recover" and increase the capacity to infiltrate water.…

• Infiltrated water replenishes soil moisture and groundwater reservoirs. Infiltrated water may also resurface to become surface flow.

• … attempt to account for infiltration by estimating excess precipitation (the difference between precipitation and excess being considered infiltration), for example, the Soil Conservation Service (SCS) runoff curve number method

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationInfiltration -SubsurfaceStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

Subsurface FlowSubsurface Flow

• …water may move via several paths.

• …subsurface flow can be evaporated if there is a well maintained transfer mechanism to the surface. This is particularly true for areas of high ground water table (the free water surface of the groundwater) which is within the limits of the capillary action or transport abilities.

• …Vegetation may also transpire or use the water.

• …The subsurface flow may also continue to move with the groundwater table as a subsurface reservoir, which the natural system uses during periods of low precipitation.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationInfiltration -Subsurface-SubsurfaceStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

Storage - SubsurfaceStorage - Subsurface

• … The infiltrated water may continue downward in the vertical, may move through subsurface layers in a horizontal fashion, or a combination of the two directions.

• … Movement through the subsurface system is much slower than the surface and thus there are storage delays. The water may also reach an aquifer, where it may be stored for a very long period of time.

• … In the NWS River Forecast System (RFS), the subsurface storage is represented by imaginary zones or "tanks". These tanks release the stored water at a given or calibrated rate. The released water from the subsurface zones is added to the surface runoff for convolution with the unit hydrograph.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

RunoffRunoff

• … runoff will be used to collectively describe the precipitation that is not directly infiltrated into the groundwater system.

• … is generally characterized by overland, gully and rill, swale, and channel flows.

• … is that portion of a precipitation event that "quickly" reaches the stream system. The term "quickly" is used with caution as there may be great variability in response times for various flow mechanisms.

• … Runoff producing events are usually thought of as those that saturate the soil column or occur during a period when the soil is already saturated. Thus infiltration is halted or limited and excess precipitation occurs. This may also occur when the intensity rate of the precipitation is greater than the infiltration capacity.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-Reservoirs

Overland FlowOverland Flow

•… Overland flow or surface flow is that precipitation that either fails to penetrate into the soil or that resurfaces at a later point due to subsurface conditions.

•… often referred to as "sheet" flow.

•… for the purposes of this discussion, overland flow (sheet and surface flow, as well) is considered to be the flow that has not had a chance to collect and begin to form gullies, rills, swales

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementWater Movement -Overland flow-Overland flow -Gullies and Rills -Swales -Channel Flow -Stream ChannelsStreamflowStorage-Reservoirs

Overland Flow (cont.)Overland Flow (cont.)•… will eventually reach defined channels and the stream system.

•… may also be infiltrated if it reaches an area that has the infiltration capacity to do so.

•… Overland flow distances are rather limited in length - National Engineering Handbook (1972) - overland flow will concentrate into gullies in less than 1000 feet.

•… Other (Seybert, Kibler, and White 1993) recommend a distance of 100 feet or less.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementWater Movement -Overland flow-Overland flow -Gullies and Rills -Swales -Channel Flow -Stream ChannelsStreamflowStorage-Reservoirs

Gullies & RillsGullies & Rills

• ... sheet flow or overland flow will soon concentrate into gullies and rills in the process of flowing towards the stream network. The location of these gullies and rills may vary from storm to storm, depending on storm patterns, intensities, current soil and land use conditions.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementWater Movement -Overland flow -Gullies and Rills-Gullies and Rills -Swales -Channel Flow -Stream ChannelsStreamflowStorage-Reservoirs

SwalesSwales

• … swales are of a more constant or permanent nature.

• … do not vary in location from storm to storm.

• … Swales are a natural part of the landscape or topography that are often more apparent than gullies and rills.

• … Flow conditions and behaviors in swales are very close to that which is seen in channels.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementWater Movement -Overland flow -Gullies and Rills -Swales-Swales -Channel Flow -Stream ChannelsStreamflowStorage-Reservoirs

Channel FlowChannel Flow

• … Excess precipitation ultimately reaches the stream channel system.

• … the stream system is generally more defined, it is by no means a constant or permanent entity.

• … The stream bed is constantly changing and evolving via aggredation and degradation.

• … Stream channels convey the waters of the basin to the outlet and into the next basin.

• … attenuation of the runoff hydrograph takes place.

• … Stream channel properties (flow properties) also vary with the magnitude of the flow.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementWater Movement -Overland flow -Gullies and Rills -Swales -Channel Flow-Channel Flow -Stream ChannelsStreamflowStorage-Reservoirs

Stream ChannelsStream Channels

• … Channels are commonly broken into main channel areas and overbank areas.

• … overbank areas are often referred to as floodplains.

• … Stream gaging stations are used to determine flows based on elevations in the channel and/or floodplain.

• … Bank full is often thought of as flood stage although more rigorous definitions are more applicable as they pertain to human activity and potential loss of life and property.

• … It is worth noting that the 2-year return interval flow is often thought of as "bank-full".

