| Climate change has significant influences on the hydrologic cycle for forested areas in the southeastern United States because of the relationship between a forest's physiology and the climate condition, as well as the intensive forest cover rate in the region. A physiological and hydrological model (PnET-II) was used in this work to study the characteristics of the hydrologic cycle for forested watersheds in the southeastern United States. The model's systematic biases, including overestimation of runoff in wet seasons and underestimation of runoff in growing seasons, were summarized for 17 experimental watersheds. A new model version, called PnET-II3SL, was developed with four revisions: (1) replacing the original single-layer soil moisture scheme with a three-layer soil moisture scheme, (2) adjusting the overland (fast) flow fraction based on antecedent moisture condition and precipitation intensity, (3) introducing a factor to adjust the water use efficiency for growing seasons, and (4) parameterizing proper values for growing-degree-day drivers. PnET-II3SL exhibited substantial improvements in simulations of runoff for the 17 experimental watersheds.; The validated PnET-II3SL model was then employed for all forested portions of the southeastern United States under two climate scenarios, HadCM2 and CGCM1, to attempt to evaluate future changes in water availability in two river systems (the Mississippi River and the Alabama River) contributed by forested areas. The PnET-II3SL predicted that forested areas will produce less evapotranspiration, and thus more water availability, in these two river systems in the 21st century. |
