Quantifying impacts of climate change and variability on groundwater recharge in Tulelake, California

Abstract

Understanding how climate change and variability will interact with groundwater resources is crucial for sustainable groundwater management. To better understand this relationship at the management scale, I investigate the effects of interannual to multidecadal climate variability on groundwater resources of the Tulelake Irrigation District (TID) in northern California using singular spectrum analysis (SSA) and lag correlations. The SSA results indicate that while the El Nino-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) influence precipitation, streamflow, and groundwater levels, there is a larger contribution of PDO-like variability in the groundwater levels but with low to moderate lag correlation coefficients. I also explored the effects of future climate change by using HYDRUS-1 D models to simulate recharge beneath the two dominant soil textures (Tulebasin clay and Laki sandy loam) of the TID from 1950 to 2099 using downscaled output from four global climate models (GCMs) at representative concentration pathways (RCP) 4.5 and 8.5. Median recharge values range from 77 to 182 mm/year for the Tulebasin and Laki sediments, with the greatest recharge rates simulated during the historic time period (1950 to 2005). Although simulated recharge rates under the RCP 4.5 and 8.5 scenarios are not statistically different, they are statistically significant and as much as 50% lower simulated recharge during the end of the 21 st century as compared to the historic time period. The simulated decreasing trend in future recharge is more sensitive to decreases in air temperature and evapotranspiration than increases in average annual precipitation. Moreover, the semi-arid climate and relatively shallow depth to water ( 4 m) of the TID create recharge dynamics that are relatively sensitive to forecasted air temperature and evapotranspiration. The findings from this thesis can be used by water resources managers in the TID or other similar agencies to forecast future groundwater budgets and plan for the likely decreases in recharge over the coming decades of the 21st century.