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 climate adaptation are topics of great interest worldwide. Mediterranean climates are particularly vulnerable to these climate-related issues because of the strong seasonality of precipitation, high human demand for water, and predicted increasingly variable worldwide climate. I will address some of these issues in Mediterranean climates from research on the Sacramento River, the San Joaquin River, and the California Bay-Delta in the western USA. The Sacramento River and San Joaquin River converge 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 the drinking water for ∼25 million people and sustain more than a million hectares of irrigated agriculture. Interannual variability in river discharge is linked to Pacific climate forcing in the late fall, winter, and early spring with peak discharge from rainstorms and snowmelt in the winter and spring. Warming coupled with drought has caused substantive change in the timing of runoff and in the composition of upland vegetation in large areas of the catchment. Human adaptation to water supply risks involves shifts to groundwater supplies, increased conservation, and water reuse or desalinization. Many of the indicator variables used to assess the ecological condition of aquatic ecosystems are highly sensitive to drought and climate change. Factoring variability and climate change into integrated ecological assessments is an ongoing challenge and effort. Finally, some of the insights from managing and researching these river ecosystems and the Delta in California, USA are discussed in the context of water resource challenges in Mediterranean climates in general.