Water Quality FAQs
Ag. Water Quality- Frequently Asked Questions
Irrigation/Water Management Advisor- University of California Cooperative Extension-Imperial County
Water transfer and agricultural use of Colorado River water in the Imperial Valley have generated much speculation about agriculture, water use, water quality, and the future of the Salton Sea. I often receive questions about salinity, water quality, and water use in the Valley. Here are some of the questions that are related to water use and water quality. I shall address these questions in non-technical format:
- How important is Colorado River water to the Imperial Valley?
Colorado River water is the lifeblood of the Imperial Valley. It is the only source of water for irrigation and domestic uses in major cities in the Imperial County. Without this water, the entire agriculture in the Valley will vanish. The average annual rainfall in the Valley is less than 3 inches. We can't depend on rainfall to grow any crop. For example, the minimum amount of water that you need to grow a commercial crop in the Valley is about 36" and that is for a short growing season (3-4 months).
- How much water is needed to grow alfalfa? Why do we use more water for alfalfa than in other areas in California?
Water is needed to satisfy the photosynthesis needs of plants and to drain salts from the plant root zone. To grow forages in the Imperial Valley you need a minimum of 48 acre-inches/ac. Alfalfa water use is approximately 72-78 acre-inches/ac of water per year. The water we need to grow a crop is applied to meet crop water requirements as well as to maintain soil productivity (minimize the impact of salinity on crop yield and quality). Crop water demand depends on two main factors; weather factors and crop physiology. Therefore the amount of water use for a given crop depends on the weather factors. For example, a cool-season grass that is grown in the coastal areas of California uses about 0.03 inches of water per day in January and about 0.15 inches/day in June. If you plant the same grass in the Valley, the water use is approximately 0.08 inches/day in January and 0.32 inches in June. The difference in water use is mainly due to weather factors (solar radiation, humidity, temperature, and wind speed). The simple answer is that crop evapotranspiration depends on the amount of energy available for direct evaporation of water from the soil surface and transpiration through leafs. While the amount of water needed to grow alfalfa in the Imperial Valley is high compared to other regions in California, the tonnage of alfalfa produced is higher as well. Alfalfa fields in the Valley generate vegetative production almost every day of the year; alfalfa grown in cool areas in California remains mostly dormant during the winter months.
- Are there any Imperial County crops that could conceivably be grown without the use of Colorado River water?
The simple answer is no, in a climate like ours (dry to relatively dry region or arid to semi arid region), the only way to grow crop is to apply water (irrigation). In general, there are two sources of water for irrigation; surface sources like Colorado River water or subsurface sources like groundwater. The groundwater in this region is too salty to grow any crop. If the salinity (total dissolved salts) in irrigation water is high, then you can't use the water for irrigation or drinking. The salinity of Colorado River water is about 650 ppm (or 650 mg/L). Most crops will die if you use saline water (salinity in excess of 1300 mg/L) for irrigation. The salinity of the Pacific Ocean is about 35,000 mg/L.
- Can we use drainage water or Salton Sea water for irrigation?
There are two sources for drainage water; surface drainage also known as “tailwater” and subsurface drainage also known as “leach water”. Surface drainage water can be reused, under certain circumstances, for irrigation. In the Imperial Valley, there are approximately 20-30 runoff recovery systems in use. Subsurface drainage water is generally too salty to grow commercial crops. Salton Sea water is too salty to grow crops; the salinity of the Salton Sea is approximately 44,000 mg/L (higher than the salinity of the Pacific Ocean).
- I understand that there are concerns that if the Imperial Irrigation District sells water to Southern California cities, there might be unwanted effects on the Salton Sea caused by decreased runoff. Can you help explain this?
In general, about one-third of Colorado River water that comes to the Valley ends up in the Salton Sea as drainage water (surface runoff and subsurface drainage). This drainage water is needed for two things: 1- surface runoff water is needed to ensure that we have uniform application of water in the field. You need to apply enough water to meet crop water requirements. On most soils, if you eliminate surface runoff, you will not have uniform application of water to the field, the lower part of the field will not get enough water to meet crop water requirements. Therefore you need to apply additional water to make sure you have enough water at the lower end of the field. 2- subsurface drainage is needed to maintain soil salinity. Colorado River water is salty; therefore, when water is used up by plant through transpiration and direct evaporation, the salts are left behind in the root zone. Additional water is needed to remove the extra salts in the root zone (leaching) to maintain soil salinity at levels that are below the salinity threshold for commercial crops. In general, vegetable crops (example lettuce) are more sensitive to salinity while field crops (example wheat) are more tolerant to salinity. But we need to leach the soil to maintain low salinity levels for all commercial crops. Drainage water that leaves the fields in the Valley ends up in the Salton Sea, while the salinity of this water is too high to use again to grow crops (salinity level of drainage water is about 2000-5000 ppm) it helps maintain the Salton Sea. Salton Sea salinity is about 44,000 ppm and keeps going up because of evaporation (in excess of a million acre-ft per year). Therefore, the salinity of the Sea increases by a rate of about 1 to 1.5% per year as a result of evaporation and drainage water. If we eliminate the drainage water (relatively fresh water compare to Salton Sea water), then the salinity of the Sea will increase at a much faster rate and Salton Sea elevation will drop.
For additional information about Salton Sea salinity, please see our 8/29/02 In-Our-Field article or send us an email to obtain a copy of that article (firstname.lastname@example.org). The Cooperative Extension Program serves all residents of the Imperial Valley.