SPATIAL AND TEMPORAL VARIATIONS OF SOIL MOISTURE IN WEST-CENTRAL OKLAHOMA

From May 7 to August 13, 1985, soil moisture was measured at 12 study sites located along a 200-km east-west trending transect in west-central Oklahoma. Soil moisture was sampled at three depths at each site: 15 cm, 61 cm, and 91 cm. Study site location and the time (week) of data collection were analyzed through correlation and regression analysis in order to assess their impact on soil moisture variability measured at the three sampled depths. Along the transect for the study period, soil moisture increased with depth; soil moisture also increased with depth from west to east along the transect during the sample period. The correlation between the location of the sample site and soil moisture was weak at the 15 cm depth (0.48), but was stronger at greater depths (0.78 at 61 cm; 0.65 at 91 cm). The location of the study site along the transect explained 25% of the variation in soil moisture at a 15 cm depth; 62% at a 61 cm depth; and 51% at a 91 cm depth. The time (week variable) of data collection at each sample site was less useful in explaining the variability in soil moisture than site location. Time explains 15, 23, and 16% of the variability observed in soil moisture along the transect for the depths of 15, 61, and 91 cm, respectively. A combination of time and location variables, however, explained 46% of the variability in soil moisture for all three depths. The same time and location variables explained 55%, 76%, and 52% of the variability observed in soil moisture for the three individual depths: 15, 61, and 91 cm, respectively. Unusual precipitation events affected the transect throughout the study period and diminished the impact of location as a significant explanatory variable for describing variability in soil moisture.