Modeling Hydrologic and Water Quality Responses to Changing Climate and Land Use/Cover in the Wolf Bay Watershed, South Alabama
Type of Degreethesis
Forestry and Wildlife Sciences
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Land use/cover (LULC) and climate change are two main factors directly affecting regional hydrology and water quality. In this study, the future potential impacts of LULC and climate change on the hydrologic regimes and water quality in Wolf Bay watershed, South Alabama were explored independently and mutually by using the Soil and Water Assessment Tool (SWAT). Due to lack of measured data, SWAT was calibrated in a nearby watershed, and the calibrated model parameters were transferred to the Wolf Bay watershed. It was shown that using data from nearby watersheds improves the model performance under limited data conditions in the study watershed. The choice of the parameter set, whether it is the default model parameters or those from a donor watershed, has a marginal effect on modeling the impacts of different LULC scenarios. SWAT with the transferred parameters was then employed to investigate the potential impacts of LULC and climate change on the hydrology and water quality of the Wolf Bay watershed. While four Global Circulation Models (GCMs) under three Green House gas emission scenarios were used to reflect variability in future climate conditions, three future LULC maps generated mainly based on different population growth rate assumptions were used to represent the uncertainty in future LULC conditions. In general, the Wolf Bay watershed is expected to experience increasing precipitation in the future, especially in fall, and temperature is expected to be higher, especially in summer and fall months. Further, the watershed is expected to undergo dramatic urbanization, with percentage of urban areas nearly doubling in future. Results showed that both climate change and LULC change would cause a redistribution of streamflow. Higher flows were projected to increase, while small flows are expected to decrease. No clear trend of extreme large flow was detected when only climate change was considered. Under combined change scenarios, a more noticeable uneven distribution of streamflow was observed. Monthly average streamflow was projected to increase in spring, fall, and winter, especially during the fall, while no clear trend was observed in summer. LULC change did not significantly affect monthly streamflow, but changed the partitioning of streamflow to baseflow and surface runoff. Surface runoff was predicted to increase every month, while for baseflow an evident decreasing trend was detected. When climate was combined with LULC effect, a more dramatic increasing trend in monthly average streamflows was detected. Furthermore, a visible increasing trend in surface runoff and more dramatic decreasing trend in baseflow were detected. Monthly distribution of sediment and nutrients are affected by both flow and management practices. Projected variations of TSS, TN, and TP loadings follow the same pattern as flow. No evident difference in annual average N:P ratio was predicted when only climate change was considered. LULC change increased TSS loadings but decreased TN loadings for all months. TP loadings were projected to decrease in summer, but increase in other months. N:P ratio was projected to decrease significantly. Results of this study indicate that if future loadings are expected/predicted to increase/decrease under either climate or LULC change scenario, then their combined impact is to intensify that trend. On the other hand, if their effects are in opposite directions, that is while one predicts an increase and the other predicts a decrease, then their mutual effect has an offsetting impact. The combined LULC and climate change effect was in general synergistic, i.e. the total effect was greater than the sum of the individual effects.