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Soil water management is critical for the production of high quality vegetables. Even short periods of moisture stress can affect a crop’s performance. Irrigation is essential in the Intermountain West due to high temperatures and high rates of evapotranspiration. Moisture deficiencies can occur early in the crop production cycle before local irrigation is available, which may delay or reduce emergence or slow early growth. Shortages later in the season often decrease fruit set, size, or quality. Over-irrigating is as detrimental to the crop as water shortage. Too much water can delay harvest, reduce quality, and shorten postharvest life. Lists of the critical periods when water is critical for high quality vegetable production shown below in Critical Period for Water Table.

A crop’s water requirement, termed evapotranspiration (ET), is equal to the quantity of water evaporated (E) from the soil surface and the quantity lost from the plant (transpiration=T). Many factors must be considered when estimating ET. Most weather services provide an estimate of ET based on solar radiation, air temperature, wind speed, and humidity level. Therefore, using ET can improve irrigation management, and taking time to better understand crop water needs can greatly improve yield and quality.

There are many things that affect irrigation requirements. These include crop species and variety, canopy size, plant population, rooting depth, and stage of growth. These all influence transpiration, light absorption, and the rate that water evaporates from the soil. Mature plants use more water than crops which do not have a complete canopy (immature plants, recently transplanted crops). Rooting depths vary with crop species and determines the volume of soil from which the crop can draw water (Effective Root Depth Table ).

Plant growth stage influences susceptibility to moisture stress (Critical Period for Water Table). Irrigation is beneficial for newly seeded or transplanted crops as their root systems are not well established. Irrigation after

Cultural practices also influence ET and irrigation requirements. Cultivation, mulching, weed growth, and method of irrigation are factors to consider. Cultivation generally increases soil evaporation. Shallow cultivation helps eliminate soil crusts and may improve water infiltration, but if crop roots are damaged by the cultivator, water uptake may be reduced. Plastic or organic mulches generally reduce water use because they reduce evaporation. Weeds compete with the crop for water. Sprinkler irrigation systems which wet the whole field have greater evaporation loss than drip systems that wet only the area around the plant.