TIME-SERIES ANALYSIS OF SOIL-MOISTURE AND RAINFALL ALONG A LINE TRANSECT IN ARID RANGELAND

Soil water content and rainfall were measured at 30 m intervals along a 2730 m transect on the New Mexico State University College ranch, 40 km northeast of Las Cruces. The data were collected at 2-week intervals. Time-series techniques (autocorrelations and cross-correlations) were used to examine the relationships between the water content and rainfall and to compare the relationship of the overall average (the entire transect) soil moisture content with that of individual transect segments. Correlograms indicated the range of dependence in time for soil moisture and rainfall was different for the entire transect when compared to individual transect segments. In general, water content showed a significant correlation in time between 3 to 5 lags, whereas rainfall showed a significant correlation in time between 2 to 25 lags. Soil moisture lagged behind rainfall by a few lags. The response time to rainfall was short in the surface soil but increased with depth to as much as 10 lags at 135 cm. Wetting of dry, arid rangeland soils is a slow process and is largely a function of antecedent water content. In addition, this wetting process pattern was different for the entire transect when compared to individual transect segments.