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementWater Movement -Overland flow -Gullies and Rills -Swales -Channel Flow -Stream Channels-Stream ChannelsStreamflowStorage-Reservoirs

StreamflowStreamflow

• … in the public eye -> the most important aspect of flooding and hydrology.

• … flooding from streams and rivers have the greatest potential to impact human property and lives; although overland flow flooding, mudslides, and landslides are often just as devastating.

• … Subsurface flow also enters the stream; although in some instances and regions, stream channels lose water to the groundwater table - regardless, this must be accounted for in the modeling of the stream channel.

• … Channels also offer a storage mechanism and the resulting effect is most often an attenuation of the flood hydrograph.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStreamflowStorage-Reservoirs

Storage - ReservoirsStorage - Reservoirs

• … Lakes, reservoirs, & structures, etc. are given a separate category in the discussion of the hydrologic cycle due to the potential impact on forecasting procedures and outcomes.

• … provide a substantial storage mechanism and depending on the intended purpose of the structure will have varying impacts on the final hydrograph, as well as flooding levels.

• … This effect can vary greatly depending on the type of reservoir, the outlet configuration, and the purpose of the reservoir.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-ReservoirsStorage-Reservoirs

Storage - Reservoirs (cont.)Storage - Reservoirs (cont.)

• … Flood control dams are used to attenuate and store potentially destructive runoff events.

• … Other structures may adverse effects. For example, bridges may cause additional "backwater" effects and enhance the level of flooding upstream of the bridge.

• … a catastrophic failure of a structure often has devastating effects on loss of life and property.

PrecipitationEvaporationTranspirationStorage-surfaceInfiltrationStorage - SubsurfaceRunoffWater MovementStreamflowStorage-ReservoirsStorage-Reservoirs

NWS - Forecast TerminologyNWS - Forecast Terminology

Hydrology TerminologyHydrology TerminologyTopicsTopicsWatershedStream flowReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

Hydrology TerminologyHydrology TerminologyWatershedWatershed -drainage area -drainage basin -sub-basin -sub-areaStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

• A watershedwatershed is an area of land that drains to a single outlet and is separated from other watersheds by a divide. • Every watershed has a drainage areadrainage area.• Related terms: drainage basindrainage basin, sub-sub-basinbasin, sub-areasub-area.

Hydrology TerminologyHydrology TerminologyWatershed

StreamflowStreamflow -cross-section area -Manning’s “n”RoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

•StreamflowStreamflow is the movement of water through a channel.•The cross-sectional areacross-sectional area of a stream is the region bounded by the walls of the stream and the water surface. The cross-sectional area is illustrated below.•See also Manning’s “n”.Manning’s “n”.

Stream Flow

Cross-sectional Area

Hydrology TerminologyHydrology TerminologyWatershed

StreamflowStreamflow -cross-section area-cross-section area -Manning’s “n”-Manning’s “n”RoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

•Manning’s “n”Manning’s “n” is a measure of the roughness of a surface, and in streamflow it is the roughness of the channel bottom and it’s sides.

Diagram 2 will have a higher Manning’s “n”Manning’s “n” because it has rougher surface due to the jagged bottom and pebbles.

Diagram 1 Diagram 2

Hydrology TerminologyHydrology TerminologyWatershedStreamflow

RoutingRouting -Hydrologic-Hydrologic -Hydraulic-HydraulicReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

HydrologicHydrologic HydraulicHydraulic

RoutingRouting

•RoutingRouting is used to account for storage and translation effects.

Hydrology TerminologyHydrology TerminologyWatershedStreamflow

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Generalized effect of routing

Hydrologic RoutingHydrologic RoutingWatershedStreamflow

•Hydrologic routingHydrologic routing is the more simple of the two techniques.•Based on the continuity equation which says Inflow - Outflow = Change in StorageInflow - Outflow = Change in Storage - or -

•A second relationship is also required which relates storage to discharge. This relationship is usually assumed, empirical, or analytical in nature.•Two types of hydrologic routing, RiverRiver and Reservoir RoutingReservoir Routing.

Hydraulic RoutingHydraulic RoutingWatershedStreamflow

•Hydraulic routingHydraulic routing is more complex and generally considered more accurate than hydrologic routing.

•Based on the simultaneous solution of the continuity equation and the momentum equation, commonly called the St. Venant equationsSt. Venant equations.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRouting

ReservoirsReservoirs -Storage -routingChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

•Reservoir storageReservoir storage attenuates the flow and delays the impact of flood waters. Reservoirs are generally used for flood control, drinking water

supply, hydropower, and recreation.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRouting

ReservoirsReservoirs -Storage -routingChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

•Reservoir routing is generally easier to perform than river routing because storage-discharge relations for pipes, weirs, and spillways are single-valued functions independent of flow. •Storage indication method or Puls Method

•Other Methods: Runge-Kutta Method

22)( n

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirs

ChannelChannel -Muskingum -Muskingum-Cunge -dynamicPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

• Channel routing can be broken into hydrologic and hydraulic methods.

