Excel-Based Computational Template for Irrigation Scheduling Using Dual Crop Coefficients

Introduction

Evapotranspiration (ET) controls the moisture content of soil, one of the most important freshwater storages in the earth (McColl et al., 2017). Thus, estimating the amount of water evaporated from soil and transpired from vegetation is a necessary process in many engineering and science fields, such as agricultural water management, water resources planning, and hydrologic analysis. A convenient way to calculate an actual ET rate for crop plants (ET_c) is to use a crop coefficient (K_cb) method based on a reference ET rate estimate and the types of plants located in an area of interest (Irmak & Haman, 2003; Morgan, Obreza, Scholberg, Parsons, & Wheaton, 2006; Zotarelli, Dukes, Romero, Migliaccio, & Morgan, 2010). There are two approaches to the crop coefficient method. With the single crop coefficient approach, the single coefficient used is the averaged effects of evaporation from soil and transpiration from vegetation for the cropped surface. With the dual crop coefficient approach, evaporation from soil and transpiration from vegetation are considered as two independent coefficients (Allen, Pereira, Raes, & Smith, 1998). The single crop coefficient approach is relatively simple, but the dual crop coefficient approach can more effectively address a soil moisture condition and plant drought stress that can limit evaporation and transpiration, respectively, in an actual ET rate estimation. Despite its expected accuracy, the dual crop coefficient approach has not been widely used in practice due to the complexity of the required calculation procedures. To address this situation, we developed an Excel-based computational template that Extension professionals and irrigation practitioners can use to develop irrigation scheduling based on the dual crop coefficient method. Excel-based computing templates have been developed as tools that aid Extension professionals in their capacity building efforts (Barbosa, 2013; Bowser, Holcomb, & Kerr, 2018; Johnson & Dahlke, 2015; Patterson, 2011; Raper, DeVuyst, & Doye, 2010). The Excel template and two sample application examples are provided as supplementary material for this article (https://joe.org/joe/2019february/Computational_Template_for_Irrigation_Scheduling_Using_Dual_Crop_Coefficients.xlsx) to help Extension educators and irrigation practitioners understand and apply the calculation procedures for the ET-based irrigation scheduling. Additionally, the article describes the calculations and assumptions on which the tool is based.

FAO-56 Dual Crop Coefficient Method

The dual crop coefficient approach was proposed as a part of the FAO-56 method for improving the accuracy of ET_c estimation using the basal crop and soil evaporation coefficients (Allen, 2000; Allen et al., 1998). The basal crop coefficient (K_cb) represents the ratio of ET_c for specific vegetation to the reference ET in the cropped areas where soils hold moisture well enough to sustain full plant transpiration. The soil evaporation coefficient (K_e) addresses evaporation from the soil surface. The relationship among the coefficients is

where K_c is an actual crop coefficient, K_cb is a basal crop coefficient, K_s is a water stress coefficient, and K_e is a soil evaporation coefficient. The basal crop coefficient (K_cb) is expressed as a function of weather variables:

where K_cb(std) is a basal crop coefficient for the standard condition, is a mean daily wind speed at 2 m height (m/s), RH_min is a mean daily minimum relative humidity (%), and h is a mean plant height. The soil evaporation coefficient (K_e) is determined by considering soil moisture condition:

where K_r is a dimensionless evaporation reduction coefficient dependent on the cumulative depth of water depleted or evaporated from the topsoil, K_c,max is the maximum value of K_c following a rain or irrigation event, and f_ew is a fraction of the soil that is both exposed and wetted (the fraction of the soil surface from which most evaporation occurs).

Parameters and Assumptions

ET rates are calculated on the basis of the reference ET (ET₀) and the amount of water infiltrated into the soil in the FAO-56 method. The ET₀ is determined by weather, and it is estimated through use of the Penman-Monteith ET equation (Zotarelli et al., 2010). The infiltration rate may be computed using an infiltration model such as Horton's equation or the Natural Resources Conservation Service's curve number method combined with a continuity equation. Assumptions are that irrigation and a rainfall event occur early in a day and that a rainfall event is not followed by an irrigation application. The depth of the surface soil layer that is subject to drying by way of evaporation is set to 0.10 m (0.33 ft) in the template, and a user may want to adjust it to somewhere between 0.10 m and 0.15 m depending on soil textures (0.10 m for coarse soils and 0.15 m for fine soils [Allen et al., 1998]). The amount of the total evaporable water from the soil and the amount of readily evaporable soil water are calculated automatically on the basis of the soil texture (Table 19 of Allen et al., 1998). The parameter values are determined by data the user enters in the "Crop and Soil Characteristics" section in the template. For instance, the amounts of total evaporable water that can be evaporated from the soil and readily evaporable soil water are calculated as functions of field capacity, wilting point, and soil depth, which are determined on the basis of soil texture types specified in the template by the user (Allen et al., 1998).

