Long-term water balance at the catchment scale is predominately controlled by vegetation and climate. Over the past 40 years, woody shrubs - primarily Carolina willow (Salix caroliniana Michx.) - have expanded in the headwater region of the Upper St. Johns River (USJR) basin in east-central Florida, USA. To evaluate the impact of willow on evapotranspiration (ET), we monitored soil moisture, groundwater level and climatic variables, including rainfall, solar radiation, temperature, humidity and wind speed, on control plots and plots sprayed in back-to-back years with either the herbicide Aquasweep followed by Ecomazapyr or two applications of Clearcast. Each treatment was replicated four times in a total of twelve, 100 m x 100 m plots, which were arranged in four blocks. Two blocks were in willow stands ~ 5 m tall while the other site had willows ~ 7 m tall. We estimated daily ET at each plot based on the Penman-Monteith equation in wet conditions or water balance in dry conditions. We modeled annual ET using the Budyko equation, which uses as inputs annual precipitation, potential ET and a single parameter that is mainly controlled by vegetation.
Results/Conclusions
Mean annual ET in control plots (1.40 m/y ± 0.03 m, 95% CI) was significantly higher than in herbicides applied plots (Aquasweep followed by Ecomazapyr, 1.04 m/y ± 0.04 m; Clearcast in back-to-back years, 0.70 m/y ± 0.02 m) and differences were more pronounced during the May – October rainy season. Analysis of annual ET shows that the parameter of the Budyko model is correlated with plant size (i.e., leaf area), which provides a method for quantitative evaluation of regional hydrological responses to vegetation change.