| Quantifying the hydrologic and agricultural pollutant runoff response to an increased atmospheric CO2 concentration and climate change is critical for proper management of water resources within agricultural systems. This research takes this challenge by simulating the effects of climate change on the hydrologic cycle and agricultural pollutant transport in the Central Valley of California using the Soil and Water Assessment Tool (SWAT) water quality model and the HYDRUS soil water transport model. Specifically, changes in hydrology (streamflow, surface runoff, groundwater recharge, evapotranspiration, and irrigation water use) and agricultural pollutant runoff (sediment, nitrate, phosphorus, chlorpyrifos, and diazinon) were assessed. For the first three studies, hydrological responses were modeled in the San Joaquin River watershed using variations of atmospheric CO2 (550 and 970 ppm), temperature (+1.1 and +6.4°C), and precipitation (0%, +/-10%, and +/-20%) based on Intergovernmental Panel on Climate Change projections. The fourth study used a calibration and an uncertainty analysis technique for the calibration of the Sacramento River watershed. This study confirmed that SWAT was able to capture the large amount of uncertainty within the Sacramento River watershed and successfully simulate streamflow, sediment, nitrate, chlorpyrifos and diazinon loads. The final study uses a novel stochastic climate change analysis technique to bracket the 95% confidence interval of potential climate changes. For all studies, increases in precipitation generally changed the hydrological cycle and agricultural runoff proportionally, where increases in precipitation resulted in increases in surface runoff and thus agricultural runoff and vice-versa. Also, for all studies, increasing temperature caused a temporal shift in plant growth patterns and redistributed evapotranspiration and irrigation water demand earlier in the year. This lead to an increase in streamflow during the summer months compared to the present-day climate due to decreased irrigation demand. Increasing CO2 concentration to 970 ppm and temperature by 6.4°C in the San Joaquin River watershed caused watershed-wide average evapotranspiration, averaged over 50 simulated years, to decrease by 37.5%, resulting in increases of water yield by 36.5% and stream flow by 23.5% compared to the present-day climate. Solely increasing CO2 concentration in the San Joaquin River watershed resulted in an increase in nitrate, phosphorus, and chlorpyrifos yield by 4.2, 7.8, and 6.4%, respectively, and a decrease in sediment and diazinon yield by 6.3 and 5.3%, respectively, in comparison to the presentday reference scenario. Only increasing temperature reduced yields of all agricultural runoff components. Elevating atmospheric CO 2 concentrations generally decreased groundwater recharge under almonds, alfalfa, and tomatoes in the San Joaquin Valley due to decreased evapotranspiration resulting in decreased irrigation water use. Increasing average daily temperature by 1.1 and 6.4°C and atmospheric CO2 concentration to 550 and 970 ppm led to a decrease in cumulative groundwater recharge for most scenarios. For the final study, 95% confidence interval (CI) results from stochastic climate change simulations indicate that streamflow (3% for the upper CI limit, 9.5% for the lower CI limit) and sediment runoff (20% for the upper CI limit, 26% for the lower CI limit) in the Sacramento River watershed is more likely to decrease under climate changes compared to present-day, while the increase and decrease for nitrate runoff was found to be equal (13% for the upper CI limit, 13% for the lower CI limit). For the San Joaquin River watershed, streamflow slightly decreased under climate change (27% for the upper CI limit, 28% for the lower CI limit), while sediment (73% for the upper CI limit, 49% for the lower CI limit) and nitrate (28% for the upper CI limit, 26% for the lower CI limit) increased compared to present-day climate. Comparisons of watershed sensitivities indicate that San Joaquin River watershed is more sensitive to climate changes than the Sacramento River watershed largely due to differences in land use and soil properties. This research improves the understanding between climate change and hydrology and agricultural pollutant runoff within the Central Valley of California. Theses climate change analyses may be used by water resource managers to evaluate the potential effects of climate change. |
