A simplified model for the transient water budget of a shallow unsaturatedzone

A simplified model describing the transient water budget of a shallow unsat urated soil profile is presented and applied to an example of long-term cli matic conditions for a site near Coshocton, Ohio. Its conceptualization is simplified because the instantaneous redistribution of infiltrated water in a uniform soil profile is imposed. The model's formulation is a revision o f that originally presented by Kim et al. [1996]. Runoff from a soil profil e with uniform hydraulic properties is described by using an infiltration e quation derived by Salvucci [1996] for ponded conditions. During storm peri ods the infiltration rate equals the rainfall intensity, unless that rate e xceeds the saturated hydraulic conductivity, and then the input rate is det ermined by the infiltration equation for ponded conditions, with the excess applied water being accounted for as runoff. During interstorm periods, wh en the soil profile is drying due to evapotranspiration (ET) and drainage, the ET rate equals the potential evapotranspiration rate E-p, until the soi l profile relative saturation s drops below a certain falling value of satu ration S-f. When s < S-f then the ET rate is linearly proportional to s as given by (s/S-f) E-p. Drainage flux from a profile equals the hydraulic con ductivity under a unit hydraulic gradient. Drainage for a 50-year-long reco rd of climate on a daily basis for the Coshocton site compared well with me asurements from four lysimeters. It is shown that the effective saturated h ydraulic conductivity for the profile is determined by the runoff amount ov er the simulation period, and the value of S-f can be set so that the predi cted drainage approximates that measured by lysimeters. This determines the appropriate cumulative ET for the site consistent with the climate data. T he appropriate value of S-f also depends on the presumed profile depth, whi ch is the plant root depth over which ET is extracted.