FAQ - Crop Water Use


Q: What is crop water use?
A: Essentially all plants require water for survival. Crop water use is the water used by a crop for growth and cooling purposes. Water used by crops serves several purposes including: translocation of minerals from the soil into plant tissue; relocation of carbohydrates and other plant-produced stubstances from the leaves to stems, roots, fruit, and storage organs; and plant cooling by means of evaporation. The process whereby water is drawn into plants through the roots, moved through the plant, and evaporated from the plant leaf surfaces is termed transpiration. Evaporation and transpiration are physically similar processes (Taylor and Ashcroft, 1972. Physical Edaphology).
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Q: What is meant by consumptive use, when referred to in the context of irrigation or ag water conservation?
A: The official definition of the U.S. Geological Survey for 'consumptive use' is: water lost from the immediate water environment through evaporation, plant transpiration, incorporation in products or crops, or consumption by humans and livestock. In the context of agricultural crops, and particularly irrigated crops, consumptive use generally refers to water actually 'used' by the crop through evapotranspiration. Not all water which is pumped, withdrawn, or diverted for irrigation and not all water which falls on an agricultural crop field is 'consumed' or 'lost' from the immediate water environment.
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Q: What is evapotranspiration?
A: Evapotranspiration, abbreviated as ET, it is the amount of water that transpires through a plant's leaves, combined with the amount of water that evaporates from the soil in which the plant is growing (GreenWeb). Evaporation is the conversion of water into vapor and its transfer from a soil or water surface to the atmosphere. It is extremely difficult to measure evaporation separately from transpiration in a soil in which plants are growing. Also, evaporation probably cannot be influenced independently or transpiration within a plant community. The two processes are therefore considered together (Taylor and Ashcroft, 1972. Physical Edaphology).
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Q: What is the difference between crop water use and evapotranspiration (ET)?
A: The term "ET" is often used interchangeably with crop water use. Technically, one might argue that crop water use is synonymous with transpiration and evapotranspiration is something more than crop water use, because of the added effect of evaporation. However, as previously noted, it is extremely difficult to measure evaporation separately from transpiration and, additionally, the amount of evaporation directly affects transpiration.
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Q: What controls crop water use?
A: Crop water use is a function of three factors in combination: the climate (weather conditions over the growing season), the crop, and the soil; basically how much water is stored in the soil and how tightly that water is held by the soil. Climate, and particularly temperature, humidity, and solar radiation, dictates how much water can be evaporated from a surface over a specified time. Climate conditions of high temperatures and low humidity contribute to high rates of transpiration and evaporation. Various crops also have different water use requirements, depending primarily upon the length of their growing season and the season of growth.
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Q: How is crop water use measured?
A: Crop water use is measured much like keeping track of fuel usage by a vehicle or an aircraft, with the soil being somewhat synonymous with the fuel tank. The amount of water stored in the soil at the beginning of a crop growing season is measured. Then the total amount of water supplied to the plant by rainfall and irrigation during the growing season is recorded. Finally, the amount of water stored in the soil at the end of the crop growing season is measured. The difference between the beginning soil water and end soil water, plus rainfall and irrigation, is approximately equal to crop water use.
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Q: How is evapotranspiration determined?
A: Evapotranspiration is often determined in much the same way crop water use is determined – by measuring how much water is removed from the soil, plus how much water is supplied by rainfall and irrigation. Frequently, however, evapotranspiration is estimated by monitoring and measuring evaporation from controlled 'reference' sites multiplied by a crop coefficient to adjust for crop type and canopy cover.
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Q: What is reference crop evapotranspiration and how is it determined?
A: Reference crop evapotranspiration is the rate of evapotranspiration from a large uniform area, covered by green grass, 8 to 15 cm tall, which grows actively, completely shades the ground and which is not short of water (Food & Agriculture Organization of the United Nations, Crop Water Needs). In humid and semi-humid areas where water usually is not a limiting factor, grass is used as a reference ET crop. In arid or semiarid areas, alfalfa is often used as a reference crop. (Iowa State University Extension)
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Q: What is soil water content?
A: Soil water content or moisture content is the quantity of water contained in soil. Soil water content is expressed as either a percentage of the volume or mass of a soil material, or as an 'equivalent depth', similar to how the weather reporter expresses precipitation. Soil, as a physical entity on the landscape, is actually a very porous material, with approximately half of the volume of a soil material consisting of pore spaces, filled with either water or gases. For example, when 25% of the soil volume is filled with water, the soil water content is said to be 25% by volume. Only a portion of the total soil water is plant available.
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Q: How is soil water content measured?
A: Soil water content can be measured either directly by collecting a soil sample, weighing, drying, and reweighing the soil sample, or by using one of several types of equipment which indirectly measure the amount of soil water present. Soil water content most commonly is expressed as percent water in the soil by weight, percent water in the soil by volume, or inches equivalent depth of water per specified depth of soil. Other units such as inches of water per inch of soil also are used. By determining the equivalent depth of water for each layer or horizon of soil, the total water in the soil profile can be estimated by adding all the values for the profile together (South Dakota Cooperative Extension Service).

