Long-term trends in evapotranspiration and runoff over the drainage basins of the Gulf of Mexico during 19012008
The Gulf of Mexico (GOM) is facing large pressures from environmental changes since the beginning of the last century. However, the magnitude and long-term trend of total water discharge to the GOM and the underlying processes are not well understood. In this study, the dynamic land ecosystem model (DLEM) has been improved and applied to investigate spatial and temporal variations of evapotranspiration (ET) and runoff (R) over drainage basins of the GOM during 1901–2008. Modeled ET and discharge were evaluated against upscaled data sets and gauge observations. Simulated results demonstrated a significant decrease in ET at a rate of 15 mm yr1 century1 and an insignificant trend in runoff/precipitation (R/P) and river discharge over the whole region during 1901–2008. However, the trends in estimated water fluxes show substantial spatial and temporal heterogeneities across the study region. Generally, in the west arid area, ET, R, and R/P decreased; while they increased in the eastern part of the study area during the last 108 years. In the recent 30 years, this region experienced a substantial decrease in R. Factorial simulation experiments indicate that climate change, particularly P, was the dominant factor controlling interannual variations of ET and R; while land use change had the same magnitude of effects on long-term trends in water fluxes as climate change did. To eliminate modeling uncertainties, high-resolution historical meteorological data sets and model parameterizations on anthropogenic effects, such as water use and dam constructions, should be developed.