Sparse plant community effects on soil water balance of an arid site

A model to account for the effect of a cheatgrass (Bromus tectorum L.) and Sandberg's bluegrass (Poa sandbergii Vasey) plant community at an arid site on soil water balance has been developed. The model simulates water and heat transport in the soil-plant-atmosphere system, and plant growth and phenology. The model is driven by daily weather parameters: Total solar radiation, maximum and minimum air temperatures, vapor density, rainfall, and wind speed. The model code was written in Turbo-PASCAL v. 5.5 and runs on a microcomputer.; To calibrate and validate the model, field data were collected in 1989 from a site on the Department of Energy's Hanford Site in south-central Washington. Measurements included components of the energy budget (radiation, evapotranspiration, sensible heat, and soil heat flux), stomatal conductance, leaf water potential, root and shoot biomass, leaf area, root length, canopy radiation interception, phenology, and soil water contents.; Predicted evapotranspiration compared well with eddy correlation measurements. Soil water storage predictions agreed closely with neutron probe measurements. Water use by plants in the first half of the year is important in reducing soil water drainage. Predicted soil water storage in the upper 3 m by Day 150 for bare soil conditions was 30.4 mm more than that of the cheatgrass/bluegrass community. This increase in soil water storage would eventually become drainage to the groundwater. The model indicates that soil evaporation dominates over plant transpiration early in the year before the exponential growth phase of the plants.