Climatic and water data are readily accessible from a variety of sources. There are several indices that are helpful in assessing drought severity and water availability. Before water from rain or snow touches the ground, some of it may be lost due to wind or evaporation. Some of the water that reaches the ground or comes from melting snow may run off and settle in lower areas of the landscape; some will percolate into the ground and some will be used directly by the plants or evaporate into the atmosphere. Plants take up available moisture mainly through their roots and use it in metabolic processes to stay healthy.
Water that evaporates from the soil or is transpired by the plant is referred to as evapotranspiration (ET). The relationship between ET and crop growth as measured by dry matter is often linear. The relationship between precipitation amount (irrigation and rainfall) is often curvilinear, since crop yield levels off at a maximum amount of precipitation.
It takes approximately 5 inches of consumptive use (ET) to produce 1 ton of alfalfa hay in the intermountain region of the Southwest. More water will be required to provide 5 inches of water, depending on the irrigation system. For example, approximately 6.7 inches of water (minus effective precipitation and change in soil moisture) will need to be applied with a sideroll that has an irrigation efficiency (total amount applied times 100 divided by the amount that enters the soil) of 75 percent to produce 1 ton of alfalfa hay.
The difference between average water consumptive use (C.U.) and effective precipitation is the net amount of irrigation water required to satisfy crop C.U. The gross amount of irrigation that will need to be applied is:
Net irrigation amount times 100 divided by irrigation efficiency in percent.
Crop C.U. or ET usually peaks at flowering to seed formation for grain crops and during hot, dry and windy days for crops like alfalfa. Thus, not only different crops may have different water requirements for maximum growth, they may also reach their peak water use (demand) at different times of the year.
Furthermore, water use efficiency or WUE (crop production divided by amount of water applied or used by the crop per unit area) usually follows the law of minimum return, meaning WUE increases with the amount of water applied to a certain point or growth stage, then levels off or even decreases afterwards. Applying more water than is needed for optimum crop production could be wasted (some of it may be stored in the soil for later use) through runoff or deep percolation and might even be harmful to the crop by promoting diseases.
Crop selection (which crops to grow) and crop allocation (where to grow them and how many acres to allocate to each crop) are always important in any farming operation. They are even more important when water is in short supply.
Proper water management involves excellent irrigation system design, operation and maintenance; sound irrigation scheduling; and commitment to management. According to Dr. Bob Hill of Utah State University, a well-designed irrigation system will deliver enough water to meet the crop(s) needs, spread the water uniformly across the field and allow maximum water storage in the root zone. In addition, a well-designed irrigation system will minimize water losses due to evaporation, runoff and deep percolation.
Evidently, irrigation scheduling plays a big role in avoiding water waste. Irrigation system efficiencies commonly range from 40 to almost 100 percent, depending on the system used, system components (such as the type of sprinkler package on a center pivot) and system operation and maintenance. Design considerations include soil type, field layout, topography and peak ET rate.
Irrigation scheduling encompasses the amount and timing of irrigation. Sound irrigation scheduling takes into account crop water needs, soil water-holding capacity and changes in soil water. When scheduling irrigation applications, it is important to know what impact a water deficit could have on the crop. Crops are more sensitive to water deficits at certain growth stages than at others. An integrated approach using ET, soil and crop information usually gives the best results.
As a rule of thumb, most crops should be irrigated when nearly 50 percent of the available soil water is depleted in the root zone.
Water may be the most limiting factor to crop production; however, crop yield potential will not be reached if other production inputs and practices are lacking or managed poorly. These include the selection of crops and crop varieties adapted to the soil and climate of the area, proper planting techniques, good pest control and soil fertility management that is based on soil testing, crop yield goal and soil-crop-water interactions.
Depending on the implements used and the timing of tillage operations, tillage can be beneficial or detrimental to crop growth and water management. Excessive tillage can cause soil erosion, accelerate soil water evaporation and increase soil compaction. Keeping tillage to a minimum and maintaining a crop cover on the soil surface will reduce evaporation, soil erosion and runoff.
Keeping water application rate at or below the soil infiltration rate can also minimize runoff. The soil amendment polyacrylamide (PAM) has been shown to reduce sediment loss drastically in furrow irrigation.
In conclusion, water is a precious and limited resource. It is paramount we manage our water responsibly. Proper water management for crop production includes careful irrigation system design, sound irrigation scheduling and best soil and crop management practices. FG
References omitted due to space but are available upon request.
—From Four Corners Irrigation Workshop Proceedings
Abdel Berrada, Research Scientist, Southwestern Colorado Research Center, Colorado State University