Issues associated with the creation of regional-scale variably saturated groundwater flow models in irrigated areas

Recent advances in groundwater modeling codes have created the opportunity to fully model groundwater flow in the saturated and unsaturated zones on the regional scale. These models can be very useful for studying irrigated regions that have a shallow water table since irrigation management is influenced by, and influences, the groundwater system. This dissertation explores three issues associated with the creation of this type of model: vertical spatial discretization and temporal discretization in the root zone, disaggregation of irrigation district deliveries to the field scale, and the effects of the presence of a shallow water table on bare soil evaporation.;Computational power limits the degree to which these problems can be spatially and temporally discretized. Experiments with numerical models are presented which explore the effects of reducing spatial discretization on predictions of flux and moisture content in the root zone. Results indicate that acceptable predictions of flux can be made with model cells that thicken with depth. The error associated with thicker layers in moisture content predictions decreases with depth. Other numerical experiments explore the differences between use of daily versus monthly applied water boundary conditions. The monthly averaged boundary conditions reduce model run times and relax spatial discretization requirements. However, accuracy in calculated flux, moisture contents, and any moisture content based boundary conditions is decreased.;A model is presented that distributes monthly district deliveries to the field scale based on predicted field evapotranspiration and water table depth. This case is demonstrated through a regional-scale variably saturated flow model of the west side of the San Joaquin Valley, CA. Sensitivity of water table extent predictions to monthly and annual updates of water table depth is presented, as is sensitivity of the combined models to the irrigation efficiency---water table depth relationship.;Increased bare soil evaporation due to the presence of a shallow water table can be a significant source of groundwater discharge. The importance of this flux can be illustrated when its magnitude is compared to the net recharge to the groundwater system. The current method for calculating bare soil evaporation due to a shallow water table in MODFLOW assumes that this flux is a linear function of water table depth. Results presented show that in transient irrigated settings, such a model is not accurate on time scales shorter than one year.