Influence of hydrogeologic structure on long-term sustainable groundwater elevations for a moderate-altitude rain-fed groundwater basin under climate change

Climate change is expected to exacerbate the severity of winter storms and the length of droughts in semi-arid regions such as California. Moderate altitude (mid-mountain) groundwater basins in these regions are dominantly rain-fed with little snowfall which makes natural recharge essential to maintaining agricultural, domestic and ecosystem water needs. Many of these basins are not connected to major water projects for importing water due to the high cost of construction in mountainous regions and due to the historical high natural recharge, it was not considered necessary. Thus, the future of moderate altitude agricultural basins may be at risk due to climate change unless outside water is sourced or if the hydrogeologic structure supports the retention of groundwater storage for later pumping. We developed a MODFLOW model of the Butte Valley, Siskiyou County, California, Groundwater System and calibrated it with 29 years of periodic groundwater level measurements and applied it under a base, 2030, 2070, 2070 wet and 2070 dry climate scenarios to investigate whether long term sustainable groundwater levels are possible given the hydrogeology. Model results suggest that the subsurface outflow to an adjacent groundwater basin will sufficiently decline due to lowered water levels in the alluvial basin and the lower hydraulic conductivity of the volcanic aquifer to maintain adequate water levels in the pumping region. Under climate change these moderate altitude basins may prove essential to continued agricultural production as they see slower warming if they can maintain sufficient available groundwater for agricultural production.