SOIL-WATER MOVEMENT UNDER NATURAL-SITE AND WASTE-SITE CONDITIONS - A MULTIPLE-YEAR FIELD-STUDY IN THE MOJAVE DESERT, NEVADA

Soil-water movement under natural-site and simulated waste-site condit ions were compared by monitoring four experimental sites in the Mojave Desert, Nevada, during a 5-year period: one vegetated soil profile, o ne soil profile where vegetation was removed, and two nonvegetated tes t trenches. Precipitation ranged from 14 to 162 mm/yr. Temporal change s in water content measured by neutron probe were limited to the upper 0.5-1 m; values ranged from 0.01 to 0.19 m(3)/m(3). Water potential a nd temperature were measured by thermocouple psychrometers; 77% remain ed operable for greater than or equal to 4.5 years. For vegetated soil , precipitation that accumulated in the upper 0.75 m of soil was remov ed by evapotranspiration: water potentials decreased seasonally by 4 t o >8 MPa. During 2 years with below-average precipitation, water poten tials below the apparent root zone decreased by 2.3 (1.2-m depth) to 0 .4 MPa (5-m depth), and the gradients became predominantly upward. Wat er potentials then rebounded during 2 years with near- and above-avera ge precipitation, and seasonally variant water potential gradients wer e reestablished above the 4.2-m depth. Under nonvegetated waste-site c onditions, data indicated the long-term accumulation and shallow, but continued, penetration of precipitation: water potentials showed moist ure penetration to depths of 0.75-1.85 m. The method of simulated-wast e drum placement (stacked versus random) and the associated difference s in subsidence showed no measurable influence on the water balance of the trenches: subsidence totaled less than or equal to 13 mm during t he study. Water potentials below the trenches and below the 2-m depth for the nonvegetated soil remained low (approximate to-5.5 to -7.5 MPa ) and indicated the persistence of typically upward driving forces for isothermal water flow. Water fluxes estimated from water potential an d temperature data suggested that isothermal liquid, isothermal vapor, and nonisothermal vapor flow need to be considered in the conceptuali zation of unsaturated flow at the field sites. Below the depth of temp oral water content change, the estimated liquid fluxes ranged from 10( -10) to 10(-15) cm/s, isothermal vapor fluxes ranged from 10(-10) to 1 0(-13) cm/s, and the nonisothermal vapor fluxes ranged from 10(-8) to 10(-10) cm/s.