at the site, twice the long-term average) and precipitation in January 1979 (4.62 cm) followed by additional rainfall in March 1979 (2.52 cm). This emphasizes the importance of antecedent water content in the soil and the sequence of precipitation events in controlling percolation. Monitoring at Beatty from 1984 to 1988 showed water movement, was restricted to the upper one meter during this period (Fischer, 1992).

Water content was monitored in a small scale ephemeral stream setting (maximum topographic relief of 0.65 m) in the Hueco Bolson of Texas (Scanlon, 1994a). Approximately monthly monitoring of water content from July 1988 to October 1990 showed that within the detection limit of the neutron probe (± 0.01 m³/m³) water content remained constant from 0.3 to 41 m depth. Although precipitation during 1989 was 50 percent of the long term mean annual precipitation, precipitation during 1990 was similar to the long term mean. Data from these desert basins indicate that penetration of water is often restricted to the shallow subsurface. The reason for the shallow penetration of water in desert soils is the large storage capacity of the surficial sediments. For example, soils similar to those at Ward Valley with an initial average volumetric soil-water content of 10 percent and a water content at saturation of 35 percent have a maximum additional storage capacity of 25 percent by volume, which is equivalent to 25 cm of water storage per meter of soil profile. To bring this into perspective, if an annual rainfall of 12.7 cm could fall in one day, the 12.7 cm of water could hypothetically be stored in only 50.8 cm of soil. In reality, the soil would not come to full saturation, but water would move deeper into the soil profile. Even if the soil-water content were to increase to 17.5 percent (which is 50 percent of saturation), the 12.7 cm annual precipitation would wet the soil down to only 170 cm. This demonstrates the great storage capacity of dry desert soils in general and of Ward Valley in particular.

In contrast to water content, water potential energy is continuous across different soil types and is typically used to infer the flow direction. Water flows from regions of higher total potential energy to regions of lower total potential energy. In areas with moderate to high subsurface water flux, gravity and matric potential are the dominant driving forces. Matric forces result from the interactions of the water and the soil matrix and include capillary and adsorptive forces. An example of such behavior is demonstrated when a sponge is placed in water. Water can move upward, against gravity, into the sponge until some equilibrium is reached.

Matric potential is expressed in meters of water, bars or megapascals (MPa). For comparison, 1 Mpa = 10 bars = 102 m of water. Because water is tightly held by unsaturated soil, the matric potential has a negative value. If soil becomes wetter, its matric potential becomes less negative until at saturation it becomes zero. Matric potential is related to soil-water content through the soil water retention curve.