Loss Methods
Overview
Loss methods compute the portion of rainfall that does not become direct runoff. These "losses" include interception by vegetation, depression storage, and infiltration into the soil. HydraLink supports two loss methods: SCS Curve Number and Green-Ampt.
SCS Curve Number Method
Theory
The SCS (now NRCS) Curve Number method is an empirical method that estimates cumulative runoff from cumulative rainfall using a single parameter, the Curve Number (CN).
Equations
Cumulative runoff:
Where:
- Q = cumulative runoff depth (inches)
- P = cumulative rainfall depth (inches)
- S = potential maximum retention = (1000 / CN) − 10 (inches)
- Ia = initial abstraction = λ × S (inches)
The initial abstraction Ia represents all losses before runoff begins (interception, depression storage, initial infiltration). The ratio λ defaults to 0.2, the classic NRCS value, and can be set per basin with the Ia/S Ratio row; 0.05 is the common alternative.
Incremental Excess Rainfall
For hydrograph computation, HydraLink computes incremental excess rainfall at each time step:
- At each time step, compute cumulative rainfall P(t)
- Compute cumulative runoff Q(t) from the SCS equation
- Incremental excess = Q(t) − Q(t−1)
Curve Number Selection
CN depends on land cover, treatment, hydrologic condition, and hydrologic soil group (HSG).
Table values are for antecedent runoff condition II (average) — the standard design condition, and the one HydraLink runs. TR-55 itself carries no antecedent-condition adjustment.
Hydrologic Soil Groups
| Group | Infiltration Rate | Soil Textures |
|---|---|---|
| A | High (> 0.30 in/hr) | Sand, loamy sand |
| B | Moderate (0.15–0.30 in/hr) | Sandy loam, loam |
| C | Low (0.05–0.15 in/hr) | Sandy clay loam |
| D | Very Low (< 0.05 in/hr) | Clay loam, silty clay, clay |
Example CN Values (AMC-II)
| Cover Description | A | B | C | D |
|---|---|---|---|---|
| Open space (good condition) | 39 | 61 | 74 | 80 |
| Impervious areas | 98 | 98 | 98 | 98 |
| Residential (1/4 acre) | 51 | 68 | 79 | 84 |
| Residential (1 acre) | 46 | 65 | 77 | 82 |
| Commercial (85% imp) | 89 | 92 | 94 | 95 |
| Woods (good condition) | 30 | 55 | 70 | 77 |
| Pasture (good condition) | 39 | 61 | 74 | 80 |
HydraLink provides a CN lookup dialog with the TR-55 values built in; you can also create and save your own CN tables.
Green-Ampt Method
Theory
The Green-Ampt method is a physically-based infiltration model that represents the wetting front as a sharp boundary moving downward through the soil. It provides a more detailed representation of infiltration than the CN method, especially for long-duration storms.
Equations
Infiltration rate:
Cumulative infiltration:
Where:
- f(t) = infiltration rate at time t (in/hr)
- K = saturated hydraulic conductivity (in/hr)
- ψ = wetting front suction head (in)
- Δθ = initial moisture deficit (dimensionless, 0–1)
- F(t) = cumulative infiltration at time t (in)
The cumulative infiltration equation is implicit in F and is solved iteratively.
Parameters
| Parameter | Units | Description |
|---|---|---|
| K | in/hr | Saturated hydraulic conductivity: the rate at which water moves through saturated soil |
| ψ (Psi) | inches | Wetting front suction head, the capillary pressure pulling water into dry soil |
| Δθ (Delta Theta) | dimensionless | Initial moisture deficit: the difference between porosity and initial moisture content (0–1) |
Typical Values by USDA Soil Texture Class
| Texture Class | K (in/hr) | ψ (in) | Porosity |
|---|---|---|---|
| Sand | 4.74 | 1.95 | 0.437 |
| Loamy Sand | 1.18 | 2.41 | 0.437 |
| Sandy Loam | 0.43 | 4.33 | 0.453 |
| Loam | 0.13 | 3.50 | 0.463 |
| Silt Loam | 0.26 | 6.57 | 0.501 |
| Sandy Clay Loam | 0.06 | 8.60 | 0.398 |
| Clay Loam | 0.04 | 8.22 | 0.464 |
| Silty Clay Loam | 0.04 | 10.75 | 0.471 |
| Sandy Clay | 0.02 | 9.41 | 0.430 |
| Silty Clay | 0.02 | 11.50 | 0.479 |
| Clay | 0.01 | 12.45 | 0.475 |
HydraLink provides a Green-Ampt lookup dialog by soil texture class; picking a texture fills in the hydraulic conductivity, suction head, and moisture deficit together.
Choosing a Loss Method
| Criterion | SCS CN | Green-Ampt |
|---|---|---|
| Simplicity | Single parameter (CN) | Three parameters (K, ψ, Δθ) |
| Physical basis | Empirical | Physically-based |
| Best for | Standard practice, most applications | Detailed analysis, known soil properties |
| Long storms | May underestimate infiltration | Better representation of infiltration recovery |
| Data needs | Land cover + soil group | Soil hydraulic properties |
References
- NRCS (2004). National Engineering Handbook, Part 630, Chapter 10, Estimation of Direct Runoff from Storm Rainfall.
- Green, W.H. and Ampt, G. (1911). "Studies of Soil Physics, Part I." Journal of Agricultural Science.
- Rawls, W.J. et al. (1983). "Green-Ampt Infiltration Parameters from Soils Data." ASCE Journal of Hydraulic Engineering.