Predictions of Soil-Water Potentials in the North-Western Sonoran Desert

Abstract

(1) A single computer model was developed to predict soil-water potential at a Sonoran Desert site in the south-western U.S.A. The variability of precipitation there, coupled with the low water-holding capacity of the sandy soil, result in large temporal and spatial variations in soil-water potential. (2) Predicted soil-water potentials for depths of 5, 10 and 20 cm were in close agreement with measured values as the soil dried after an application of water. Predicted values at a depth of 10 cm, the mean rooting depth of Agave deserti and other succulents common at the study sites, also agreed with soil-water potentials measured in the field throughout 1 year. Both soil-water potential and evaporation from the soil surface were very sensitive to stimulated changes in the hydraulic conductivity of the soil. (3) The annual duration of soil moisture adequate for succulents was dependent on the rainfall as well as on the spacing and amount of individual rainfalls. The portion of annual precipitation evaporated from the soil surface varied from 73% in a dry year (77 mm precipitation) to 59% in a wet year (597 mm). (4) Besides using the actual precipitation events, simulations were performed using the figures for total monthly precipitation. Based on the average number of rainfalls for a particular month, the rainfall was distributed throughout the month in the model. Predictions using both daily and monthly inputs were in close agreement, especially for the number of days during a year when the soil-water potential was sufficient for water absorption by the succulent plants (above -0.5 MPa).