Even without climate change, the expected increase in population will lead to an increase in domestic demand as well as changes in land use (i.e. increased urbanization) (J.D. Landis and M. Reilly, 2003, unpublished paper). This will intensify pressure on water resources in the Sacramento watershed. Bulletin 160-98 of the California Water Plan Update estimates that at 1995 levels of development, water shortages already exist and are in the order of 2000 million cubic metres (106 m3) in average water years for the entire state. In drought years, the shortage more than triples to 7000 X 106 m3. By 2020, due to population-driven demand growth alone, it is estimated that the shortages will be in the order of 3000 X 106 m3 in an average water year and 80 00 X 106 m3 in drought years for the state of California, and 105 X 106 m3 and 1220 X 106 m3 respectively for the Sacramento watershed (Department of Water Resources, 1998). The Sacramento is in part vulnerable to water shortages as substantial supplies are exported to meet demands in other parts of the state. An aspect of the future that has not explicitly been explored in the state so far is the impact of land use changes on the hydrology of the system, and in particular a shift of land use from agriculture to urban areas.
Model results show much higher shortfalls than the Department of Water Resources (DWR) Sacramento projections of deficit for an average year without climate change. Modelled shortfalls are 505 X 106 m3. One possible explanation is that the DWR water budget did not consider exports from the Sacramento region to other parts of the state. The export from the Sacramento delta to the SanJoaquin Valley and Los Angeles are currently modelled as major demands (i.e. with a combined urban and agricultural demand of approximately 7400 X 106 m3). Another possible explanation for this difference is that the California state projections do not account for the impact of land use change on the basin hydrology as predicted by Landis and Reilly (2003, unpublished). This impact is included this study and is probably negatively impacted.
Of the 505 X 106 m3 shortfall, agricultural demand accounts for 48 2 X 106 m3, while the remainder is urban demand. Furthermore, in-stream flow requirements for the anadromous fish recovery programme (AFRP), particularly for the American River tributary, are consistently not met. On average, flow requirements in the month of July are not met 69% of the time. This is consistent with current conditions in this river.
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