Consequences of Climate Variability and Human Water Demand on Freshwater Ecosystems: A Mediterranean Perspective from the United States

Abstract

Climate variability, climate change, climate risk, and climateadaptation are topics of great interest worldwide. Mediterraneanclimates are particularly vulnerable to these climate-related issuesbecause of the strong seasonality of precipitation, high human demandfor water, and predicted increasingly variable worldwide climate. I willaddress some of these issues in Mediterranean climates from research onthe Sacramento River, the San Joaquin River, and the CaliforniaBay-Delta in the western USA. The Sacramento River and San Joaquin Riverconverge to form the California Delta. Waters from these catchments,which drain 40% of the landmass of California and discharge about 47%of the available water from California, are extensively dammed,diverted, and exported. Exports from the Delta provide a portion of thedrinking water for similar to 25 million people and sustain more than amillion hectares of irrigated agriculture. Interannual variability inriver discharge is linked to Pacific climate forcing in the late fall,winter, and early spring with peak discharge from rainstorms andsnowmelt in the winter and spring. Warming coupled with drought hascaused substantive change in the timing of runoff and in the compositionof upland vegetation in large areas of the catchment. Human adaptationto water supply risks involves shifts to groundwater supplies, increasedconservation, and water reuse or desalinization. Many of the indicatorvariables used to assess the ecological condition of aquatic ecosystemsare highly sensitive to drought and climate change. Factoringvariability and climate change into integrated ecological assessments isan ongoing challenge and effort. Finally, some of the insights frommanaging and researching these river ecosystems and the Delta inCalifornia, USA are discussed in the context of water resourcechallenges in Mediterranean climates in general.