Water content by weight is determined by dividing the weight of water in a soil sample by the dry weight of that same soil sample. This ratio is then converted to percent by multiplying the ratio by 100%. Water content by volume is obtained by multiplying the water content of a soil sample by weight by the bulk density of that same soil sample. Bulk density is the ratio of the dry mass of soil per unit volume to the volume of that same soil, Bulk density for typical soils usually varies between 1.3 and 1.6. g/cm3 This is equivalent to approximately 80 to 100 pounds/ft3 of dry soil.
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Q: What is permanent wilting point?
A: Permanent wilting point is the soil moisture content at which most plants cannot extract further water from the soil. A plant is defined as permanently wilted when the plant will not recover from wilting after 24 hours without moisture stress. As a general guideline, permanent wilting occurs when the tension with which water held in the soil equals or exceeds about 225 pounds per square inch. It would take the equivalent to the amount of pressure that would be exerted by the weight of 15 atmospheres to remove this water. In the metric system, this is equivalent to 15 bars tension.
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Q: What is field capacity?
A: Field capacity is the soil water content at which the gravitational or easily drained water has drained from the soil. This is usually considered to occur 24 to 48 hours after significant rainfall or irrigation. The general guideline is that field capacity is the soil water content resulting when a tension of approximately 5 pounds per square inch is applied to wet soil. It would take the equivalent to the amount of pressure that would be exerted by 1/3 the weight of the atmosphere, applied to a wet soil, to result in this water content. However, field capacity for many irrigated, coarse-textured, sandy soils is approximately 1/10 atmosphere or bar tension (South Dakota Cooperative Extension Service).
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Q: What is readily available water?
A: Readily available soil water is water which can be removed from the soil with minimal energy applied. (www.irrig8right.com.au/Glossary/Glossary.htm). It is common to consider about 50% of the available water as readily available water. Even though all of the available water can be used by the plant, the closer the soil is to the wilting point, the greater the stress is that the plant experiences when water is being removed from the soil. Plant stress and yield loss are possible after the readily available water has been depleted
(South Dakota Cooperative Extension Service).
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Q: What is maximum allowable depletion (MAD)?
A: Maximum allowable depletion, or MAD, is a term used in reference to what is the allowable amount of water that can be withdrawn from the soil between irrigation events without stressing the crop to the point where significant reductions in crop yield or quality are experienced. Another explanation of MAD is: the maximum level of depletion to which the soil can dry without causing water deficit stress in a crop that has a fully expanded root zone; the sum of the readily available water in each soil horizon within the plant root zone with an allowance made for the soil water extraction pattern of the crop (Australian Government National Water Commission).
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Q: What is plant-available water?
A: Plant-available water is that portion of the soil water in the soil between the field capacity water content and the permanent wilting point water content. Table 1 gives common ranges of available water for soil types (South Dakota Cooperative Extension Service).

The following table shows the maximum allowable depletion, as a percentage of available water and the approximate rooting depth for selected crops which are typically irrigated.

