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Crop Coefficients for Midsouth Soybeans

In a previous blog post, I proposed that atmometers be considered as a tool for Midsouth crop irrigation management. In that post, I made the statement that “Data from atmometers is most useful when used in coordination with crop coefficients (CC); however, those have not been developed for Midsouth crops”. The following narrative briefly describes CC’s, and provides background information for understanding them, how they can be determined, and how they can be used with atmometer data to estimate crop water use over a defined time period.

Many of the terms used in this narrative are referred to by their abbreviations. These are given here.

•   Transpiration (T): The process by which water that enters plant roots is carried to stems and leaves and then passes through the leaves to the atmosphere in the form of water vapor.

•   Evaporation (E): The process of water leaving any surface–i.e., soil, water, and plant leaves–and being absorbed into the drier surrounding air. In a crop environment, water lost through soil evaporation is of no benefit since it does not contribute to crop growth, development, and yield. Thus, cropping practices that reduce E from the soil should benefit a developing crop.

•   Pan Evaporation (PE). A measure of the amount of water that evaporates from a water surface. It usually is obtained from a Class A evaporation pan that holds water for the determination of the amount of evaporation at a given location.

•   Evapotranspiration (ET). A dynamic variable that defines the transfer of water in the form of water vapor from the surface of soil and plants (leaves) to the surrounding atmosphere–i.e., crop water use. It is a combination of water that is evaporated (E) from the soil surface and water that moves from the soil through plant leaves to the atmosphere via transpiration (T). It is generally the largest component of the hydrologic cycle. ET increases with increasing air temperature and solar radiation, the two main drivers of ET. ET will be highest from irrigated plants, or plants that experience minimal water-deficit stress by accessing adequate, readily available soil water. E comprises the greatest proportion of ET from young crops, but decreases with increasing T as the crop grows. Reference ET can be measured using evaporation pans and atmometers.

•   Vapor Pressure Deficit (VPD). An indication of the dryness of the air. VPD is the difference between the amount of water in the air and how much water the air can hold when it is saturated at a given temperature. As the VPD increases, root extraction of water from the soil must increase to meet the increased demand of air for water from the plant. A reduction in relative humidity (drier air) increases the VPD, which results in a corresponding increase in ET. Higher ET will always need to occur to meet the demand of the air for moisture when VPD increases. The VPD is a function of both relative humidity and air temperature.

•   Relative Humidity (RH). RH is the ratio of water in the air to the amount of water that air will hold at a given temperature. In essence, it is the amount of water in an air-water mixture and is usually expressed as a percentage. High RH reduces ET, and conversely low RH increases ET because low RH increases the VPD of the air surrounding leaves.

In general, most ET early in the season of any crop occurs as evaporation from the soil. As the crop canopy develops and covers the soil surface, E from the soil decreases and T increases. Thus, factors such as row spacing will affect how the ratio of E to T will change during crop development.

Crop Coefficients account for the difference in potential ET (PET–usually obtained from pan evaporation or an atmometer) and actual ET. CC is the coefficient for a given crop and its stage of development, and is usually obtained experimentally. Crop coefficients are used in conjunction with PET to estimate ET at a given growth or developmental stage of the crop in question so that water deficits or crop water use for defined periods of soybean development can be determined.

Presently, there are no calculated CC’s for Midsouth soybeans. Estimates of CC’s that have been calculated or used previously in Midsouth soybeans are 0.5, 0.7, and 0.95 for the periods planting to R1, R1 to R3, and R3 to R6, respectively (click here–Table 1), and 0.21, 0.67, and 0.94 for pre-R1, R1 to R3, and R3 to R6, respectively (click here).

A University of Nebraska publication (NebGuide G1994) provides CC estimates based on specific vegetative and reproductive growth stages of soybeans. They are 0.20, 0.40, and 0.60 for first, second, and third node stages, 0.90 for R1, 1.00 for full bloom (R2), 1.10 for R3 to R6, and 0.90 for R7. These values were obtained from the High Plains Regional Climate Center, and are alfalfa-reference crop coefficients.

Results from irrigation research conducted in Miss. indicate that soybeans will rarely require irrigation prior to bloom; thus, CC’s for the bloom to R6.5 period are the ones that are most important for Midsouth soybeans. Until these are developed, a good CC estimate for the R1 to R7 reproductive period is 1.00+0.10. However, it is likely that this CC estimate will be different between early and late soybean plantings that will be in reproductive phases at significantly different times of the season. For example, it is not unreasonable to assume that the CC for the R1 to R6.5 period of ESPS plantings of MG IV varieties might be 0.90, while the CC for the same period of late-May/early-June plantings of MG IV and/or MG V varieties might be 1.10. Such a difference, if in fact it does exist at this magnitude, is significant in terms of water requirement for the two soybean cropping systems.

Crop Coefficients can be developed using PE data. However, data from PE’s and Atmometers will theoretically be different for a given set of conditions because of the different surfaces from which evaporation is measured. Also, atmometers should be easier to maintain to ensure accuracy of obtained data. The important point to remember is that CC’s developed using data from the two different sources will be different by some amount, so it is important that CC’s that might be developed in the future give the reference source for those calculations.

Quantification of ET is necessary for determining crop productivity response to units of water applied and/or received. Atmometer data combined with crop-specific coefficients can be used for estimating this crop water use (actual crop ET) during a defined period of crop development. These data may prove invaluable if deficit irrigation (less than that to replace full ET) vs. full irrigation is required to conserve water in the region.

Take Home Message


There are noteworthy points to consider from the above.

1. The surface irrigation that is predominantly used for irrigating soybeans growing on the shrink-swell clay soils is likely best scheduled with soil moisture sensors as shown by results from MSPB-funded research. This is because these soils that are irrigated by surface methods absorb the amount of water used by the crop since the last irrigation; i.e., the amount of water applied with each irrigation is controlled by the amount of water these cracking soils will hold.

2. Use of the atmometer/CC scheduling system seems made to order for overhead irrigation of soybeans, especially those grown on silty (non-clayey) soils. This is because 1) overhead irrigation systems can be set to apply a known amount of water and thus can be set to apply what the atmometer/CC methodology determines is the amount needed to replace what has been used since the last irrigation, and 2) many of these soils have a low infiltration rate, and thus can only handle small amounts of irrigation water that should be applied frequently. Thus, soil moisture sensors placed at the recommended 6, 12, and 24 in. depths likely will not be effective for scheduling overhead irrigation on these soils.

3. Data from the atmometer/CC methodology can be invaluable for accurately estimating how much water an irrigated soybean crop actually uses regardless of how it was irrigated or what soil it was grown on.

4. Atmometer/CC data can be used to accurately estimate how much a known water deficit reduces productivity of nonirrigated soybeans. This will enable nonirrigated producers to adopt practices that will allow their soybean crop to be grown in periods with the least deficit during the growing season so the crop can be managed to produce the highest, most consistent dryland yields.

Composed by Larry G. Heatherly, June 2018,