Download - CE 3354 Engineering Hydrology Lecture 11: Watershed Loss Processes.

CE 3354 Engineering Hydrology

Lecture 11: Watershed Loss Processes

OutlineLoss Processes

EvapotranspirationWriting Workshop

Precipitation (Input)

Runoff (Output)

Loss

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Module 2

Hydrologic Cycle

Loss Processes – Evapotranspiration

Process ConceptsUseful Models:

Blaney-Criddle Thornwaithe


Process ConceptsHeat EnergyVapor transport

Relative humidity Wind speed


Process ConceptsThermodynamicsFluid Mechanics

Energy Method Aerodynamic Method Combined Method

Data Requirements Extensive

CMM pp 80-91


MeasurementEvaporation Pans

Used worldwideFlux Instruments

Eddy Covariance Instruments

Evaporation PansUsed in conjunction with

lysimeter or EC Flux instruments to calibrate.

Then make measurements with a pan

Evaporation PansClass A - Circular. Colorado Sunken

Dug into ground, rectangular

Evaporation Pan Operation (1 of 2)The pan is installed in the fieldThe pan is filled with a known quantity of waterThe water is allowed to evaporate during a certain

period of time (usually 24 hours).The rainfall, if any, is measured simultaneously

Every 24 hours, the remaining quantity of water (i.e. water depth) is measured

Evaporation Pan Operation (2 of 2)The amount of evaporation per time unit (the

dffierence between the two measured water depths) is calculated; this is the pan evaporation: Epan (in mm=24 hours)

The Epan is multiplied by a pan coecient, Kpan, to obtain the ETo.

Reset the pan for next time interval to desired level

Don’t forget to dress well for the measurement. You are a scientist/engineer. STEM == TIES

Pan ConstantsNeed to be determined by lysimeter or Eddy

Covariance instruments

Evapotranspiration – Models

Models are used to estimate ET for practical cases where measurements are not availableBlaney-CriddleTurkThornwaithe

All similar in that they are correlations to averaged measurements at different locations

All are just approximations, but are used in practice and when ET matters they may be only tool available

Blaney-Criddle ModelSimple formula – Temperature and latitude driven

only!Estimates daily rate for a particular month

Temperature is an average from daily values for a month

Blaney-Criddle ModelP- value by latitude and month

Loss Processes – InfiltrationInfiltration

Process ConceptsModels:

Hortonian Loss Model Green-Ampt Loss Model NRCS Runoff Generation Model Initial Abstraction, Constant Rate Model

InfiltrationInfiltration is water that soaks into the ground. This water

is considered removed from the runoff process.

Largest contribution to losses during a storm event, hence most loss models are some form of an infiltration accounting

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Loss ModelsHEC-HMS

Losses are infiltration losses. Evaporation is modeled as a component of meterology.

Infiltration accounting defined by soil properties and ground cover. Soil type (sand, clay, silt, etc.) Land use (percent impervious, etc.)

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Hortonian InfiltrationInfiltration Excess Concept

Rate has an initial and asymptotic value.

Integral of rate is total depth (volume) lost

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Loss ModelsDetailed Discussion

NRCS Curve Number Green-Ampt Initial Abstraction, Constant Loss

Other Methods Exponential Model Phi-Index (and proportional rainfall) Soil Moisture Accounting Deficit/Constant

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Loss Model: NRCS CNNRCS Runoff Curve Number

Is really a runoff generation model, but same result as a loss model.

Uses tables for soil properties and land use properties.

Each type (A,B,C, or D) and land use is assigned a CN between 10 and 100

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Loss Model: NRCS CNThe CN approaches 100 for impervious

The CN approaches zero for no runoff generation.

Reminder: The CN is NOT a percent impervious. The CN is NOT a percent of precipitation.

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Loss Model: NRCS CNNRCS CN method

Separate computation of impervious cover then applied to pre-development land use or

Use a composite CN that already accounts for impervious cover.

