Holtan’s Equation
Holtan (1961) developed a procedure based on the concept that the amount of runoff that could
be infiltrated was proportional to the unfilled capacity of the soil to hold water.
Where
- f is the infiltration
- fc is the final infiltration rate
- Fp is the unfilled capacity of the soil
- coefficients “a” and “n” are constants
- The exponent n is typically 1.4 for many soils
Fp is the most interesting coefficient. It can be range from the maximum available
water capacity (AWC) to zero. Values for it are given in Agricultural
Service publications, US Dept of Agriculture.
Values of the “a” coefficient are taken from the following table developed by Holtan et al (1975):
| Estimates of Holtan “a” coefficient |
|---|
| Basal Area Rating (note 1) |
|---|
| Land Use or cover | Poor Condition | Good Condition |
|---|
| Fallow (note 2) | 0.10 | 0.30 |
|---|
| Row corps | 0.10 | 0.20 |
|---|
| Small grains | 0.20 | 0.30 |
| Hay (legumes) | 0.20 | 0.40 |
|---|
| Hay (sod) | 0.40 | 0.60 |
|---|
| Pasture (bunchgrass) | 0.20 | 0.40 |
|---|
| Temporary Pasture (sod) | 0.40 | 0.60 |
|---|
| Permanent Pasture (sod) | 0.80 | 1.00 |
|---|
| Woods & Forests | 0.80 | 1.00 |
|---|
| 1 Adjustments needed for “weeds” and “grazing.” |
| 2 For fallow land only, “poor condition” means “after row crop,” and “good condition” means “after sod.” |
Estimates for fc coefficient, the final infiltration rate can be based on the SCS Hydrologic soils group as follows:
HSG A: 0.45 to 0.30 inches per hour
HSG B: 0.30 to 0.15 inches per hour
HSG C: 0.15 to 0.05 inches per hour
HSG D: 0.05 or less inches per hour