Crops and Water Use
If water shortages appear to be a certainty, it is important to try to anticipate when those water shortages will occur and recognize the difference in the impact that a short of water will have on different crops. The crops to be least impacted by moisture shortages are the perennial forages - alfalfa, grasses, and hay crops. These crops have well established, deep root systems and are biomass accumulators - that means they produce biomass (forage) in direct response to the amount of water transpired through the plant. This is in contrast to seed producing crops such as wheat, barley, corn, for grain, sunflower, safflower, dry beans, sorghum and millet for grain, which require a specific amount of water to develop to the seed-producing stage and then require additional water to produce high quality seed. Similarly, root and storage crops, such as sugar beet and potatoes, accumulate storage in direct proportion to the amount of water transpired through the plant, but a significant degree of stress at specific stages will result in a significant reduction in both yield and quality.
One way to look at crop water needs is to determine the number of pounds of water needed to produce a pound of dry matter or yield. The list of crops included here shows a wide variation in water needs. From the relatively small amount of water needed by millet, sorghum and corn to the very large amount needed by alfalfa, bromegrass and quackgrass. Irrigators also need to consider that each crop may have vulnerable or critical periods when limited moisture is likely to reduce yields.
Millet is often considered a drought resistant crop but it is more correctly a fast maturing crop that develops so quickly that it utilizes any moisture stored in the relatively shallow soil layers. Beans mature fairly early and are somewhat similar to millet. Navy beans blossom over a short period of time and are damaged by high temperatures during blossoming so yields can be reduced.
Pinto beans, soybeans and buckwheat blossom over a prolonged period of time; thus, the critical period for pollination and filling is spread over a greater time period, giving some degree of drought resistance. While sunflowers are considered to be drought resistant, soybeans are able to withstand higher moisture tensions than sunflowers. Sunflowers, like safflower, however, apparently have an extensive root system that is available to explore the root zone more completely and extract more of the available water from it. Wheat appears to be able to do comparatively well under limited moisture conditions, because of extensive root development to a depth of 5'. Wheat has the ability to produce under quite dry conditions.
Sorghum has the capacity to recover from drought stress and resume growth. Yield under these circumstances is often reduced, but some recovery is made when moisture comes. Sorghum exposed to severe moisture shortage and high temperature over a prolonged period is not likely to recover sufficiently to produce economic yields even though death of the plant does not occur.
Corn, oats and flax show varying degrees of drought resistance. Moisture at pollination time and from then to maturity is critical for corn, wheat, and barley harvested for grain. On the other hand, crops like corn, wheat, and barley may prove to be excellent forage crops is not allowed to go to maturity and cut as high quality, high moisture, high protein crops before maturity.
For grain crops, the components that make up yield (the number of heads (ears) per acre, number of kernels per head and the weight per kernel) are the critical components. Environmental conditions have a large influence on these components of yield at different stages in a plant's life cycle.
Water Requirement of Plants
Source: Modified version of Ag. Notes from Dr. Jim Bauder, Montana State Univ.