Effects of Warm Period Timing and Coastal Low Clouds on Water Deliveries in Coastal Southern California

The impacts of climate change on water resource availability will be felt both directly and indirectly through changes in water supply and water demand, respectively. Physical water supply changes due to climate stem from modified precipitation, temperature and evaporation, and streamflow, while changes in water demand stem from the same, as well as additional land use and land cover and socioeconomic features. As urban and agricultural water demands are projected to increase under climate change, a regional understanding of both water supply and demand responses to climate change will be necessary to equip water resource managers with locally-relevant, research-driven insights to guide adaptation. In previous work, we investigated the water delivery response to temperature and precipitation changes within the semi-arid San Diego County, located in the Southern California region of the U.S. There, we established that water agency-scale water deliveries are sensitive to temperature and background hydrologic conditions (i.e., antecedent precipitation), and that the temperature sensitivity of water deliveries is mediated by geographic and demographic features such as land cover. Here, we build on this research to further investigate the role of climate in mediating water deliveries in the Southern California region. Specifically, we investigate the hypothesis that the timing of a warm period additionally mediates water deliveries depending on agency attributes such as land cover. For example, agricultural agencies may respond differently than urban agencies to warm periods that occur during pivotal crop growing stages. Additionally, we hypothesize that coastal low clouds may impact water deliveries through the modulation of temperatures during warm periods. We investigate these hypotheses for 20 San Diego region water agencies using daily records of water deliveries made to the agencies from a regional wholesale water supplier, temperature, coastal low cloud coverage, annual precipitation, and agency-level attributes such as income and land cover from May to September for the years 2007 - 2021. This study of a representative arid urban region improves our understanding of coupled human and water system responses to climate variability and change in order to support adaptive water resources management in water-stressed environments.