• Hydrologic routing again uses the storage or continuity equation:

• This formula subtracts the average outflow from an average inflow to determine the change in storage over a given time period.

SSOOII

122121 2

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirs

• Common methods of hydrologic routing :

•Lag & K•Tatum•Mod-Puls•Kinematic Wave•Muskingum• Muskingum-Cunge**

Hydrology TerminologyHydrology Terminology

WatershedStreamflowRoutingReservoirs

•Hydraulic river routing includes solving the continuity equation and the momentum equation simultaneously.•Dynamic routing is an example of this.•DAMBRK & FLDWAV, as well as, UNET are dynamic routing models

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannel

PrecipitationPrecipitation -excess -intensity -patternsSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

•PrecipitationPrecipitation is water that falls to the earth in the form of rain, snow, hail or sleet.•Excess precipitationExcess precipitation is the precipitation that is not infiltrated into the soil and becomes available as a rapid runoff component in the hydrologic response of a basin.

•The intensityintensity of the precipitation is the rate at which it is raining, and is measured in length/time. A radar

picture of rainfall intensity can be seen below.

•Precipitation can fall in many different patternspatterns, which influences the hydrologic response.

•For example, a storm may be:•Uniform over the entire watershed•A storm may move up the watershed•A storm may move down the watershed•A storm may only rain on a portion of the watershed.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitation

SnowSnow -snowfall -snowmelt -snowpackRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

• SnowfallSnowfall is a form of precipitation that comes down in white or translucent ice crystals. •SnowmeltSnowmelt is the excess water produced by the melting of snow. This leads to flooding possibilities in the spring when temperatures begin to rise. There is generally a delay in the snowmelt response of a basin due to the melting process and travel times.•SnowpackSnowpack is the amount of annual accumulation at

higher elevations.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnow

RunoffRunoff -overland flow -sub-surface flow -baseflowInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

•RunoffRunoff is the excess precipitation and is often considered a “fast” response.•Overland flowOverland flow is the flow of water across the land surface.•Sub-surface flowSub-surface flow is the flow of water through the soil layers to the stream.•BaseflowBaseflow is the flow in a channel due to ground water or subsurface supplies. The baseflow is generally increased by precipitation events that produce enough infiltration.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoff

InfiltrationInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand UseFrequency

• InfiltrationInfiltration is the movement of water from the surface into the soil.•The rate of infiltration is based on a number of factors, including but not limited to:

•soil types•current conditions•precipitation intensity

•The are many methods to estimate infiltration and/or excess precipitation. To name a few :

• indexindex•Horton’s Horton’s •Green-AmptGreen-Ampt•SCS - curve number *SCS - curve number *•Continuous simulations (SAC-SMA)Continuous simulations (SAC-SMA)

WatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltration

Unit hydrographUnit hydrograph -derived -syntheticTimingFloodingFlowGrade linesLand UseFrequency

• The unit hydrographunit hydrograph is the hydrograph for 1 unit of runoff in a given specified time or duration of runoff.

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Baseflow

Surface Response

• The unit hydrographunit hydrograph is a “transfer” mechanism for transforming excess precipitation into streamflow.

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Baseflow

Surface Response

Derived Unit HydrographDerived Unit Hydrograph

Rules of Thumb•… the storm should be fairly uniform in nature and the excess precipitation should be equally as uniform throughout the basin. This may require the initial conditions throughout the basin to be spatially similar.

•… Second, the storm should be relatively constant in time, meaning that there should be no breaks or periods of no precipitation. • •… Finally, the storm should produce at least an inch of excess precipitation (the area under the

hydrograph after correcting for baseflow).

Synthetic Unit HydrographSynthetic Unit Hydrograph

• SCS

• Snyder

• Clark - (time-area)

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrograph

TimingTiming -lag time -time of concentration -durationFloodingFlowGrade linesLand UseFrequency

•Lag TimeLag Time is the time from the center of mass of the rainfall to the peak of the unit hydrograph. •Time of concentrationTime of concentration is the time at which outflow from a basin is equal to the inflow. It is often considered the longest travel time from any point in the watershed.•DurationDuration is the time span of the rainfall.

Lag time

Time of concentration

Duration of excess precipitation

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTiming

FloodingFlooding -bank-fullFlowGrade linesLand UseFrequency

•FloodingFlooding is the main concern of forecasters.

•Bank-fullBank-full flooding is often thought of as the two-year return flow or Q2. •The effects of flooding can drastically effect an ecosystem, which can be seen in the next two pictures.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTiming

FloodingFlooding -bank-fullFlowGrade linesLand UseFrequency

BeforeBefore

AfterAfter

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFlooding

FlowFlow -quantity -timing -velocity -”wave” speedGrade linesLand UseFrequency

•The flow and its effect on the environment and the human population depends on quantity, timing, velocity, and wave speed.•The quantityquantity of the flow is the volume of water, while the peak flow is generally of greatest interest.•The timingtiming of the flow is based on when a storm event occurs. If it occurs when a river is already close to flood stage, it will have a greater impact than if it occurred over a river that was relatively low. The time to peak, timeof concentration, lag time, response time, and duration are all of great concern.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFlooding

FlowFlow -quantity -timing -velocity -”wave” speedGrade linesLand UseFrequency

•The velocityvelocity of the flow is based on the slope of the stream bottom. The greater the slope the greater the potential velocity of the flow.

