Submitted to: Irrigation and Drainage International Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 3, 1996
Publication Date: N/A
Interpretive Summary: Drainage water from irrigated areas is impacting water quality and water users downstream. Irrigators are being told to reduce the amounts of potential contaminants they discharge. Although some drainage is necessary to maintain the soil salt balance and sustain productivity, the drainage volume often exceeds this requirement. Irrigation efficiency and drainage discharge were studied in an irrigation district located on the west side of the San Joaquin Valley. Results of the study demonstrated that significant reductions in the mass of salt and other elements being discharged could be achieved by improved irrigation water management. Improvements were needed in the practice of pre-plant irrigation and irrigation of water stress sensitive crops such as tomato. Also, improved water management was needed for areas without shallow ground water problems which were contributing to the shallow ground water in adjacent areas. The load-flow relationship for salt was found by regression analysis to be linear which means that the load will be reduced directly proportionately to the reduction in flow.
Technical Abstract: The irrigation efficiency and drainage discharge were studied on a 15,300 ha irrigation district located on the west side of the San Joaquin Valley. Results of the study indicated that the primary sources of deep percolation losses were from pre-plant irrigation and irrigation of water stress sensitive crop such as tomato. Tap rooted crops such as alfalfa and cotton had higher irrigation efficiencies than did tomato, in part because of their ability to use water from the shallow ground water. Irrigated lands having no shallow ground water present had lower irrigation efficiencies than those with shallow ground water and drainage. The volume of drainage flow was cyclic, increasing in the summer and decreasing in the winter, in response to the applied irrigation water. The electrical conductivity of the drainage water increased from 3.2 to 4.4 dS/m in a five year period as a result of less dilution from surface water. The relationship between the salt load in the drainage water and the discharge was determined to be linear using regression analysis. This means that the reduction in salt load being discharged from the district is a linear relationship to the reduction in flow.