Water movement through st thick unsaturated zone underlying an intermittent stream in the western Mojave Desert, southern California, USA

Previous studies indicated that small amounts of recharge occur as infiltra tion of intermittent streamflow in washes in the upper Mojave River basin, in the western Mojave Desert, near Victorville, California. These washes fl ow only a few days each year after large storms. To reach the water table, water must pass through an unsaturated zone that is more than 130 m thick. Results of this study, done in 1994-1998, show that infiltration to depths below the root zone did not occur at control sites away from the wash. At t hese sites, volumetric water contents were as low as 0.01 and water potenti als (measured as the combination of solute and matric potentials using a wa ter activity meter) were as negative as - 14,000 kPa. Water-vapor movement was controlled by highly negative solute potentials associated with the acc umulation of soluble salts in the unsaturated zone. Highly negative matric potentials above and below the zone of maximum solute accumulation result f rom movement of water vapor toward the highly negative solute potentials at that depth. The delta O-18 and deltaD (delta oxygen-18 and delta deuterium ) isotopic composition of water in coarse-grained deposits plots dong a Ray leigh distillation line consistent with removal of water in coarse-grained layers by vapor transport Beneath Oro Grande Wash, water moved to depths be low the root zone and, presumably, to the water table about 130 m below lan d surface. Underneath Oro Grande Wash, volumetric water contents were as hi gh as 0.27 and water potentials (measured as matric potential using tensiom eters) were between -1.8 and -50 kPa. On the basis of tritium data, water r equires at least 180-260 years to infiltrate to the water table. Clay layer s impede the downward movement of water. Seasonal changes in water vapor co mposition underneath the wash are consistent with the rapid infiltration of a small quantity of water to great depths and subsequent equilibration of vapor with water in the surrounding material. It may be possible to supplem ent natural recharge from the wash with imported water. Recharge to the was h may be advantageous because the unsaturated zone is not as dry as most ar eas in the desert and concentrations of soluble salts are generally lower u nderneath the wash. Published by Elsevier Science B.V.