•The “wave” speed“wave” speed is the velocity of the flood wave down the channel. The speed of this wave affects how quickly the downstream area will effected.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlow

Grade linesGrade lines -EGL -HGLLand UseFrequency

Energy Grade Line

Hydraulic Grade Line(water surface)

Channel Bottom

headloss

Elevation Head

Depth1

Depth2

•The energy grade lineenergy grade line represents the depth of the water surface and the velocity component of the Bernoulli equation. •The hydraulic grade linehydraulic grade line represents the depth of the water surface.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade lines

Land UseLand Use -land cover -urbanization -karst -slopeFrequency

•Land UseLand Use is a major contributor to runoff behavior.•If the land is covered by trees, it will behave differently than if it was a pasture or a meadow.

•UrbanizationUrbanization also changes runoff patterns by the increase in artificial materials which decrease infiltration and increase flow response time.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade lines

Land UseLand Use -land cover -urbanization -karst -slopeFrequency

•Karst hydrology Karst hydrology is caused by pores and holes in limestone formations. This increases the infiltration into the limestone, reducing the runoff potential.

•The slopeslope changes the speed of runoff and therefore effects collection times.

Hydrology TerminologyHydrology TerminologyWatershedStreamflowRoutingReservoirsChannelPrecipitationSnowRunoffInfiltrationUnit hydrographTimingFloodingFlowGrade linesLand Use

FrequencyFrequency -return period -probability

•The frequencyfrequency of a storm event is described by its return periodreturn period. For example a two year storm event has a 1 in 2 chance of occurring in any given year.

•The probabilityprobability is also affected by the return period. Thus the probability of a 2 year storm occurring is 50%. The probability of a 100-year

event occurring is 1/100 or 1%

TopicsTopics

Energy Head

Momentum

Open Channel

Fluid ConceptsFluid Concepts

Energy or Energy HeadEnergy or Energy Head

• Elevation head

• Velocity head

• Total head

Energy HeadEnergy Head

-Elevation Head-Elevation Head

-Velocity Head-Velocity Head

-Total Head-Total Head

Momentum

Open Channel

Energy or Energy HeadEnergy or Energy Head

Momentum

Open Channel

•The total energy of water moving through a channel is expressed in total headtotal head in feet of water.•This is simply the sum of the the elevation above a datum (elevation head), the pressure head and the velocity head.•The elevation headelevation head is the vertical distance from a datum to a point in the stream.•The velocity head is expressed by:

VadVelocityhe

Energy Grade Line

Hydraulic Grade Line(water surface)

Channel Bottom

headloss

Elevation HeadElevation Head

Depth1

Depth2

Momentum

Open Channel

VeloctiyVeloctiyheadhead

Graphical depiction of elevation head, velocity head, and total head. Total head is the sum of velocity head, depth and elevation head.

Momentum EquationMomentum Equation

)Q(V 12 VForcesx Energy Head

MomentumMomentum

-Equation-Equation

-Forces-Forces

Open Channel

forces external weight friction forces chydrostati xForces

Hydrostatic Forces Friction Forces Weight External Forces

Hydrostatic ForcesHydrostatic Forces

Hydrostatic ForcesHydrostatic Forces Friction Forces Weight External Forces

Control Volume

HydrostaticForces

•Hydrostatic ForcesHydrostatic Forces are the forces placed on a control volume by the surrounding water. •The strength of the force is based on depth and can be seen in the following relationship:

P=P=HHH

Energy Head

MomentumMomentum

-Equation-Equation

-Forces-Forces

Open Channel

Friction ForcesFriction Forces

Hydrostatic Forces Friction ForcesFriction Forces Weight External Forces

Friction Force

The friction forcefriction force on a control volume is due to the water passing the channel bottom and depends on the roughness of the channel.

Control Volume

Energy Head

MomentumMomentum

-Equation-Equation

-Forces-Forces

Open Channel

WeightWeight

Hydrostatic Forces Friction Forces WeightWeight External Forces

Control Volume

The weightweight of a control volume is due to the gravitational pull on the its mass.

Weight

Weight = mg

Energy Head

MomentumMomentum

-Equation-Equation

-Forces-Forces

Open Channel

External ForcesExternal Forces

Hydrostatic Forces Friction Forces Weight External ForcesExternal Forces

Top View of Control Volume

Streamflow direction Fd

External Forces (FExternal Forces (Fdd) ) the forces created by a control volume striking a stationary object. External Forces can be explained by the following equation:

Fd=1/2CdAv2

Energy Head

MomentumMomentum

-Equation-Equation

-Forces-Forces

Open Channel

Steady vs. Unsteady FlowSteady vs. Unsteady Flow

Energy Head

Momentum

Open ChannelOpen Channel -Steady -vs- Unsteady-Steady -vs- Unsteady

-Uniform -vs- Nonuniform

-Supercitical -vs- subcritical

-Equations

•Fluid properties including velocity, pressure, temperature, density, and viscosity vary in time and space.•A fluid it termed steadysteady if the depth of flow does not change or can be assumed constant during a specific time interval.•Flow is considered unsteadyunsteady if the depth changes with time.

