THE EFFECT OF PEDO-TRANSFER FUNCTIONS ON ESTIMATED RATES AND PATTERNS OF POTENTIAL RECHARGE

Abstract

Accurate estimates of recharge rates and areal distributions of recharge within basins are necessary for water-resources management and aquifer protection. Given the difficulties involved with measuring recharge, basin-scale methods of estimating potential recharge (water available to recharge the saturated zone) have become popular. Using a waterbalance approach, basin-scale models partition water among potential recharge, runoff, and storage. Key soil properties in the soil-moisture balance are porosity (water content at saturation) and field capacity (water content at 10 kPa), which are commonly obtained from spatially referenced soil datasets through pedo-transfer functions (PTFs). PTFs translate soil data, such as particle-size distributions and bulk density, into soil hydraulic parameters. Rillito Creek watershed, near Tucson, Arizona, was used to illustrate the effect of using different PTFs in a basin-scale soil-moisture balance to estimate the areal distribution and rate of potential recharge. Using 10 common PTFs, the estimated rate of potential recharge for the Rillito Creek watershed varied by nearly an order of magnitude, and the area over which recharge occurred varied by a factor of two. These large ranges demonstrate the sensitivity of soil-moisture balance models to soil-property estimates derived from PTFs.