1. Basic Hydrology & Hydraulics: DES 601
Module 9
Loss Models

2. Loss models
A loss model represents all of the processes that abstract or remove water from the gross rainfall volume.
Precipitation
Losses
Loss Model
Excess Precipitation
Response (Transform)
Runoff
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3. Loss models
Clouds
Surface Water Body
Lake or Stream
Ocean
Groundwater Flow
Surface Runoff
Precipitation
Transpiration
Evaporation
Sun
What remains after losses is what runs off- “EXCESS rainfall”
Losses are a function of TIME
Losses include:
Infiltration
Evaporation
Transpiration
Infiltration
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4. Loss models
A loss model represents all of the processes that abstract or remove water from the gross rainfall volume.
What remains after losses is what runs off- “EXCESS rainfall”.
Losses are a function of TIME
Loss models are often confused with infiltration models. They are not the same thing.
Infiltration is a component of losses, but alone does not account for all observed losses.
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5. Infiltration models commonly incorporated into loss models
Horton’s Infiltration Model
Initial Abstraction, Constant Loss Model
Green-Ampt Infiltration Model
NRCS Curve Number Model
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6. Horton’s Infiltration Model
Loss rate has initial and asymptotic value
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7. Initial Abstraction, Constant Loss
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.
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8. Initial Abstraction, Constant Loss
Typical values, Ia:
Sandy soils: to 1.50 inches
Clay soils : 0.40 to 1.00 inches
Typical values, Cl
Sandy soils: to 0.30 inches/hour
Clay soils : 0.05 to 0.15 inches/hour
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9. Initial Abstraction, Constant Loss
Two parameters, the initial abstraction and the constant loss rate.
Parameter estimation:
Calibration
TxDOT
Local guidance
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10. Assumes soil behaves like a permeameter.
Green-Ampt Loss Model
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:
Initial and saturated water content
Soil suction and saturated hydraulic conductivity
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11. Green-Ampt Loss Model
Flux (infiltration rate); Governed by saturated hydraulic conductivity, soil suction, and accumulated infiltration.
Volume infiltrated over time;
Governed by flux, change in water content.
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12. Parameter estimation Local guidance Green-Ampt Loss Model
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
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13. NRCS Curve Number Model
NRCS Runoff Curve Number
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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14. NRCS Curve Number Model
The CN approaches 100 for impervious cover
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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15. NRCS Curve Number Model
NRCS 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 2011)
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16. NRCS Curve Number Model
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17. NRCS Curve Number Model
Composite CN equation
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18. NRCS Curve Number Model
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19. Summary Basin loss models in the HDM include:
Initial and Constant rate loss
Green-Ampt
NRCS Curve Number
Table 4-29 in HDM provides use guidance
Loss
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