Impact of Nitrogen Fertilizer on Maize Evapotranspiration Crop Coefficients under Fully Irrigated, Limited Irrigation, and Rainfed Settings

One of the common methods for estimating actual evapotranspiration (ETa) is the two-step approach, which relates crop-specific crop coefficients (K-c) to a reference surface ET, typically alfalfa or grass (ETr and ETo, respectively). Minimal, if any, study has reported K-c values for water, nutrient, and both water and nutrient deficiencies. In this study, alfalfa (K-cr) and grass (K-co) reference maize (Zea mays L.) K-c values were developed as a function of growing degree days (GDDs) for 0, 84, 140, 196, and 252 kg ha(-1) nitrogen (N) treatments under fully irrigated (FIT), limited irrigation (75% FIT), and rainfed conditions at the University of Nebraska-Lincoln South Central Agricultural Laboratory (SCAL) near Clay Center, Nebraska, for the 2011 and 2012 growing seasons. The research also investigated a stress factor (K-stress) to assess the reduction in crop water use as compared with a nonlimiting water and N treatment (reference). In 2011, maximum Kc(r) values ranged from 0.95 to 1.27 and occurred between GDD values of 995 and 1,163 degrees C (late July to early August), which corresponded to the R1 to R3 growth stages, whereas in 2012 (much drier), maximum Kcr values ranged from 0.84 to 1.19 for 75% FIT and FIT and existed between GDD values of 1,111 and 1,267 degrees C (R2 to R4 growth stages). On average, greater Kcr values existed at higher N rates (e.g., 196 and 252 kgN ha(-1)) compared with lower N rates. Lower N treatments typically reached their maximum Kcr value earlier in the growing season and began to decrease towards harvest. Rainfed and 75% FIT experienced a greater reduction in Kstress as compared with FIT as well as lower N rate treatments as compared with higher N treatments. A water stress factor (K-w) was calculated to determine the portion of Kstress attributed with water stress alone. The monthly average values often experienced lower Kstress compared with Kw values, indicating that Kw alone was unable to account for the total reduction in Kcr from a nonlimiting water and N reference. Thus, an N stress factor (K-n) was also quantified by assuming Kstress was the product of water and N stress (e.g., K-stress K-w x K-n). The seasonal average K-n was 1.15 in 2011 and 0.64 in 2012. Values of K-n were always lower in the drier year in 2012 than in 2011, ranging from 0.45 towards the end of the season in 2012 to a maximum of 1.27 in August 2011. In general, K-n decreased as N rate decreased and K-n had a decreasing trend (e.g., greater N stress) throughout the growing season, especially in the drier year in 2012. The reduction in K-n over time was due to the temporal reduction in readily available N as well as compounding effects of reduced N on plant growth and consequently crop water uptake over the growing season. (C) 2014 American Society of Civil Engineers.