Application Examples

Sample applications of the Excel-based template for determining irrigation timing are provided in the supplementary material for this article, and screen captures of two examples that can be found in the template are shown in Figure 1. In the first example ("Example_Template_Example_1" worksheet in the template and Figure 1a herein), irrigation is to be applied when the amount of readily available soil water in the root zone is depleted. The calculation shows that there is no need to apply water to the tomato field with sandy loam soil after the irrigation of 40 mm (1.57 in) on the first day or after the rainfall event (or infiltration) of 20 mm (0.79 in) on the fifth day. In the second example ("Example_Template_Example_2" worksheet in the template and Figure 1b herein), the same weather conditions as those of the first example are assumed, but the field will need additional water applications of 13 mm on the fourth day and 14 mm on the ninth day to satisfy the crop water requirement in the sandy soil. A comparison between the two examples shows that the small field capacity makes the sandy soil require more frequent water application than the sandy loam soil.

Figure 1.
Sample Views of the Excel-Based Computational Template

(a) Irrigation Timing Calculation Example for a Sandy Loam Soil

(b) Irrigation Timing Calculation Example for a Sandy Soil

Summary

This article introduces an Excel-based irrigation scheduling template and includes a description of the relevant background theory as well as application examples. Extension educators can use the template as a handy tool for helping irrigation practitioners and growers save water while maintaining crop productivity. Copies of the template are freely available in the supplementary material for the article. Additionally, the Excel template can serve as the basis for smartphone or tablet applications Extension educators and irrigation practitioners can use in developing site-specific crop irrigation schedules.

References

Allen, R. G. (2000). Using the FAO-56 dual crop coefficient method over an irrigated region as part of an evapotranspiration intercomparison study. Journal of Hydrology, 229(1–2), 27–41. doi:10.1016/S0022-1694(99)00194-8

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). FAO irrigation and drainage paper No. 56. Rome, Italy: Food and Agriculture Organization of the United Nations.

Barbosa, R. N. (2013). SpreaderCal: An MS Excel-based computer program for spreader calibration. Journal of Extension, 51(3), Article 3TOT10. Available at: https://www.joe.org/joe/2013june/tt10.php

Bowser, T. J., & Holcomb, R. B. (2018). Financial analysis tools for on-farm and off-farm commercial kitchens. Journal of Extension, 56(2), Article 2TOT3. Available at: https://joe.org/joe/2018april/tt3.php

Irmak, S., & Haman, D. Z. (2003). Evapotranspiration: Potential or reference. IFAS Extension EDIS, ABE343. Retrieved from http://edis.ifas.ufl.edu/ae256

Johnson, S., & Dahlke, G. (2015). Estrus synchronization planner spreadsheet helps beef producers implement artificial insemination programs. Journal of Extension, 53(2), Article 3TOT9. Available at: https://www.joe.org/joe/2015june/tt9.php

McColl, K. A., Alemohammad, S. H., Akbar, R., Konings, A. G., Yueh, S., & Entekhabi, D. (2017). The global distribution and dynamics of surface soil moisture. Nature Geoscience, 10(2), 100–104. doi:10.1038/ngeo2868

Morgan, K. T., Obreza, T. A., Scholberg, J. M. S., Parsons, L. R., & Wheaton, T. A. (2006). Citrus water uptake dynamics on a sandy Florida Entisol. Soil Science Society of America Journal, 70(1), 90–97. doi:10.2136/sssaj2005.0016

Patterson, R. K. (2011). Boom sprayer calibration made easy with an Excel spreadsheet program. Journal of Extension, 49(1), Article 1TOT7. Available at: https://joe.org/joe/2011february/tt7.php

Raper, K. C., DeVuyst, E. A., & Doye, D. (2010). A beef calf retention decision tool. Journal of Extension, 48(4), Article 4TOT6. Available at: https://www.joe.org/joe/2010august/tt6.php

Zotarelli, L., Dukes, M. D., Romero, C. C., Migliaccio, K. W., & Morgan, K. T. (2010). Step by step calculation of the Penman-Monteith Evapotranspiration (FAO-56 Method). IFAS Extension EDIS, AE459. Retrieved from http://edis.ifas.ufl.edu/ae459