Analysis of Soil-Water Movement on a Sandy Hillslope

摘要

Field experiments were conducted on a virtually unvegetated, thinly stratified sand dune near Socorro, New Mexico to determine whether soil moisture movement had significant horizontal flow components. The site is in a semi-arid area where mean annual precipitation is about 20 cm. The site was instrumented with tensiometers and neutron probe access tubes to monitor soil water movement. These instruments show that near unit vertical hydraulic gradients occur and that moisture from infiltrated precipitation propagates deep into the sand, except near the dune crest. A bromide tracer buried at different locations beneath the dune slope was found by core sampling to have moved a significant distance downslope from the source. It appears that soil moisture does exhibit horizontal flow components, even through the hydraulic gradient's near vertical. A series of numerical simulations was conducted to identify the most likely explanation for the observed field behavior. The principal phenomena considered were: (a) hysteresis in the moisture characteristics curves which prolongs the retention of infiltrated water in a zone sloping nearly parallel to the dune slope; and, (b) anisotrophy which is either constant at all saturations or which increases with decreasing saturation. This finding tends to support recent theories on anisotrophy of unsaturated soil.