Table 1. Crop Rooting Depth and Maximum Allowable Depletion
Crop Rooting Depth Depletion
Alfalfa 4 feet 65%
Grass Meadow 3 feet 60%
Cereal Crops 2.5 feet 40-50%
Sugar Beets 2.5 feet 40%
Potatoes or Corn 2 feet 40%
Beans or Peas 1.5 feet 40%

(http://waterquality.montana.edu/docs/irrigation/depletion.shtml)

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Q: What parameters determine the total soil water available to plants?
A: The first parameter is the plant-available water-holding capacity, which is the amount of water available, expressed in inches of water /foot of soil (or mm/m). The second parameter is the effective root zone of the crop, which is the soil depth of the roots. Knowing the combination of soil textures and horizons by depth, it is possible to calculate total soil water available to the plant. (CSU Extension).

The following table provides a list of soil textures and available water capacity in inches of water per foot of soil.
Soil Texture Feels Like... AWC (inches/foot)
Coarse sand, sand sand, grit 0.3-0.5
Fine sand, very fine sand fine grit, sand 1.25
Loamy coarse sand, loamy sand sandy, loamy 1.0
Loamy fine sand smooth, fine grit 1.25
Coarse sandy loam, sandy loam smooth with grit 1.25-1.5
Fine sandy loam smooth, fine grit 1.5-2.0
Loam, silt loam smooth 2.0
Silt, sandy clay loam smooth, slippery 2.0-2.2
Clay loam, silty clay loam sticky but smooth 2.2
Sandy clay, silty clay smooth and sticky 2.0
Clay sticky 2.0

(http://waterquality.montana.edu/docs/irrigation/depletion.shtml).

