In Century 4 runoff was not calculated. In Century 4-VEMAP, runoff was calculated and subtracted from the total water (precipitation + irrigation) before water is applied. This calculation is now in Century 5, but with a change in the runoff equation, described in the water budget in section Water Budget, Leaching and Soil Temperature.
Soil water content calculations now utilize the new physical soil submodel. The initial number of soil water layers are specified using two site parameters. NLAYER is the initial number of layers in the soil physical submodel, and can be in the range 1-9. NLAYPG is the number of layers available for plant growth, and is at least 1 and less than or equal to NLAYER. Since the number of layers can change with erosion and deposition events, the initial value of NLAYER can change. NLAYPG will be modified, if necessary, so as not to exceed the current number of layers in the soil structure. If NLAYPG is reduced due to erosion, but deposition events later increase the number of layers, then NLAYPG will be increased also, up to its original value in the site parameter set.
The site parameter arrays specifying the wilting point and field capacity soil water contents, AWILT and AFIEL, are limited to the first 10 layers, as in Century 4. The wilting point and field capacities are recaculated after every change in soil texture and bulk density.
The output variable array ASMOS, which contains the current soil water content for each layer, and RWCF, which contains the relative water content for each layer, are limited to the uppermost 10 soil layers, as in Century 4.
The water budget submodel and soil water calculations are described further in the section Water Budget, Leaching and Soil Temperature.