Uniform and Nonuniform FlowUniform and Nonuniform Flow

Energy Head

Momentum

Open ChannelOpen Channel -Steady -vs- Unsteady

-Uniform -vs- Nonuniform-Uniform -vs- Nonuniform

-Equations

•Uniform FlowUniform Flow is an equilibrium flow such that the slope of the total energy equals the bottom slope.•Nonuniform FlowNonuniform Flow is a flow of water through a channel that gradually changes with distance.

Super -vs.- Sub CriticalSuper -vs.- Sub CriticalEnergy Head

Momentum

Open ChannelOpen Channel

-Steady-vs.-Unsteady

-Uniform-vs. Nonuniform

-Sub/Supercritical-Sub/Supercritical

-Equations

Critical flow: a demonstrationCritical flow: a demonstration

No velocity

If a stone is dropped into a body of water, with no velocity, the waves formed by the water are fairly

circular. This is similar to sub-critical flow.

Energy Head

Momentum

-Equations

Small velocity

Now, if a velocity is added to the body of water, the waves become unsymmetrical, increasing to the downstream side. This happens as the velocity approaches critical flow. Notice that the wave still moves upstream, though slower than the downstream wave.

Energy Head

Momentum

-Equations

Large velocity

Now if a large velocity is added to the body of water, the wave patterns only go in one direction. This represents the point when flow has gone beyond critical, into the supercritical region.

Energy Head

Momentum

-Equations

Froude numberFroude number

The Froude numberFroude number is a numerical value that describes the type of flow present (critical, supercritical, subcritical), and is represented by the following equation for a rectangular channel:

NF = Froude number v = mean velocity of flow g = acceleration of gravitydm = mean (hydraulic) depth

Energy Head

Momentum

-Froude number-Froude number

-Equations

Froude numberFroude numberThe generalized formula for the Froude Froude numbernumber is as follows:

Fr = Froude numberQ = Flow rate in the channelB = Top width of water surfaceA = Area of the channel

Energy Head

Momentum

-Equations

Froude number - mean depth Froude number - mean depth

B=width of the free water surface

A=cross-sectional area of the channel

•Mean depth is a ratio of the width of the free water surface to the cross-sectional area of the channel.

Energy Head

Momentum

-Equations

Froude numberFroude number

The Froude number Froude number can then be used to quantify the type of flow.

•If the Froude number is less than 1.0, the flow is subcritical. The flow would would be characterized as tranquil.•If the Froude number is equal to 1.0, the flow is critical. •If the Froude number is greater than 1.0, the flow is supercritical and would be characterized as rapid flowing. This type of flow has a high velocity which can be potentially damaging.

Energy Head

Momentum

-Equations

Super-vs.-SubcriticalSuper-vs.-Subcritical

•Critical depth can also be determined by constructing a Specific Energy Curve.•The critical depth is the point on the curve with the lowest specific energy.•Any depth greater than critical depth is subcritical flow and any depth less than is supercritical flow.

Energy Head

Momentum

-Equations

Super-vs.-SubcriticalSuper-vs.-Subcritical

Specific Energy Curve

0 2 4 6 8 10

Specific Energy, E

th, dCritical depth

Subcritical depth

Supercritical depth

Open Channel EquationsOpen Channel Equations

• Chezy Equation

• Manning’s Equation

• Bernoulli Equation

• St. Venant Equations

Energy Head

Momentum

Open ChannelOpen Channel -Steady -vs- Unsteady

-Uniform -vs- Nonuniform

-Equations:-Equations:

ChezyChezy

ManningManning

BernoulliBernoulli

St. VenantSt. Venant

Chezy EquationChezy EquationEnergy Head

Momentum

-Chezy Equation-Chezy Equation

-Manning’s

-Bernoulli

-St. Venant

•In 1769, the French engineer Antoine Chezy developed the first uniform-flow formula.

RSCV •The formula was derived based on two assumptions. First, Chezy assumed that the force resisting the flow per unit area of the stream bed is proportional to the square of the velocity (KV2), with K being a proportionality constant.

Momentum

-Manning’s

-Bernoulli

-St. Venant

•The second assumption was that the channel was undergoing uniform flow.

•The difficulty with this formula is determining the value of C, which is the Chezy resistance factor. There are three different formulas for determining C, the G.K. Formula, the Bazin Formula, and the Powell Formula.

Momentum

-Manning’s

-Bernoulli

-St. Venant

•Later on, when Manning's equation was developed in 1889, a relationship between Manning’s “n” and Chezy’s “C” was established.

•Finally in 1933, the Manning equation was suggested for international use rather than Chezy’s Equation.

6/149.1R

Manning’s EquationManning’s EquationEnergy Head

Momentum

--Chezy Equation

-Manning’s-Manning’s

-Bernoulli

-St. Venant

•In 1889 Robert Manning, an Irish engineer, presented the following formula to solve open channel flow.

