WATER-MOVEMENT IN DESERT SOIL TRACED BY HYDROGEN AND OXYGEN ISOTOPES, CHLORIDE, AND CL-36, SOUTHERN ARIZONA

Understanding soil water movement is important for water resource management and for analyzing pollutant transport in the vadose zone, Seasonal variations in shallow soil profiles may have profound effects on deep soil water and solutes. We have investigated seasonal soil water movement in the top meter of undisturbed desert soil in the southern Arizona using H-2, O-18, Cl, and bomb-Cl-36. Six soil profiles were sampled from a young terrace (Holocene) during 1991 to 1992 on a seasonal basis. One soil profile was sampled from an older terrace (latest Pleistocene) in October 1992, and one from a yet older fan surface (late Pleistocene) in March 1992, The results indicate that repeated seasonal cyclic movement of soil water in the top 60-80 cm active zone tends to produce a consistent stable isotope composition for the soil water below the active zone in different seasons. The deep delta(18)O on the oldest surface is about 6 parts per thousand heavier than on the younger surfaces, probably resulting from greater evaporative loss due to the larger proportion of fines in the older soil. This deep isotopic composition is closely related to the composition of the local average annual precipitation. An annual average evaporation rate of 35 mm year(-1) was estimated using a steady-state diffusion model, whereas the actual regional evapotranspiration rate is 175-250 mm year(-1). This suggests that about 80% of the soil water is lost through transpiration and first-stage evaporation before quasi steady-state is reached, The long-term average infiltration rate below the active zone is about 4 mm year(-1) for the younger terrace, based on both chloride mass balance and bomb-Cl-36, and 0.02 mm year(-1) for the older terrace and the fan surface, based on chloride mass balance. The correlation between the deep delta(18)O, the reduction of downward flux and the surface age suggests that increasing soil development has resulted in a significant decrease of soil water infiltration and hence an increase in evaporation and surface runoff.