Spatial and temporal soil moisture and drought variability in the Upper Colorado River Basin

This research investigates the interannual variability of soil moisture as related to large-scale climate variability and also evaluates the spatial and temporal variability of modeled deep layer (40–140 cm) soil moisture in the Upper Colorado River Basin (UCRB). A three layers hydrological model VIC-3L (Variable Infiltration Capacity Model – 3 layers) was used to generate soil moisture in the UCRB over a 50-year period. By using wavelet analysis, deep layer soil moisture was compared to the Palmer Drought Severity Index (PDSI), precipitation, and streamflow to determine whether deep soil moisture is an indicator of climate extremes. Wavelet and coherency analysis for the UCRB indicated a strong relationship between the PDSI, climate variability and the deep soil moisture. The spatial variability of soil moisture during drought, normal, and wet years was analyzed by using map analysis. Distinct regions showing higher vulnerability to drought and wet conditions were identified in the spatial analysis. The temporal variation in soil moisture was performed by utilizing map analysis in pre-drought, drought, and post-drought years for four drought events, 1953–1956, 1959–1964, 1974–1977, and 1988–1992. Less than 50% of the basin had dry conditions (soil moisture anomaly below −10 mm) for the pre-drought years. Soil moisture anomalies were lower than −10 mm for more than 50% of the basin in 15 out of 19 drought years. Generally, droughts did not end until the average soil moisture anomalies increased to positive values for two consecutive years.