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Q: How does soil water content influence crop water use?
A: As soil dries, the stress that plants experience as water is removed from the soil gradually increases. At field capacity (maximum water content), plants use water at the maximum rate determined by climatic conditions at the time. As the soil water content drops below field capacity, and progressively becomes less and less, plants use less and less water. Eventually, the tension with which water is held in the soil becomes sufficiently great that water can no longer be removed by the plant, and the plant experiences stress. Different crops have different water requirements and respond differently to water stress – both the magnitude of the stress and the timing of the stress relative to the plant growth process. The range of water use for crops also varies from one area to another (Iowa State University Extension).
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Q: What is water stress and is it the same as drought stress?
A: Water stress is a term used to refer to the circumstance when the crop's demand for water exceeds the available amount of water during a certain period or when poor water quality restricts the use of water by the crop. Drought stress is defined as the condition of a limitation on maximal plant performance imposed by limited supplies of water (Cereal Knowledge Bank).
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Q: What does a crop look like if it is suffering from drought stress, i.e., stress due to lack of water?
A: Crop appearance is one of many field indicators that can be used in irrigation scheduling, although it is likely that some yield or quality loss has been realized by the time most crops show visible signs of stress due to lack of water. The term drought stress is frequently used synonymously with the terms water stress and moisture stress. A crop suffering from water stress tends to have a darker color and exhibits curling or wilting. This is a physiological defense mechanism of the crop that is evident on hot, windy afternoons when the crop cannot transpire fast enough, even if the water is readily available in the soil. If the crop does not recover from these symptoms overnight, the crop is suffering from water stress. Using this indicator alone for irrigation scheduling is not recommended if a maximum yield is desired (CSU Extension).
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Q: What is meant by the expression 'soil water balance'?
A: The soil water balance is an accounting of the inputs of water to, outputs of water from the soil, and changes in the amount of water stored in the soil over a period of time. The water balance of an agricultural field can be determined by calculating the input, output, and storage changes of water in the soil over a specified period of time.
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Q: What are the critical growth periods for water stress for the most frequently grown crops in the Central Plains, Northern Plains, and Mountains states?
A: Table 6. Critical growth stages of selected irrigated crops.
Crop Critical period Symptoms of water stress Other considerations
Small grains Boot and bloom stages Dull green color, firing of lower leaves. Plants wilt and leaves curl. Apply last irrigation at milk stage
Barley Jointing, booting and heading. Early drought stress may cause more tillering than usual Erect leaves rolled toward the midrib. Stress after heading causes plants to wilt, darken in color and ripen prematurely Under severe stress, leaves become hard, dry, and ashen to bronze in color, and florets may abort
Wheat During and after heading Leaves wilt, yellow, then burn. Tillers abort prior to flowering. Empty, bleached white heads or partial heads Reduction of tiller roots, tillers, spikelets, florets, plant growth, and yield
Corn Tasseling, silk stage until grain is fully formed Curling of leaves by mid-morning, darkening color Needs adequate water from germination to dent stage for maximum production
Sunflower Heading, flowering, and pollination Weakened stalks may lodge Plants are predisposed to charcoal rot and stem weevil larvae
Beans (Dry) Bloom and fruit set Wilting Reduced yields
Sugar beet Post-thinning Leaves wilting during heat of the day Excessive irrigation lowers sugar content
Potato Tuber formation to harvest Wilting during heat of the day Water stress during critical period may cause cracking of tubers
Alfalfa Early spring and immediately after cutting Darkening color, then wilting Adequate water is needed between cuttings
Grass hay Early spring through 1st harvest and start of regrowth Dull grayish green color Avoid overgrazing pasture in early spring and fall
Annual forages Any extended period of limited water Reduction in forage production or quality Prolonged drought may pose a nitrate toxicity risk
Cool season grasses Early spring, early fall Dull green color, then wilting Critical period for seed production is boot to head formation
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Q: How can a soil water balance be used to help with irrigation scheduling?
A: A soil water balance – or budget, so to speak – can be used similar to a checkbook balance – to determine the amount of water remaining in the soil and available to the plant. By knowing the amount of plant available water held in the soil at the beginning of the growing season, measuring added water (in the form of irrigation or rainfall) and by accounting for water removed by evapotranspiration, it is possible to maintain a reasonably accurate estimate of the balance of water remaining in the soil available to the crop.
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Q: How can ET be used to schedule irrigation?
A: With the use of a soil water balance or budget, daily evapotranspiration (ET) amounts are withdrawn from storage (or the balance of plant available water) in the soil profile. Rainfall or irrigation amounts are added to soil water storage. Should the water balance calculations project soil water to drop below some minimum level, irrigation is indicated. Weather forecasts enable prediction of ET rates and projection of soil water balance to indicate whether irrigation is needed in the near future (CSU Extension).
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Q: Why is it important to measure and monitor soil moisture?
A: Measuring soil moisture makes it possible to determine if there is a water shortage that can reduce yields or if there is excessive water application that can result in water logging or leaching of nitrates below the root zone. Measuring soil moisture also can build knowledge of the soil and water storage and supplying capacity of each irrigated. Monitoring soil moisture levels is required for effective irrigation water management. Many tried and proven methods of estimating or measuring soil moisture are available. The method selected depends on a variety of factors such as accuracy, cost, and ease of use
(South Dakota Cooperative Extension Service).
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Q: How does climate change impact crop water use?
A: It is easier to project how climate change might affect crop water use than it is to project just what climate change will be, how severe the change will be, and how long the change will last. Obviously, as environments become warmer and growing season duration changes, crop water needs will increase. The magnitude of increase will depend on how much the environment warms up and what kind of relative humidity changes accompany the temperature changes. Evaporation generally decreases as humidity increases.
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Q: What is a reasonable expectation of different crop responses to limited or deficit irrigation?
A: Undoubtedly, limited irrigation imposes some degree of stress on a crop, compared to circumstances where neither timing nor amount of water applied are constrained. It is reasonable to expect that yield of forage crops or those harvested for biomass (grass, alfalfa hay, corn silage, sudan grass, hay barley, oat hay, switchgrass, forage peas for example) will decrease in direct proportion to the degree to which water is limited. Biomass production is generally directly and often nearly linearly correlated with evapotranspiration. Timing of the limitation of water can also have an effect on yield of these crops, which often demonstrate the highest degree of water use efficiency during cool weather and early season growth. In contrast, quality of these crops, i.e., protein, digestible matter, balance in nutritional composition, is often more adversely affected by limited water during the middle or latter part of the growing season.

Crops which are grown for the purpose of harvesting grain, vegetables, tubers or fruit (wheat, barley, corn, milo, millet, lentils, dry edible beans, soybeans, vegetable crops, fruits for example) are generally more sensitive to limited or deficit irrigation – both in terms of yield and quality. Most of these crops will reflect limited or deficit irrigation early in the growth period as yield reductions at harvest while limited or deficit irrigation during the middle or latter part of the growing season will be reflected in reduced or poor quality product.
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