Water supplies have come under increasing stress in the southeastern U.S. in recent decades due to a combination of rapid population growth, land use intensification, and climate variability. While precipitation in the region is relatively high (>1000 mm yr-1), regional conflicts over water and projections of future water scarcity suggest a growing need to evaluate land management options focused on improving watershed yield. Our goal was to determine the effects of land cover change on evapotranspiration (ET) in the Ichawaynochaway Creek basin, a 500 km2 watershed in southwest Georgia, and suggest land management options that might improve water yield (P – ET). We hypothesized that expansion of irrigated agriculture was the dominant cause of water yield reductions, but that changes in forest management practices have also affected water yield. We compiled published annual ET values for major land cover types in the region including irrigated and unirrigated agricultural crops, pasture, pine plantation, wetlands, hardwood forest, and longleaf pine savanna. We used these values to scale ET to the watershed using landcover data and validated our scaled ET estimates by comparing historic water yield with scaled ET estimates from 1948–2007.
Irrigated agricultural crops, wetlands, and pine plantations had the highest annual ET rates among major land-cover types. Restored, fire-maintained longleaf pine had the lowest ET rate of any vegetated land cover type in the watershed. Annual watershed ET in 2007 was ~829 mm, and was 9.4% higher than watershed ET from 1948–1968. This increase in ET translated to a 13% decline in water yield between 1968 and 2007. Seventy percent of the cumulative change in ET was from the proliferation of center-pivot irrigation systems, with the remaining change due to expansion of short-rotation pine plantations and fire-suppressed hardwood forest. These results suggest that managing fire-suppressed forests towards longleaf pine savanna, and/or converting from short-rotation pine plantations to longleaf pine savanna could reduce watershed ET and improve water yield. Future work will concentrate on the potential of geographically-focused longleaf pine restoration in specific sub-basins of the watershed using the SWAT (Soil and Water Assessment Tool) model. These results will inform us of the potential value of water-focused land management that emphasizes longleaf pine restoration as a tool to mitigate water scarcity.