Figure 5 shows a flow diagram for the model computations. In overview, the atmospheric potential ET (PET) is determined, then separated into principle components of interception evaporation, soil water evaporation, and plant transpiration which combine to provide an estimate of actual ET (AET). After subtracting actual ET from existing soil moisture, daily infiltration of rainfall or irrigation water is distributed among the soil layers, and deep percolation and runoff, if any, are determined. The next section briefly discusses details for each of the major processes within this model framework of Figure 5. Additional details about these processes is provided in Appendix III.
Figure 5: The computational flow chart of the SPAW field water budgeting model.