Hourly and daily single and basal evapotranspiration crop coefficients as a function of growing degree days, days after emergence, leaf area index, fractional green canopy cover, and plant phenology for soybean

Hourly evapotranspiration (ET) crop coefficients (Kc) are needed to optimize the effectiveness and efficiency of high-frequency microand sprinkler irrigation practices involving the application of water multiple times a day. However, not much is known about the daily and seasonal patterns and magnitudes in hourly Kc values for soybean. In addition, locally developed Kc values are necessary for more robust within-season irrigation management, crop ET estimation, and water balance analyses. Hourly and daily Kc functions were developed for soybean in south-central Nebraska through extensive field research. Actual crop evapotranspiration (ETa) was measured using a Bowen ratio energy balance system. Daily crop coefficients were calculated as Kc = ETa/ETref, wherein reference (potential) evapotranspiration (ETref) was calculated using the Penman-Monteith equation with a fixed canopy resistance for both alfalfa-reference (ETr) and grassreference (ETo) surfaces. The Kc values were derived in two forms: (1) a single (normal or average Kc) Kcr based on ETr, and Kco based on ETo; and (2) a basal coefficient (Kcbr) based on ETr, and Kcbo based on ETo. The seasonal patterns of variation of Kcr, Kco, Kcbr, and Kcbo were examined on five different temporal base scales: days after emergence (DAE), cumulative growing degree days (GDD), leaf area index (LAI), fractional green canopy groundcover (CC), and plant phenology (V and R stages). The 2007 and 2008 growing season ETa totals were 535 and 514 mm, respectively. Extreme hourly Kc values were frequently observed in the early morning and late afternoon hours when ETa was very low relative to ETr and ETo. Daily means of the 10 to 13 hourly values computed for Kcr ranged from 0.25 to 1.06 in 2007 and from 0.15 to 1.02 in 2008, whereas those computed for Kco ranged from 0.39 to 1.37 in 2007 and from 0.22 to 1.29 in 2008. Daily Kcr and Kco values calculated based on daily data ranged from 0.20 to 1.12 and from 0.27 to 1.47, respectively. Comparison of all daily means of hourly coefficients with the corresponding daily coefficients in one-to-one graphs and zero-origin based regression of the former on the latter revealed linear regression coefficients of 0.92 (2007 Kcr), 0.95 (2008 Kcr), 0.96 (2007 Kco), and 0.97 (2008 Kco), with R values of 0.78 or better. On average, hourly Kc values were about 4% to 8% lower that the corresponding daily values. Substantial diurnal variability was observed in Kco and Kcr measured during daylight hours (ranging from 0.1-0.2 to 1.5-1.6) from early morning to late afternoon (8:00 to 18:00), and the range of variability was substantially dependent on the coincident V and R stages. The relationship between Kc and LAI was best represented by two regression trend lines: one representing crop development from its beginning up to the start of senescence, and the other representing crop development thereafter. A similar break in the regression trend line was observed in the relationship between basal Kc and GDD. In contrast, the relationship between Kc and fractional CC was not biphasic and could be modeled with one regression trend line. The FAO-56 tabulated Kco values and those measured in this research were significantly different (p < 0.05). Thus, the FAO-56 values, if used for south-central Nebraska soil, climate, and management practices or similar conditions, would not be able to provide accurate ETa and crop water requirement estimates. Because this research proved that Kco and Kcr values are not constant during the day from dawn to dusk, using daily average Kco or Kcr values would not be able to provide robust and precise determination of crop irrigation requirements for irrigation practices delivered more than once per day. The crop coefficients developed in this research as a function of several base scales should provide crop consultants, extension service personnel, agronomists, irrigation practitioners, and other irrigation and water management professionals with robust and accurate methods for choosing and applying crop coefficients to be used for more precise determination of ETa and water requirements, thus leading to more efficient and effective seasonal soybean irrigation management.