Discussions and Closures
Discussion of “Actual Crop
Evapotranspiration and Alfalfa- and
Grass-Reference Crop Coefficients of Maize under Full and Limited Irrigation and Rainfed
Conditions” by K. Djaman and S. Irmak
Gürol Yıldırım, Ph.D.1 1Associate Professor of Civil Engineering, Head of Hydraulics Division,
Dept. of Civil Engineering, Aksaray Univ., Engineering Faculty, Central Campus, Aksaray 68100, Turkey. E-mail: email@example.com
The authors are appreciated for presenting a comprehensive study to quantify the actual evapotranspiration (ETa) of maize and the reference crop coefficients (Kco and Kcr) of grass and alfalfa.
Obviously, the research topic of the paper primarily addresses the implementation of limited and full irrigation, in addition to rainfed farming practices under soil, climate, and management conditions through extensive field campaigns conducted in south central Nebraska in 2009 and 2010. As stated in the “Introduction” section, the paper primarily focuses on the following objectives: “(1) to monitor and assess soil water dynamics under full- and limited-irrigation and rainfed settings; (2) to quantify the full and limited-irrigated and rainfed maize ETa by soil water–balance approach; and (3) to develop grass- and alfalfa-reference crop coefficients for maize under various irrigation levels and rainfed condition in a transition zone between the subhumid and semiarid climatic region of south central Nebraska.”
According to the present approach, four irrigation treatments were studied as fully irrigated treatments (FITs): 75, 60, and 50% FIT and rainfed treatments. The dual crop coefficient (Kc) values were calculated for full and limited irrigation settings and for the rainfed treatment; next, a two-step approach (ETa ¼ Kc × ETref .) was implemented by using the Food and
Agriculture Organization of the United Nations (FAO) method (Allen et al. 1998) to quantify maize ETa values for each irrigation level and rainfed conditions, and the estimates were compared with those ETa values determined by using the soil water–balance approach. Additionally, detailed investigations (Irmak and Irmak 2008; Irmak et al. 2008a, b, c) on evaluating reference and crop evapotranspiration in south central Nebraska were presented by the authors. As a matter of fact, these studies (Irmak and Irmak 2008; Irmak et al. 2008a, b, c) basically encompass the results of the present examination, for the sake of good comparison.
As a consequence of this extensive research, the concluding remarks described in the following were claimed in the “Summary and Conclusions” section: 1. “On average, the two-step approach overestimated ETa by approximately 19% in 2009 and 7.2% in 2010. The difference in ETa between the two methods ranged from 18% for the FIT to 38% for the rainfed treatment in 2009. The difference in 2010 was smaller in a wetter 2010 growing season, ranging from 12% for the rainfed treatment to 21% for the FIT.” 2. “The FAO-approach Kc was not able to fully account for the impact of limited irrigation and rainfed conditions on the crop coefficient, resulting in considerable errors in estimating ETa, emphasizing the importance and necessity of developing Kc values for full- as well as limited-irrigation settings and rainfed conditions under local soil, crop, and other management conditions to improve the accuracy of ETa estimates.” 3. “The new set of Kco and Kcr equations can be beneficial for in-season irrigation management and for estimating maize water use under fully irrigated and limited irrigation as well as rainfed conditions for the locations that have similar soil, crop, climatic, and management conditions of this study location. Because ETa for the rainfed treatment is heavily dependent on the rainfall timing and amount within a given growing season, the Kco and Kcr values developed for the rainfed treatment will also be a strong function of rainfall and may show much greater variability between the years than the Kco and Kcr values for the limited and fully irrigated settings.”
Essentially, the present study provides an opportunity to clearly evaluate the available information in the literature and provides helpful information for irrigation designers and decision makers.
The results from this extensive research are largely satisfactory; additionally, a few minor points that may need further clarification will be pointed out in this discussion.
Consideration of FAO Approach
As concluded in the paper, for estimating the ETa values, the FAO approach yielded considerable errors when fully accounting for the impact of limited irrigation and rainfed conditions on the crop coefficient. In line with this concept, some additional considerations on the FAO approach may be useful for the sake of comprehensive evaluation.
Scientifically based irrigation scheduling programs are dependent upon the accurate computation of historic crop evapotranspiration (ETc) to predict future crop water use. Many different methods and equations for calculating the ET of a crop have surfaced in recent decades, including combination, radiation, temperature, and pan-evaporation methods. The standardized
Penman-Monteith (PM ET0) method by the Environmental Water
Resources Institute (EWRI) of ASCE (ASCE EWRI 2005; Walter et al. 2005) and the FAO Manual 56 Penman-Monteith (FAO-56
PM) method (Allen et al. 1998) are the most recent combination methods and are widely accepted as better than all other methods (Itenfisu et al. 2003). The PM ET0 method (ASCE EWRI 2005) is identical to the FAO-56 PM method for short grass at a daily time step.
These two reference crop evapotranspiration (ET0) methods are the most widely accepted to estimate grass-based reference crop ET when the required climatic data are available. In 1998, the FAOPenman Monteith method was recommended as “the sole standard method for the definition and computation of reference evapotranspiration” (Allen et al. 1998). However, since the adoption of the