UC Davis

Groundwater is a crucial component of global water resources, especially in semi-arid and arid regions where it may be the only or predominant source of water. The intensive use of this resource deeply impacts hydrological systems at a basin-scale, causing widespread aquifer depletion. As consequence, recent decades have seen an increasing interest in complex hydrogeologic modeling and analysis has been observed worldwide to support the decision-making process for sustainable water resource management. These numerical groundwater models offer several capabilities to represent the physical systems and to assess different water management policies, but they are often constrained by limited data availability and several sources of model uncertainty (e.g., parameters, variables, inputs, and assumptions). This dissertation uses a physically-based groundwater model to explore to explore several strategies to overcome data gap conditions during the water resource modeling process and to evaluate the performance of spatially distributed aquifer management alternatives for the case study of the Ukiah Valley Groundwater Basin in northern California.First, the development of a three-dimensional finite-difference groundwater model is described in Chapter 1. This Chapter indicates that the bias in the model related to limited data availability could be reduced when taking specific steps to improve model representation of the system. These steps include; 1) taking advantage of the knowledge and practical insights of technical experts and locals to methodically define model inputs (e.g., the stream network consistency with the actual flowing stream segments), 2) conducting a large review and cross validation of all available geological and hydrogeologic representations is best practice for designing a robust aquifer 1 layering system; and 3) by integrating the appropriate software components (packages) to accurately represent the predominant hydrologic mechanisms within the basin. Chapter 2 aims to use the developed physical model to compare the impact on groundwater heads and stream-aquifer interaction of the coupled effects of a set of six managed aquifer recharge (MAR) alternatives combined with different surface water storage scenarios. In four MAR alternatives, the water availability was investigated from the hypothetical construction of four new small dams, and in two MAR alternatives, water availability was explored from the reoperation of the existing reservoir. Numerical modeling results confirm that four out of six of the proposed recharge alternatives in the alluvial aquifer have an important impact on hydraulic heads, with substantial (greater than 7 m) increases that last over an average of 4 months and smaller increases (greater than 0.01 m) that are visible for most of the year. These results highlight how the combination of a smaller infiltration basin with a larger reservoir capacity improves the groundwater levels basin-wide more than the opposite scheme, with a larger infiltration basin and smaller reservoir capacity. Additionally, combining expansive infiltration basins and high flows from reservoirs can considerably increase the net aquifer-to-stream flux along the main stem and tributaries, depending on the location of the reservoirs. In Chapter 3, we develop a method to support decision making regarding efficient data collection that could address the model uncertainties for this case study (e.g., parameters). We used a time variant global sensitivity (TVSA) analysis to assign the variation in model outputs (i.e., RMSE metrics for simulated groundwater head, NSE metrics for simulated streamflows) to the variations in model inputs (i.e., a set of 11 parameters). we then perform these TVSA methods across four well observation subsets (i.e., W1, W2, W3, and W4) and three stream gage observation subsets (i.e., SG1, SG2, and SG3) to evaluate the independent effects of record length and number of 2

observations (i.e., groundwater head and streamflows) on the temporal (i.e., annual and seasonal) parameters sensitivities. We find that, though the error in the heads and flows exhibited some differences in temporal trends, drought cycles largely governed the variation of parameter sensitivities in both metrics. Findings suggest that the length of record of monitoring data are more important than the number of wells in screening the parameter temporal sensitivities, and more data could be collected for the regulated segments of the main stem of the watershed, particularly during dry years. These highlight how sensitivity analysis methods can be expanded to inform decision-making in term of data prioritization.The methods developed during this dissertation could be valuable tools to apply in other Mediterranean or semi-arid alluvial basins and to respond to different groundwater modeling challenges. Specifically, the frameworks developed can be used to overcome limited groundwater elevation data availability, to evaluate managed aquifer recharge alternatives impacts on the hydrologic system, and to apply time variant sensitivity analysis for supporting the design of new data acquisition. Such analyses could assist communities as they invest in surface- and groundwater modeling to adapt to unpredictable water supplies and a changing future climate.