Water Relations and Water use Patterns for Artemisia Tridentata Nutt. In Wet and Dry Years

Plant H"2O relations and soil moisture depletion and recharge were followed in a stand of Artemisia tridentata near Washtucna, Washington during 1973 and 1974. Precipitation during the 1972-73 recharge season was 14.5 cm, 11 cm below normal. The 1973-74 precipitation was 35.7 cm, or 10 cm above normal. The 2 yr were therefore ideal for comparing plant behavior on wet vs. dry years. Soil moisture was depleted to around -70 bars in 1973 and -60 bars in 1974 to depths of 2.5 m. Leaf H"2O potentials were -10 bars in the spring and decreased to -50 to -60 bars in the summer of 1973. In 1974, summer leaf H"2O potential was -30 bars. Osmotic potentials were around -20 bars in the spring of 1973 and throughout the spring and summer of 1974. In the summer of 1973 osmotic potentials dropped to -60 bars. Stomatal diffusive conductances of 0.5 cm/s (resistance of 2 s/cm) were common during the spring and summer of 1974 and the spring of 1973. Summer conductances in 1973 were .05 cm/s (resistance of 20 s/cm). A simple model is proposed which predicts evaporation and transpiration from daily precipitation and potential evapotranspiration. Model parameters included soil hydraulic properties and maximum transpiration rate as a function of available soil moisture. The agreement between model prediction and measured values is within the uncertainties imposed by the input data. The model predicts 6.5 and 8.2 cm of evaporation for 1973 and 1974 respectively. In 1973 this was half of the total precipitation received. In 1974 it was only one fourth of the total.