Applicability of the Steady State Flow Assumption for Solute Advection in Field Soils

Abstract

A comparison between solute travel times predicted by a transient and a steady state flow model is made. Data for five different soil profiles with detailed measurements of their hydraulic properties and their variation with depth are used. Daily measurements of meteorological data are used as input parameters in the transient simulations that include snow and frost dynamics, interception of precipitation, and evapotranspiration. The parameters of the steady state flow model are related to the measured soil properties and the hydrological characteristics of each transient simulation. Furthermore, the influence of solute injection time on the predicted travel time is analyzed, and the effect of root water uptake on the applicability of the steady state flow assumption for solute advection is investigated. The results indicate that the steady state flow model may provide estimates of the mean solute advection that are compatible with those of the transient flow model. The constant rate of recharge in the steady state flow model should then be interpreted as the average annual effective infiltration (i.e., infiltration minus actual evapotranspiration). When root water uptake is accounted for, an arithmetic depth‐averaging of the soil parameters appears to yield steady state estimates of arrival time that are closest to the transient predictions. When root water uptake is neglected, a harmonic depth‐averaging of the soil parameters provides the best steady state results. The discrepancy between the arrival times predicted with the two flow models decreases with the travel distance from the soil surface.