Ag. Water Quality- Frequently Asked Questions

Khaled M. Bali

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 (kmbali@ucdavis.edu). The Cooperative Extension Program serves all
residents of the Imperial Valley.