A TRANSIENT FLUX MODEL FOR CONVECTIVE INFILTRATION - FORWARD AND INVERSE SOLUTIONS FOR CHLORIDE MASS-BALANCE STUDIES

Forward and inverse solutions are provided for analysis of inert trace r profiles resulting from one-dimensional convective transport under f luxes which vary with time and space separately. The approach is devel oped as an extension of conventional chloride mass balance techniques used to analyze vertical unsaturated aqueous phase transport over larg e timescales in arid environments. This generalized chloride mass bala nce (GCMB) approach allows incorporation of transient fluxes and bound ary values of precipitation and chloride mass deposition and allows an alysis of a tracer profile which does not remain constant with depth b elow the extraction zone, in terms of a purely convective water transp ort model. The conventional quasi-steady state chloride mass balance ( CMB) can be derived from the transient GCMB model developed here. By s pecifying a link between precipitation and recharge, closed-form forwa rd and inverse solutions relating soil water chloride concentrations t o transient boundary fluxes are obtained. This link is necessary for q uantitative analysis of variable chloride profiles arising from climat ic change. The GCMB can use transient chloride mass deposition rates, transient precipitation, and transient evapotranspiration sates. If tw o of these quantities are known or if the time frame is constrained su ch that a quantity can be treated as constant, then the inverse model can be used to determine the third. When mixing processes are limited, the GCMB can provide an alternative approach for estimating paleoprec ipitation for performance-assessment modeling. The GCMB model is demon strated with the following applications: (1) determination of time-var ying precipitation from a field chloride profile and (2) evaluation of transient changes in water extraction by evapotranspiration and trans ient recharge associated with a change in land use.