213249.1fh SR

V = mean velocity in fpsR = hydraulic radius in feetS = the slope of the energy linen = coefficient of roughness

The hydraulic radius (R) is a ratio of the water area to the wetted perimeter.

Manning’s EquationManning’s Equation

213249.1fh

Energy Head

Momentum

-Chezy Equation

-Bernoulli

-St. Venant

•This formula was later adapted to obtain a flow measurement. This is done by multiplying both sides by the area.

•Manning’s equation is the most widely used of all uniform-flow formulas for open channel flow, because of its simplicity and satisfactory results it produces in real-world applications.

Manning’s EquationManning’s EquationEnergy Head

Momentum

-Chezy Equation

-Bernoulli

-St. Venant

•Note that the equation expressed in the previous slide was the English version of Manning’s equation.•There is also a metric version of Manning’s equation, which replaces the 1.49 with 1. This is done because of unit conversions.•The metric equation is:

21321fh SAR

Bernoulli EquationBernoulli EquationEnergy Head

Momentum

-Chezy Equation

-Manning’s

-Bernoulli-Bernoulli

-St. Venant

•The Bernoulli equation is developed from the following equation:

This equation states that the elevation (z) plus the depth (y) plus the velocity head (V1

2/2g) is a constant. The difference being the headlosses - hL

Bernoulli EquationBernoulli Equation

Energy Head

Momentum

-Chezy Equation

-Manning’s

-St. Venant

•This equation was then adapted by making a few assumptions.•First, the head loss due to friction is equal to zero. This means the channel is perfectly frictionless surface.•Second, that alpha1 is equal to alpha2 which is equal to 1. The alpha’s are in the original equation to account for a non-uniform velocity distribution. In this case we will assume a uniform distribution which produces the following equation:

constant22

V constant z

Energy Head

Momentum

-Chezy Equation

-Manning’s

-St. Venant

A simplified version of the formula is given below:

Some comments on the Bernoulli equation

•Energy only

•Headloss in terms of energy

•Cannot calculate forces

•Limited Effect in “rapidly varying flow”

Energy Head

Momentum

-Chezy Equation

-Manning’s

-St. Venant

St. Venant EquationsSt. Venant EquationsEnergy Head

Momentum

-Chezy Equation

-Manning’s

-Bernoulli

-St. Venant-St. Venant

ySS of

The two equations used in modeling are the continuity equation and the momentum equation.

Continuity equation

Momentum Equation

St. Venant EquationsSt. Venant EquationsEnergy Head

Momentum

-Chezy Equation

-Manning’s

-Bernoulli

-St. Venant-St. Venant

y - S = S of

Unsteady -Nonuniform

Steady - Nonuniform

Diffusion or noninertial

Kinematicof SS

The Momentum Equation can often be simplified based on the conditions of the model.

Simulating the Hydrologic Simulating the Hydrologic ResponseResponse

Model TypesPrecipitationLossesModeling LossesModel Components

Model TypesModel Types

• Empirical

• Lumped

• Distributed

Model TypesModel TypesPrecipitationLossesModeling LossesModel Components

PrecipitationPrecipitation• … magnitude, intensity, location, patterns,

and future estimates of the precipitation.

• … In lumped models, the precipitation is input in the form of average values over the basin. These average values are often referred to as mean aerial precipitation (MAP) values.

• … MAP's are estimated either from 1) precipitation gage data or 2) NEXRAD precipitation fields.

Model TypesPrecipitationPrecipitation -Thiessen -Isohyetal -NexradLossesModeling LossesModel Components

Precipitation (cont.)Precipitation (cont.)• … If precipitation gage data is used, then the

MAP's are usually calculated by a weighting scheme.

• … a gage (or set of gages) has influence over an area and the amount of rain having been recorded at a particular gage (or set of gages) is assigned to an area.

• … Thiessen method and the isohyetal method are two of the more popular methods.

Model TypesPrecipitationPrecipitation -Thiessen -Isohyetal -NexradLossesModeling LossesModel Components

ThiessenThiessen

•Thiessen methodThiessen method is a method for areally weighting rainfall through graphical means.

Model TypesPrecipitationPrecipitation -Thiessen-Thiessen -Isohyetal -NexradLossesModeling LossesModel Components

IsohyetalIsohyetal

•Isohyetal methodIsohyetal method is a method for areally weighting rainfall using contours of equal rainfall (isohyets).

Model TypesPrecipitationPrecipitation -Thiessen -Isohyetal-Isohyetal -NexradLossesModeling LossesModel Components

NEXRADNEXRAD

•NexradNexrad is a method of areally weighting rainfall using satellite imaging of

the intensity of the rain during a storm.

Model TypesPrecipitationPrecipitation -Thiessen -Isohyetal -Nexrad-NexradLossesModeling LossesModel Components

LossesLosses• … modeled in order to account for the destiny of the

precipitation that falls and the potential of the precipitation to affect the hydrograph.

