Computer estimates of natural recharge through soils in southern Arizona, U.S.A.

A conventional water-balance method, employing long-term average values of rainfall, runoff and evapotranspiration, yields values of natural recharge which have low correlation with rainfall for the Sonoita and Cienega Basins in Arizona. A different approach for calculating recharge, through soils based on the characteristics of these basins and the rainfall regime, is proposed. In particular, the rainfall in the basins is characterized by intense thunderstorms of short duration (30-60 min) in the summer and rainy days of long duration (average 2 days) in the winter. The hydraulic prameters of the mountain soils in these basins may cause considerable amounts of water to infiltrate through the soil profile, thereby escaping evapotranspiration during the following day in the summer or the following dry period in the winter. The proposed model deals with individual rainfall events, considering moisture distribution, evapotranspiration and deep drainage as initial and boundary conditions. The mathematical model solves the flow equation, using the finite-difference technique. The solution includes a sink term and was solved for the simultaneous process of infiltration redistribution and evapotranspiration. The results obtained showed that although winter rainfall is smaller than the summer rainfall, its computed contribution to recharge through soils is considerably higher. No recharge was found in summer through soils exceeding a depth of 10 cm, and in winter, exceeding a depth of 40 cm. Various factors affecting the recharge are also discussed.