Interannual and seasonal variability of modelled soil moisture in Oklahoma

The temporal variability of soil moisture is examined for Oklahoma, where diverse land surface, soils, and precipitation regimes exist. Using readily available meteorological, vegetation, and soils data, a hydrologic model was used to estimate daily soil moisture at 258 evaluation locations for the 30 year period from 1 January 1962 through 31 December 1991. A geographic information system (GIS) was used for the analysis, to take advantage of its powerful analysis capabilities and visualization tools. Results document the long-term average soil moisture conditions, the variability that occurred during this 30 year period and the temporal scale of soil moisture variability for Oklahoma. On average, the soil moisture conditions in both winter and spring in Oklahoma are about 70% of field capacity and decrease to below 30% during the summer. Thus, the seasonal cycle is pronounced, although periods of wetter and drier conditions are evident. Overall, the period from 1962 through to early 1967, 1970-72, and 1976 through to 1981 were drier than the long-term average soil moisture conditions. By contrast, wetter soil moisture conditions prevailed in late 1972 through to early 1975 and 1985 to the early months of 1988, where winter and spring soil moisture values often exceeded the long-term average. An empirical estimate of the temporal consistency in soil moisture over Oklahoma is 53 days (1.8 months), and the amount of variance explained by the soil moisture (red noise) signal is 86%. No significant seasonal differences in the temporal autocorrelation were observed. Copyright (C) 2003 Royal Meteorological Society.