• … losses include interception, evapotranspiration, depression storage, and infiltration.

• … Interception is that precipitation that is caught by the vegetative canopy and does not reach the ground for eventual infiltration or runoff.

• … Evapotranspiration is a combination of evaporation and transpiration and was previously discussed.

• … Depression storage is that precipitation that reaches the ground, yet, as the name suggests, is stored in small surface depressions and is generally satisfied during the early portion of a storm event.

Model TypesPrecipitationLossesLossesModeling LossesModel Components

Modeling LossesModeling Losses

• … simplistic methods such as a constant loss method may be used.

• … A constant loss approach assumes that the soil can constantly infiltrate the same amount of precipitation throughout the storm event. The obvious weaknesses are the neglecting of spatial variability, temporal variability, and recovery potential.

• Other methods include exponential decays (the infiltration rate decays exponentially), empirical methods, and physically based methods.

• … There are also combinations of these methods. For example, empirical coefficients may be combined with a more physically based equation. (SAC-SMA for example)

Model TypesPrecipitationLossesModeling LossesModeling Losses -SAC-SMAModel Components

Simulating Watershed ResponseSimulating Watershed Response

InfiltrationInfiltrationLong Term –vs.- ShortLong Term –vs.- Short InfiltrationInfiltrationEvapotranspirationEvapotranspirationUnit HydrographUnit HydrographTimingTimingRoutingRouting

Infiltration or “losses” - this section describes the action of the precipitation infiltrating into the ground. It also covers the concept of initial abstraction, as it is generally considered necessary to satisfy the initial abstraction before the infiltration process begins.

Initial Abstraction - It is generally assumed that the initial abstractions must be satisfied before any direct storm runoff may begin. The initial abstraction is often thought of as a lumped sum (depth). Viessman (1968) found that 0.1 inches was reasonable for small urban watersheds.

Would forested & rural watersheds be more or less?

Forested & rural watersheds would probably have a higher initial abstraction.

The Soil Conservation Service (SCS) now the NRCS uses a percentage of the ultimate infiltration holding capacity of the soil - i.e. 20% of the maximum soil retention capacity.

Infiltration is a natural process that we attempt to mimic using mathematical processes. Some of the mathematical process or simulation methods are conceptual while others are more physically based.

Constant Infiltration Rate :

A constant infiltration rate is the most simple of the methods. It is often referred to as a phi-index or -index.

In some modeling situations it is used in a conservative mode.

The saturated soil conductivity may be used for the infiltration rate.

The obvious weakness is the inability to model changes in infiltration rate.

The phi-index may also be estimated from individual storm events by looking at the runoff hydrograph.

Constant Percentage Method :

Another very simplistic approach - this method assumes that the watershed is capable of infiltrating or “using” a value that is proportional to rainfall intensity.

The constant percentage rate can be “calibrated” for a basin by again considering several storms and calculating the percentage by :

excess

Constant Percentage ExampleConstant Percentage Example

Long Term –vs.- ShortLong Term –vs.- Short InfiltrationInfiltrationEvapotranspirationEvapotranspirationUnit HydrographUnit HydrographTimingTimingRoutingRouting

77.5% infiltrates

Exponential Decay: This is purely a mathematical function - of the following form:

ktecfofcfif )(

Exponential Decay:

Effect of fo or fc

Exponential Decay:

Effect of K

SCS Curve Number:

Soil Conservation Service is an empirical method of estimating EXCESS PRECIPITATION

We can imply that :

P - Pe = F

SCS (NRCS) SCS (NRCS) Runoff Curve NumberRunoff Curve Number

• The basic relationships used to develop the curve number runoff prediction technique are described here as background for subsequent discussion. The technique originates with the assumption that the following relationship describes the water balance of a storm event.

where F is the actual retention on the watershed, Q is the actual direct storm runoff, S is the potential maximum retention, and P is the potential maximum runoff

MoreMore ModificationsModifications

• At this point in the development, SCS redefines S to be the potential maximum retention

• SCS defines Ia in terms of S as : Ia = 0.2S

• and since the retention, F, equals effective precipitation minus runoff : F = (P-Ia) - Q

• Substituting gives the familiar SCS rainfall-runoff

0.8S)+(P

)0.2S-(P = Q

Estimating “S”Estimating “S”

• The difficult part of applying this method to a watershed is the estimation of the watershed’s potential maximum retention, S.

• SCS developed the concept of the dimensionless curve number, CN, to aid in the estimation of S.

• CN is related to S as follows :

10 - CN

1000 = S

CN ranges from 1 to 100 (not really!)

Determine CNDetermine CN

• The Soil Conservation Service has classified over 8,500 soil series into four hydrologic groups according to their infiltration characteristics, and the proper group is determined for the soil series found.

• The hydrologic groups have been designated as A, B, C, and D.

• Group A is composed of soils considered to have a low runoff potential. These soils have a high infiltration rate even when thoroughly wetted.

• Group B soils have a moderate infiltration rate when thoroughly wetted,

• while group C soils are those which have slow infiltration rates when thoroughly wetted.

