The water content of the soil does not decrease below the permanent wilting point. Water does not move upward from below the root zone. The chemical does not move upward anywhere in the profile. Water content in the soil root zone is adjusted for infiltration by filling consecutive layers of the profile to field capacity until all of the root zone is recharged or until the infiltrating water has been stored. Thus water passing the root zone depth passes all depths below that.
Infiltrating water redistributes instantly to field capacity.
Water content of soil below the root zone is never less than field capacity. If the infiltrating water is stored before reaching the bottom of layer J C , q is zero. If q obtained in step 3 is larger than needed to move the chemical to the bottom of the next layer, the excess must be calculated and used in equation 1 with appropriate properties of succeeding layers. Degradation of the chemical in the soil is assumed to be described by first-order processes. The degradation process is a first-order process.