Interannual variation in long-term center pivot-irrigated maize evapotranspiration and various water productivity response indices. II: Irrigation water use efficiency, crop WUE, evapotranspiration WUE, irrigation-evapotranspiration use efficiency, and precipitation use efficiency

Quantification of various water use efficiency (crop water productivity) indices for different irrigation regimes can aid in making effective in-season water management decisions and developing crop water productivity models. However, these critical variables can be affected by interannual variation in climatic conditions, and long-term research that provides such data and information is extremely rare. Maize (Zea mays L.) irrigation water use efficiency (IWUE), crop water use efficiency (CWUE), evapotranspiration water use efficiency (ETWUE), annual precipitation use efficiency (ANNPUE), growing season precipitation use efficiency (GRSPUE), and the newly developed growing season precipitation use efficiency with respect to rainfed yield (GRSPUE_rainfed) were measured under full and limited irrigation and rainfed conditions from 2005 to 2010 in south-central Nebraska. An additional new irrigation efficiency term [irrigation-evapotranspiration use efficiency (IRRETUE)] was developed and tested to evaluate the efficiency of irrigation management with respect to actual crop evapotranspiration (ETa). Four irrigation treatments were imposed [fully irrigated treatment (FIT), 75% FIT, 60% FIT, and 50% FIT] and a control rainfed treatment. CWUE varied from 1.21 kg/m³ for rainfed treatment in the dry year of 2005 to 2.51 kg/m³ for 60% FIT in the below average year of 2009. CWUE exhibited substantial interannual variation. The lowest CWUE was always obtained under rainfed production, whereas the highest value was usually observed under FIT, except in 2009. The FIT and 75% FIT CWUE values were very similar. Six-year average CWUE values were 2.34, 2.32, 2.29, 2.24, and 1.73 kg/m³ for FIT, 75% FIT, 60% FIT, 50% FIT, and rainfed, respectively, with SDs of 0.06, 0.09, 0.14, 0.12, and 0.38 kg/m³, respectively. IWUE varied from 1.75 kg/m³ for 50% FIT in the wettest year (2008) to 5.9 kg/m³ for the same treatment in 2009. The six-year average IWUE values were 4.01, 4.45, 4.48, and 4.13 kg/m³ for FIT, 75% FIT, 60% FIT, and 50% FIT, respectively, with SD values of 0.82, 0.84, 0.83, and 1.88 kg/m³ for the same treatments. There was considerable interannual variation in IWUE for the same treatment; this variation between treatments in a given year was greater than the interannual variation for the same treatment. The IWUE was maximized with limited irrigation treatments (mostly 50% FIT, followed by 75% FIT and 60% FIT). ETWUE values varied from 1.18 for 50% FIT in 2008 to 7.16 kg/m³ for 60% FIT in 2010; ETWUE decreased linearly with seasonal ETa (R² > 0.80) in 2006, 2007, and 2009 and decreased with seasonal ETa, displaying a low R² of 0.26 in 2010. In the wettest year (2008), ETWUE increased with ETa (R² = 0.31). The new efficiency index, IRRETUE, ranged from 37.8% (62.2% underirrigation) for 50% FIT in 2010 to as high as 149.6% (49.6% overirrigation) for 50% FIT in 2008. On a long-term average, IRRETUE values were 87, 93.2, 88, and 89.2% for FIT, 75% FIT, 60% FIT, and 50% FIT, respectively. The new IRRETUE can be a very effective tool to evaluate the effectiveness of irrigation management practices with respect to meeting crop ETa for maximum yield. When the annual precipitation is considered, FIT resulted in greater ANNPUE, followed closely by 75% FIT and other limited irrigation treatments. The GRSPUE_rainfed values on all-treatment average basis were 32, 46, 66, 90, 61, and 82% (for FIT, 75% FIT, 60% FIT, and 50% FIT) lower than the GRSPUE values from 2005 through 2010. May precipitation consistently had the greatest influence on GRSPUE in all years, with high R² of 0.73, 0.72, 0.71, and 0.76 for FIT, 75% FIT, 60% FIT, and 50% FIT, respectively. The rainfed treatment, however, was most influenced by April precipitation (R² = 0.62). May precipitation was also the most influential on the GRSPUE_rainfed, with high R² values of 0.67, 0.77, 0.87, and 0.82 for FIT, 75% FIT, 60% FIT, and 50% FIT, respectively. These research results are some of the first long-term research results reported in the literature for full and limited irrigated and rainfed maize productivity indices and can provide invaluable data and information for developing effective in-season water management decisions and determining crop productivity responses to different irrigation/water management conditions.