Winter wheat (Triticum aestivum L.) evapotranspiration and single (normal) and basal crop coefficients

Locally measured crop coefficients (K_c) are critical for accurately quantifying and evaluating crop evapotranspiration (ET_c) under local climate, soil, and crop management practices. Data and information on winter wheat K_c values do not exist in Nebraska and are limited in the U.S. Great Plains in general. The objectives of this research were to measure ET_c rates and develop growth-stage-specific single (normal) (K_c) and basal (K_cb) crop coefficients for winter wheat (Triticum aestivum L.) to provide data and information to producers, their advisors, and state and federal water regulatory and management agencies about the water use dynamics of winter wheat. Field experiments were conducted during two consecutive winter wheat growing seasons in 2008-2009 and 2009-2010 at the University of Nebraska-Lincoln South Central Agricultural Laboratory near Clay Center, Nebraska, in a 13.6 ha field. Hourly evapotranspiration was measured using a Bowen Ratio Energy Balance System (BREBS), and growth-specific single crop coefficients [grass-reference (K_co) and alfalfa-reference (K_cr)] and grass- and alfalfa-reference basal crop coefficients (K_cbo and K_cbr, respectively) were developed from measured ET_c and estimated grass- and alfalfa-reference (potential) evapotranspiration (ET_o and ET_r, respectively). The K_co, K_cr, K_cbo, and K_cbr values were developed as a function of days after planting (DAP) and thermal unit (TU, also known as growing degree days). Winter wheat grain yield averaged 4.55 tons ha^-1 in both growing seasons. Water productivity (water use efficiency) was 0.76 kg m^-3 in 2008-2009 and 0.93 kg m^-3 in 2009-2010. Daily ET_c increased with DAP in the pre-winter and post-winter periods. Maximum daily ET_c was measured on 129 DAP (19 May 2009; 9.5 mm d^-1) in 2008-2009 and on 137 DAP (24 May 2010; 10.6 mm d^-1) in 2009-2010. Seasonal cumulative ET_c was 600 mm during 2008-2009 and 490 mm during 2009-2010, and seasonal daily average ET_c was 2.1 and 1.6 mm d^-1 for the two growing seasons, respectively. Crop coefficients varied substantially with the growth and development stages. Two-year average K_co values were 0.60, 1.30, and 0.30, and K_cr values were 0.40, 1.10, and 0.20 for the early-season, mid-season, and late-season crop growth and development stages, respectively. Two-year average K_cbo values were 0.45, 1.30, and 0.30, while K_cbr values were 0.30, 1.05, and 0.20 for the same growth stages, respectively. On average, K_co and K_cbo were about 20% greater than K_cr and K_cbr, and K_co and K_cr were about 10% to 11% greater than K_cbo and K_cbr. The seasonal average K_cbo was 87% of K_co, and K_cbr was 89% of K_cr. Therefore, transpiration is expected to be about 87% to 89% of ET_c, and soil evaporation would be expected to be about 11% to 13% of ET_c, but these percentages varied during the season. For example, soil evaporation during the winter wheat dormancy period represented 21% and 10% of seasonal ET_c in 2008-2009 and 2009-2010, respectively. The ratio of K_cbo to K_co ranged from 0.69 to 0.97, and the ratio of K_cbr to K_cr ranged from 0.63 to 0.98. A detailed K_c table was developed to present K_co, K_cr, K_cbo, and K_cbr values for different phenological and crop development stages. These values can be used by winter wheat producers, water regulatory agencies, and managers to evaluate water use of winter wheat with respect to availability and demand of water resources in soil and crop management and climatic conditions similar to those observed in south central Nebraska.