LARGE-SCALE PROPERTIES OF SIMULATED SOIL-WATER VARIABILITY

Surface water balance computations were applied to historical data fro m the central and southeastern United States. The focus of the study w as to characterize soil water variability across spatial scales in the se two different parts of the United States. The regions studied in ea ch area span the range of spatial scales from 10(6) km(2) to 10(3) km( 2). Routinely collected precipitation, temperature, pan evaporation, a nd discharge data were utilized together with widely used parameteriza tions to reconstruct soil water (depth-integrated and areal-averaged s oil moisture) for the period from January 1960 through December 1988. The results suggest that in the central United States, larger regions possess a greater range of temporal soil water variability than smalle r embedded regions and that extreme soil water anomalies possess scale s of spatial coherence that range from 10(5) km(2) to 10(3) km(2), wit h stronger anomalies characterized by smaller scales. In the Mississip pi-Missouri region, variability of aggregate soil water cannot be full y characterized by soil water variability in a few ''representative'' catchments. Remarkable similarity of the spatiotemporal variability of soil water across scales (10(6) km(2) to 10(3) km(2)) was found for t he southeastern United States. In this area the variability across sca les was characterized by a frequency scaling ratio which decreased lin early with soil water anomaly strength and which remained substantiall y higher than that corresponding to random occurrence, even for extrem e soil water anomalies. In both regions of the United States and for a ll spatial scales, extreme positive anomalies show smaller spatial coh erence than negative anomalies of the same magnitude.