One of the common methods for estimating actual evapotranspiration (ETa) is the two-step approach, which relates crop-specific crop coefficients (Kc) to a reference surface ET, typically alfalfa or grass (ETr and ETo, respectively). Minimal, if any, study has reported Kc values for water, nutrient, and both water and nutrient deficiencies. In this study, alfalfa (Kcr) and grass (Kco) reference maize (Zea mays L.) Kc 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 (Kstress) to assess the reduction in crop water use as compared with a nonlimiting water and N treatment (reference). In 2011, maximum Kcr 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 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°C (R2 to R4 growth stages). On average, greater Kcr values existed at higher N rates (e.g., 196 and 252 kgNha-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 (Kw) was calculated to determine the portion of Kstress attributed with water stress alone. The monthly average values often experienced lower Kstress compared with Kwvalues, 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 (Kn) was also quantified by assuming Kstress was the product of water and N stress (e.g., Kstress = Kw × Kn). The seasonal average Kn was 1.15 in 2011 and 0.64 in 2012. Values of Kn 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, Kn decreased as N rate decreased and Kn had a decreasing trend (e.g., greater N stress) throughout the growing season, especially in the drier year in 2012. The reduction in Kn 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.
|Original language||English (US)|
|Journal||Journal of Irrigation and Drainage Engineering|
|State||Published - Dec 1 2014|
All Science Journal Classification (ASJC) codes
- Civil and Structural Engineering
- Water Science and Technology
- Agricultural and Biological Sciences (miscellaneous)