• Group D soils are those which are considered to have a high potential for runoff, since they have very slow infiltration rates when thoroughly wetted (SCS, 1972).

Adjust CN’sAdjust CN’s

CN for AMC II Corresponding CN’s

AMC I AMC III

100 100 100

95 87 98

90 78 96

85 70 94

80 63 91

75 57 88

70 51 85

65 45 82

60 40 78

55 35 74

50 31 70

SAC-SMASAC-SMA

• … The Sacramento Soil Moisture Accounting Model (SAC-SMA) is a conceptual model of soil moisture accounting that uses empiricism and lumped coefficients to attempt to mimic the physical constraints of water movement in a natural system.

Tension Free

Tension Free - Primary

Free - Supplemental

Upper Zone

Lower Zone

Model TypesPrecipitationLossesModeling LossesModeling Losses -SAC-SMA-SAC-SMAModel Components

RunoffRunoff

• … Runoff is essentially the excess precipitation - the precipitation minus the losses.

• … In the NWSRFS, runoff is modeled through the use of the SAC-SMA or an antecedent precipitation index (API) model.

• … Runoff is transformed to streamflow at the basin outlet via a unit hydrograph.

• … In actuality, all forms of surface and subsurface flow that reach a stream channel and eventually the outlet are modeled through the use of the unit hydrograph.

Model TypesPrecipitationLossesModeling LossesModel ComponentsModel Components -Runoff-Runoff -Unit Hydrograph

Unit HydrographUnit Hydrograph

• The hydrograph that results from 1-inch of excess precipitation (or runoff) spread uniformly in space and time over a watershed for a given duration.

• The key points :• 1-inch of EXCESS precipitation• Spread uniformly over space - evenly over

the watershed• Uniformly in time - the excess rate is

constant over the time interval• There is a given duration

Model TypesPrecipitationLossesModeling LossesModel ComponentsModel Components -Runoff -Unit Hydrograph-Unit Hydrograph

Linearity of Unit HydrographLinearity of Unit Hydrograph• … In addition, when unit hydrograph theory is applied, it is

assumed that the watershed responds linearly.

• … Meaning that peak flow from 2 inches of excess will be twice that of 1 inch of excess

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0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000

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Baseflow

Surface Response

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Total Hydrograph

Surface Response

Baseflow

•Rules of Thumb :•… the storm should be fairly uniform in nature and the excess precipitation should be equally as uniform throughout the basin. This may require the initial conditions throughout the basin to be spatially similar. •… Second, the storm should be relatively constant in time, meaning that there should be no breaks or periods of no precipitation. •… Finally, the storm should produce at least an inch of excess precipitation (the area under the hydrograph after

correcting for baseflow).

Synthetic Unit HydrographSynthetic Unit Hydrograph

• SCS

• Snyder

• Clark - (time-area)

SCS - Dimensionless UHGSCS - Dimensionless UHG

Time-AreaTime-Area

Q % Area

Timeof conc.

Time-AreaTime-Area

Stream RoutingStream Routing

• ... stream routing is used to account for storage and translation effects as a runoff hydrograph travels from the outlet of one basin through the next downstream basin.

• … Most of the time, channels act as reservoirs and have the effect of attenuating the hydrograph.

• … 2 basic types of flow or channel routing :

• hydrologic

• hydraulic

Typical Effect of RoutingTypical Effect of Routing

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Lakes, Reservoirs, Impoundments,Lakes, Reservoirs, Impoundments,

• ...have the effect of storing flow and attenuating hydrographs.

• … Reservoirs (and impoundments) are modeled with some form of routing.

• … hydrologic and hydraulic routing may be applicable; although most often, hydrologic routing is used in reservoir routing for normal flow conditions.

• … During failure scenarios an unsteady flow model (hydraulic routing) is usually necessary due to the nature of the flow, which is

rapidly changing.

Factors Affecting the Factors Affecting the Hydrologic ResponseHydrologic Response

• Current Conditions• Precipitation Patterns• Land Use• Channel Changes• Others…..

Current ConditionsCurrent Conditions

• Wet

• Dry

• Update model states

• subjective

Precipitation PatternsPrecipitation Patterns

• … The pattern is both temporal and spatial.

• … A storm moving away from an outlet will have a very different result than the identical storm pattern (spatially) moving towards the outlet.

• … Lumped hydrologic models have a very difficult time in simulating spatially and temporally varied storm events.

• … The very nature of MAP values - indicates one of the problems.

• … A forecaster must understand the potential of precipitation patterns to affect the forecast

Land UseLand Use

• Urban

• Agricultural

• Anything that changes the infiltration, runoff, etc...

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After"urbanization"

Before"urbanization"

Channel ChangesChannel Changes

• Slopes

• Storage

• Rating Curve

• Ice!!!

Rating CurvesRating Curves

• Rating curves establish a relationship between depth and the amount of flow in a channel.

Rating Curve for a sample watershed

0 2000 4000 6000 8000 10000 12000 14000 16000

Q total (cfs)

NWS-COMET Hydrometeorology Course 13 - 28 October 1999 Hydrology Primer

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