CE 3354 Engineering Hydrology Lecture 11: Watershed Loss Processes

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Precipitation (Input) Runoff (Output) Loss 3 Module 2 Hydrologic Cycle

Transcript of CE 3354 Engineering Hydrology Lecture 11: Watershed Loss Processes

CE 3354 Engineering HydrologyOutline
Loss Processes – Evapotranspiration
Evaporation Pans
Used in conjunction with lysimeter or EC Flux instruments to calibrate.
Then make measurements with a pan
The global standard for measuring evaporation is the evaporation pan.
A glorified stock tank that thousands of cooperative observers (volunteers!) have operated for about 150 years.
Most of the Pan section is lifted directly from the UN Training manual for irrigation water management (Brouwer and Heibloem, 1986.)
Evaporation pans provide a measurement of the combined effect of temperature, humidity, windspeed and sunshine on the reference crop evapotranspiration ET0.
Figure is a schematic of the relationship of the pan to a crop.
Generally the idea is that pan measurements are correlated to a particular crop - usually in a lysimeter setting.
Once the pan is 'calibrated' it can then be used to manage crop watering.
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Dug into ground, rectangular
Evaporation pans have other uses and their data is crucial to understanding the evaporative component of any regional water budget.
Many different types of evaporation pans are being used.
The best known pans are the Class A evaporation pan (circular pan), and the Sunken Colorado pan (square pan)
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The pan is installed in the field
The pan is filled with a known quantity of water
The 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 Constants
Need to be determined by lysimeter or Eddy Covariance instruments
Evapotranspiration – Models
Models are used to estimate ET for practical cases where measurements are not available
Blaney-Criddle
Turk
Thornwaithe
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 Model
Temperature is an average from daily values for a month
Blaney-Criddle Model
Loss Processes – Infiltration
Infiltration
Infiltration 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
Module 4
HEC-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.)
Module 4
Integral of rate is total depth (volume) lost
Module 4
Loss Model: NRCS CN
NRCS 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
Module 4
The CN approaches zero for no runoff generation.
Reminder:
The CN is NOT a percent of precipitation.
Module 4
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
Module 4
(included on reference flash-drive)
Composite CN equation
P = precipitation depth (inches)
Detailed development of the model, Chapter 10
Estimation of CN, Chapter 9
FHWA-NHI-02-001 (Highway hydrology)
Most hydrology textbooks
Module 4
Disadvantages
Same loss for given rainfall regardless of duration.
HEC-HMS User Manual 3.5 pg 137
Module 4
Loss Model: IaCl
Assumes 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.
Module 4
Typical values, Cl
Module 4
Two parameters, the initial abstraction and the constant loss rate.
Parameter estimation:
Local guidance (i.e. Harris County, circa 2003)
Module 4
Complexity appropriate for many studies
Disadvantages
May be too simplified for some studies
HEC-HMS User Manual 3.5, pg 136
“Initial and Constant Loss”
Loss Model: Green-Ampt
Infiltration 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:
Soil suction and saturated hydraulic conductivity
Module 4
Flux (infiltration rate); Governed by saturated hydraulic conductivity, soil suction, and accumulated infiltration.
Module 4
Saturated water content
Saturated hydraulic conductivity
Module 4
Documented soil saturation theory
Parameters can be estimated either by measurement or textural soils description
Disadvantages
HEC-HMS User Manual 3.5, pg 133
Module 4
Not in HEC-HMS, analyst prepares excess precipitation time series externally.
Documented in most hydrology textbooks.
Module 4
Similar to IaCl. Ia “rebounds” after period of zero precipitation.
HEC-HMS User Manual 3.5 pg 130
Exponential Model
Needs local calibration, popular in coastal communities (long history of calibration)
HEC-HMS User Manual 3.5 pg 130
Module 4
A soil science approach more complex than Green-Ampt, similar concepts.
Nine parameters
Soil Moisture Accounting
Three-layer soil storage model. Evapotranspiration used to dry upper layer.
14 parameters
Module 4
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
The rational method (Mulvaney, 1850; Kuichling, 1889) is a tool for estimating peak (maximum)
discharge from relatively small drainage areas.
It [the rational method] predates the automobile age.
Typical guidance is that the rational method should be applied to watersheds with drainage areas of 200 acres or less.
The rational method relates peak discharge to contributing drainage area, average rainfall intensity for a duration equal to a watershed response time (typically the time of concentration), and a coefficient that represents hydrologic abstractions and hydrograph
attenuation.
The coefficient is generally termed the runoff coefficient and has a range from 0 (no
peak discharge or runoff produced for a given rainfall intensity) to 1 (perfect conversion of rainfall
intensity to a peak discharge).
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Assumptions
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.
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