FAQs - 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).

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 (boldweb.com/gw/index.php). 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).

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.

Q: What controls crop water use?
A: Crop water use is a function of three factors in combination: the climate (weather conditions), 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 day length, 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, with some crops having relatively low water use requirements while other crops have high water use requirements.

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 amount of water supplied to the plant by rainfall and irrigation 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.

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 or conditions or by calculating approximations using proven models and equations.

Q: What is reference crop evapotranspiration and how is it determined?
A: Reference crop evapotranspiration is the rate of evapotranspiration from a large area, covered by green grass, 8 to 15 cm tall, which grows actively, completely shades the ground and which is not short of water (FAO, http://www.fao.org/docrep/S2022E/s2022e07.htm). In humid and semihumid areas where water usually is not a limiting factor, grass is used as a reference ET crop. In arid or semiarid areas, alfalfa is more suitable as a reference ET crop because it has a deeper root system (Iowa State University Extension, http://www.ipm.iastate.edu/ipm/icm/2000/5-29-2000/wateruse.html).

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.

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, http://agbiopubs.sdstate.edu/articles/FS876.pdf).
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.

Q: What is permanent wilting point?
A: Permanent wilting point is the soil moisture content at which most plants experience permanent wilting. 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.

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, http://agbiopubs.sdstate.edu/articles/FS876.pdf).

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. http://agbiopubs.sdstate.edu/articles/FS876.pdf). Table 1 gives common ranges of available water for soil types (South Dakota Cooperative Extension Service, http://agbiopubs.sdstate.edu/articles/FS876.pdf). ??????

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).

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. (http://dictionary.nwc.gov.au/water_dictionary/item.cfm?id=580&cRefer=2&sRefer=60). The following table shows the maximum allowable depletion, as a percentage of available waterm and the approximate rooting depth for selected crops which are typically irrigated.

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

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, http://www.ext.colostate.edu/pubs/crops/04707.html). The following table provides a list of soil textures and available water capacity in inches of water per foot of soil.

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

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, http://www.ipm.iastate.edu/ipm/icm/2000/5-29-2000/wateruse.html).

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 irrigated1.

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 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. (www.knowledgebank.irri.org/glossary/Glossary/D.htm)

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, http://www.ext.colostate.edu/pubs/Crops/04715.html).

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.

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, and by periodically added water (in the form of irrigation or rainfall) and by periodically accounting for water removed by evapotranspiration, it is possible to maintain a reasonably accurate estimate of the balance of water remaining in the soil throughout the crop growing season.

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, http://www.ext.colostate.edu/pubs/crops/04707.html).

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, http://agbiopubs.sdstate.edu/articles/FS876.pdf).

Q: How does climate change impact crop water use?
A: It is easier to hypothesize or project how climate change will 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 due to climate change, 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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