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100 Part I California Water Ennow- 2.3 Freshwater ecosystem services in California Ecosystems provide many economic services.A major global study done for the United Nations considered four overlapping categories:provisioning,regulating, cultural,and supporting services(Millennium Ecosystem Assessment 2005).Some services are easier to measure than others. Provisioning services.Provisioning involves the production of(1)food,both from irrigated agriculture and fisheries;(2)materials,including timber and cotton; (3)fresh water,for household,industrial,and service uses;and(4)hydropower. Provisioning services have the longest tradition of economic valuation and are regularly calculated for water management projects. Regulating services.Freshwater ecosystems also regulate a range of environ- mental conditions that affect human well-being.Some prominent examples in California include(1)flow regulation,including use of watersheds and floodplains to recharge groundwater basins and reduce downstream harm from floods; (2)water quality regulation,including the use of wetlands and rivers to remove nutrients and pesticides from waterways;and(3)climate regulation,including regional air quality(e.g.,reducing airborne particulates and summer temperatures) and carbon sequestration in floodplain wetlands and riparian forests.Economic benefits from these services are rarely measured. Cultural services.Some cultural services have direct,measurable market value: recreation,ecotourism,and the aesthetic values of scenic views and parks.Cultural services with nonmonetized value are more difficult to measure:spiritual renewal, religious and cultural values,and the use of freshwater ecosystems for formal and informal education. Supporting services.Many of California's freshwater ecosystems provide support for other economic activities that are only realized over very long periods of time or through indirect connections to other ecosystem services.Supporting services include soil formation and fertility,particularly in floodplain and wetland settings subject to seasonal flooding;removal of carbon dioxide through photosynthesis; nutrient cycling(the natural cycling of nutrients necessary to sustain life in freshwater ecosystems);and water cycling(regulating the rates of movement and pathways of water through the hydrologic cycle).Supporting services are rarely measured. forestry accounted for$7.6 billion of gross state product(2008$).25 Other ser- vices are essentially public,free for use,such as recreation,and must be valued using nonmarket methods,which can generate wide ranges of estimates.Some services,particularly support services,have no easy method for measurement. 25. Bureau of Economic Analysis gross state product data(current values,converted to 2008 values using the ratio of nominal to real U.S.gross domestic product).