Watershed Management and Irrigated Agriculture

Watershed Management and Irrigated Agriculture

Harley H. Davis
Central Valley Regional Water Quality Control Board Irrigated agriculture is not traditionally thought of in terms of watershed management. But irrigated agriculture is a form of watershed management that in many ways is not unlike that practiced in wildlands.

I have been working for the Central Valley Regional Water Quality Control Board on water quality regulation of agricultural land and I have found that many concepts are the same as those practiced in the management of forest, range and chaparral watersheds. Concepts like Best Management Practices (BMPs) and cumulative effects in mixed ownerships apply to irrigated agriculture just as they do in other settings.

The Regional Board is encouraging the use of Best Management Practices (a local management approach) to improve water quality in the San Joaquin River Basin. Best Management Practices include the application of irrigation techniques to improve water quality. These BMPs focus on water conservation, reuse of irrigated water, and crop management. The object of implementing these practices is to reduce drainage waters containing contaminants such as pesticides and salts. Boron, molybdenum and selenium are the constituents for which water quality objectives are established in the Basin.

Each practice is oriented toward reducing the amount of poor quality water percolating to groundwater and in cropland drainageways. An interesting application is the use of eucalyptus trees and other salt-tolerant woody species. Eucalyptus can remove up to 5 AF/acre (5 feet) of water, immobilizing the associated salts. This brand of agroforestry is practiced mostly in areas with high water tables and extensive selenium. Because of concerns that stem from deformities of waterfowl resulting from high concentrations of selenium in Kesterson Reservoir, research is ongoing that will quantify the effects of BMPs in controlling these problems.

An irrigated watershed usually contains a multitude of ownerships where adverse effects can accumulate. For example, on the west side off the San Joaquin Valley, irrigation drainage flows from various land uses. Water also drains from private duck clubs and public wildlife refuges. Towns and dairies also contribute to salt loads. Channelization of streams, changing direction of flows in canals, interaction of chemicals with sediment, and water use by riparian vegetation make analysis of cumulative effects complex. An ongoing analysis of mass loadings will give a better idea of the mechanisms involved.

Harley can be reached at (916) 361-5691