Composite CN described in TxDOT Hydraulic Design Manual (circa 2009)

Composite common in TxDOT applications

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Loss Model: NRCS CN

Rural: Table from NEH-630-Chapter 9 (included on reference flash-drive)

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Loss Model: NRCS CN

Urban: Table from NEH-630-Chapter 9 (included on reference flash drive)

Composite CN equation

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Loss Model: NRCS CNRunoff generated by

)8.0()2.0( 2

SPSPq

where,q = depth of direct runoff (inches)P = precipitation depth (inches)

CNCNS )101000(

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Loss Model: NRCS CNGraphical runoff

generation model

From NEH-630-Chapter 10

Depth

DepthModule 4

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Loss Model: NRCS CNParameter Estimation

NEH 630 Chapters 9 and 10 Detailed development of the model, Chapter 10 Estimation of CN, Chapter 9

FHWA-NHI-02-001 (Highway hydrology) Most hydrology textbooks TxDOT Hydraulics Design Manual (circa 2009)

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Loss Model: NRCS CNAdvantages

Simple, documented approach Widely used and established across the USA

Disadvantages Losses approach zero for moderate duration storms Same loss for given rainfall regardless of duration.

HEC-HMS User Manual 3.5 pg 137

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Loss Model: IaClAssumes soil has an initial capacity to absorb a

prescribed depth.

Once the initial depth is satisfied, then a constant loss rate thereafter. No recovery of initial capacity during periods of no

precipitation.

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Loss Model: IaClTypical values, Ia:

Sandy soils: 0.80 to 1.50 inches Clay soils : 0.40 to 1.00 inches

Typical values, Cl Sandy soils: 0.10 to 0.30 inches/hour Clay soils : 0.05 to 0.15 inches/hour

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Loss Model: IaClTwo parameters, the initial abstraction and the constant

loss rate.

Parameter estimation: Calibration TxDOT 0-4193-7 (HEC-HMS Example 2) Local guidance (i.e. Harris County, circa 2003)

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Loss Model: IaClAdvantages

Simple to set up and use Complexity appropriate for many studies

Disadvantages Parameter estimation (outside of 0-4193-7) May be too simplified for some studies

HEC-HMS User Manual 3.5, pg 136 “Initial and Constant Loss”

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Loss Model: Green-AmptInfiltration model based on constant head or constant

vertical flux into a porous medium. Assumes soil behaves like a permeameter. Uses Darcy’s law (adjusted for soil suction).

Four parameters: Initial and saturated water content Soil suction and saturated hydraulic conductivity

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Loss Model: Green-Ampt

Volume infiltrated over time;Governed by flux, change in water content.

Flux (infiltration rate); Governed by saturated hydraulic conductivity, soil suction, and accumulated infiltration.

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Loss Model: Green-AmptParameter estimation

Initial water content wilting point is a good lower bound for modeling

Saturated water content porosity is a good approximation

Saturated hydraulic conductivity Infiltrometer measurements

Soil suction Textural description Hanging column measurements

Local guidance (e.g. Harris County has suggested GA parameter values)

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Loss Model: Green-AmptAdvantages

Documented soil saturation theory Parameters can be estimated either by measurement or

textural soils description

Disadvantages Parameter estimates NON-TRIVIAL. More complex than rest of hydrologic model.

HEC-HMS User Manual 3.5, pg 133

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Other Loss ModelsDeficit and Constant

Exponential Model

Smith Parlange

Soil Moisture Accounting

Phi-Index (and proportional rainfall) Not in HEC-HMS, analyst prepares excess precipitation time

series externally. Documented in most hydrology textbooks.

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Other Loss ModelsDeficit and Constant

Similar to IaCl. Ia “rebounds” after period of zero precipitation.

HEC-HMS User Manual 3.5 pg 130

Exponential Model Exponential decay of infiltration rate Needs local calibration, popular in coastal communities

(long history of calibration) HEC-HMS User Manual 3.5 pg 130

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Other Loss ModelsSmith Parlange

A soil science approach more complex than Green-Ampt, similar concepts.

Nine parametersHEC-HMS User Manual 3.5, pg 138

Soil Moisture AccountingThree-layer soil storage model. Evapotranspiration

used to dry upper layer.14 parametersHEC-HMS User Manual 3.5, pg 139

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SCS Curve Number Model

The rational method is a tool for estimating peak discharge from relatively small drainage areas. (Mulvaney, 1850; Kuichling, 1889)

CMM pp. 496-502

AssumptionsRainfall is distributed uniformly over the

drainage area.Rainfall intensity is uniform throughout the

duration of the storm.Response time for the drainage area is less

than the duration of peak rainfall intensity.

Next TimeWriting WorkshopHEC-HMS Workshop (bring laptops)