Closing the Water Balance for Arid Soils - First Results from a Large Lysimeter Study

Many ecological and hydrological processes cannot be fully assessed without full closure of the water balance. The weighing lysimeter facility in Boulder City, NV provides an excellent opportunity to study water infiltration, redistribution, storage and evaporation of bare soils at the intermediate (meter) scale under well-defined boundary conditions. Each of three lysimeters is weighed on separate balances, with a resolution of roughly 100 g or 0.025 mm of water. Each lysimeter contains 12 m3 of repacked homogenized and layered desert soil (dimensions: 2.26 m diameter and 3 m deep) and is instrumented with 13 different sensor technologies to measure state variables including water content, matric potential, and thermal properties at 15 depth planes. An angled rhizotron tube visually monitors movement the infiltration front. Between July 2008 and 2011, 15 storm events were recorded, with the largest storm total from 19-22 December 2010 yielding 62 mm of precipitation (originally in the form of snow). By July 2011, nearly 350 mm of cumulative precipitation was recorded, and the wetting front had reached 150 cm depth. This presentation analyzes storm events over the past three years with respect to changes in total soil mass to determine the amount of infiltration, storage, and evaporation; alterations in soil moisture and matric potential profiles determined by in situ sensors; and wetting front movement observed by the rhizotron. This lysimeter facility fills a critical gap in the vadose zone hydrology of arid environments by closing the total water balance and providing discrete data on soil moisture redistribution in a 3 meter deep soil profile to evaluate and improve infiltration models.