Water Quality
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Abstract
As much as 3.0 million acre-feet of Colorado River water are used every year to irrigate more than 500,000 acres of lands in the Imperial Valley. Surface and subsurface drainage water from irrigated fields enters the Salton Sea which serves as a drainage sink for the Imperial and Coachella Valleys since its formation in 1905. The Salton Sea continues to exist because of the drainage water from agriculture in Imperial and Coachella Valleys as well as flow from agricultural drainage and untreated and partially treated wastewater from the Mexicali Valley. The salinity of the Sea is over 46,000 ppm or approximately 125% the salinity of the Pacific Ocean. More than 17% of the delivered irrigation water in the Imperial Valley becomes tailwater runoff. This water transports significant amounts of chemicals that eventually reach the Salton Sea. The suitability of water for irrigation depends on the type and concentration of salts in the irrigation water. Salinity is a common problem in arid and semi-arid regions where evaporation exceeds precipitation. Salts are naturally present in soil. However, irrigation water adds significant amounts of salts. Accumulation of salts over time, due to evaporation of pure water, results in an increase in soil salinity and a reduction in crop yield. Salts present in the soil-water system can reduce crop yield and cause decline in quality. Most vegetable crops are sensitive to salinity. Salinity can affect crop growth through specific-ion toxicities and osmotic effects. Specific-ion toxicity occurs when the concentration of one ion is high enough to cause toxicity. Boron, chloride, and sodium are few of the ions that impede plant growth and development. Specific-ion toxicity causes leaf burn on the tips and margins of crop leaves. Osmotic effects also result in decline in yield and quality due to the movement of water from the cells of plant root to the soil-water system outside the roots. If water uptake is significantly reduced, the plant slows down the growth rate. In general, salinity problems are more severe during the early stages of growth. Decline in crop yield can be predicted from average root zone salinity. In general, vegetable crops are more sensitive to salinity than field crops. Good drainage systems coupled with good irrigation management strategies are the most effective tools to control salinity in most soils. Silt and sediments transport from irrigated fields to Imperial Valley drains and rivers has been identified as the primary source of pollution in the Salton Sea Watershed. The Index of watershed Indicators (IWI) score is used to categorize watersheds based on their overall aquatic health. A score of one is given to a watershed with better water quality and lower vulnerability to stressors such as pollutant loading. The maximum score is 6, this score is given to watersheds with more serious water quality problems and higher vulnerability to stressors such as pollutant loading. The Salton Sea watershed was given a score of 5 (more serious water quality problems and lower vulnerability to stressors such as pollutant loading). Efforts are underway to reduce the sediments load in the Salton Sea watershed.