VADOSE-ZONE TECHNIQUES FOR ESTIMATING GROUNDWATER RECHARGE IN ARID AND SEMIARID REGIONS

Both physical and chemical methods have been used to estimate recharge in arid and semiarid areas. Our review indicates that indirect, physi cal approaches, such as water balance and Darcy flux measurements, are the least successful, while methods using tracers (e.g., Cl, H-3, and Cl-36) have been the most successful in estimating groundwater rechar ge in dry regions. Lysimeters, which can directly measure root-zone dr ainage, have been useful in quantifying recharge, particularly for cou rse soils, but are costly to construct and operate. Of the tracer tech niques available, Cl balance techniques appear to be the simplest, lea st expensive, and most universal for recharge estimation. In Australia n studies, under native vegetation in semiarid areas, Cl profiles were found to be remarkably uniform, indicating very low and relatively un iform rates of groundwater recharge. Following changes in land use, re charge appeared to become much more variable, increasing more than two orders of magnitude. Methods for scaling point estimates of recharge to large areas using indirect techniques (such as nondestructive elect romagnetic induction) have also been developed. In deep unsaturated zo nes, the pressure response in the soil water may be recorded in the pr ofile, and simple field measurements may be used to obtain semi-indepe ndent verification of recharge rates determined by using Cl balance te chniques.