Daily infiltration value was originally programmed to be estimated by one of two methods. If measured daily runoff was available and correctly associated with the causative precipitation, then it was entered as an item of the climatic data file and daily infiltration was computed as simply precipitation minus runoff. But, since runoff values were rarely available for other than a research setting, this approach was not included in the current model version.

The current method to estimate infiltration is to first estimate daily runoff as some percentage of daily rainfall by a modified version of the Soil Conservation Service (SCS) curve number (CN) method. (Woolhiser 1976). Daily infiltration then becomes the simple difference of rainfall or snowmelt minus runoff. This infiltrated water is budgeted to the upper soil layers according to their current storage capacity.

The SCS-CN method estimates an amount of the daily precipitation which becomes runoff by first an initial abstraction, and then a percentage of precipitation that becomes runoff, based on a series of empirical curves. The curve numbers are selected by tabulated CN values for crop-soil combinations plus an antecedent moisture adjustment. Table 1 provides the suggested CN values for the crop-soil combinations. The SCS-CN method is described in the U.S. Soil Conservation Service National Engineering Handbook (1973).

The standard SCS-CN method was modified as suggested by Woolhiser (1976) to utilize the predicted estimates of crop canopy and soil moisture. Because these are both dynamic variables computed in the SPAW model, it is no longer necessary to use average annual conditions. To adjust the CN for the current canopy, the CN values for fallow and crop conditions are entered from Appendix I. These values are then prorated according to the computed canopy ranging from the fallow value with no crop to less than the "average" crop canopy represented by the annual tabled crop values. The full canopy value is computed as a value as much below the average value as the difference between the average canopy value and the fallow value.

Similarly, the significant effect of antecedent soil moisture is dynamically considered by setting limits for the application of the antecedent conditions I and III. The soil water of the second layer (first real layer below the evaporative boundary layer) is used as the antecedent index. Based on experience simulations on research watersheds, condition I was defined for when the soil water of this layer is below 60% of FC and condition III when it is above FC. These limits were selected by several calibrations to provide approximately correct annual surface runoff volumes on experimental watersheds. Runoff and infiltration volumes can be calibrated by modifying the CN values from those suggested in Appendix I.

Daily infiltration was not given a time distribution. The water volume is added to the uppermost soil layers that can store this amount without exceeding 90% of saturation moisture content. The infiltrated water is divided into sub-daily time steps defined for the Darcy redistribution, cascaded to successive deeper layers until adequate storage is achieved, then all further redistribution is by the Darcian soil moisture redistribution routine. Should the entire profile reach 90 % saturation due to exceptional rains or restrictive soil layers, additional runoff is estimated. Without an infiltration time distribution there is no time distribution to the runoff, thus the SPAW model is not designed to provide hydrographs or stream routing.

Field Hydrologic Processes | Precipitation | Potential ET
Interception | Soil Water Evaporation | Plant Transpiration | Crop Water Stress
Root Water Uptake | ActualET | Soil Water Redistribution | Irrigation