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 (${\mathrm{ET}}_a$) is the two-step approach, which relates crop-specific crop coefficients ($K_c$) to a reference surface ET, typically alfalfa or grass (${\mathrm{ET}}_r$ and ${\mathrm{ET}}_o$, 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 $252\mathrm{kg}{\mathrm{ha}}^{-1}$ 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 ($K_{\mathrm{stress}}$) to assess the reduction in crop water use as compared with a nonlimiting water and N treatment (reference). In 2011, maximum $K_{cr}$ values ranged from 0.95 to 1.27 and occurred between GDD values of 995 and 1,163°C (late July to early August), which corresponded to the R1 to R3 growth stages, whereas in 2012 (much drier), maximum $K_{cr}$ values ranged from 0.84 to 1.19 for 75% FIT and FIT and existed between GDD values of 1,111 and 1,267°C (R2 to R4 growth stages). On average, greater $K_{cr}$ values existed at higher N rates (e.g., 196 and $252\mathrm{kg}\mathrm{N}{\mathrm{ha}}^{-1}$) compared with lower N rates. Lower N treatments typically reached their maximum $K_{cr}$ value earlier in the growing season and began to decrease towards harvest. Rainfed and 75% FIT experienced a greater reduction in $K_{\mathrm{stress}}$ 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 $K_{\mathrm{stress}}$ attributed with water stress alone. The monthly average values often experienced lower $K_{\mathrm{stress}}$ compared with $K_w$ values, indicating that $K_w$ alone was unable to account for the total reduction in $K_{cr}$ from a nonlimiting water and N reference. Thus, an N stress factor ($K_n$) was also quantified by assuming $K_{\mathrm{stress}}$ was the product of water and N stress (e.g., $K_{\mathrm{stress}}=K_w\times K_n$). 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.