Large-scale spatial and temporal variability in evapotranspiration, crop water-use efficiency, and evapotranspiration water-use efficiency of irrigated and rainfed maize and soybean

Quantification of long-term trends and magnitudes of actual crop evapotranspiration (ETc) and crop water productivity [CWP, also known as crop water use efficiency (CWUE)], especially in relation to interannual variation in climate, is critical to determine whether current crop, soil, and irrigation management practices are effective in water conservation and enhancing CWUE. This research quantified and mapped long-term spatio-temporal variability in CWUE, evapotranspiration water use efficiency (ETWUE) and environmental variables associated with it at regional scales for maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] producing counties in Nebraska, from 1986 to 2009. Declining trends were observed in seasonal and annual reference (potential) evapotranspiration (ETref) and irrigated and rainfed maize and soybean ETc. About 15 and 64 mm reduction in seasonal and annual ETref, respectively, were observed from 1986 to 2009 with maximum reduction of 284 mm observed in Zone 4 (eastern subhumid region of Nebraska). A total reduction of 20, 50, 6.7, and 43 mm was observed in irrigated maize, rainfed maize, irrigated soybean, and rainfed soybean ETc, respectively. Analyses of the relationship between annual and seasonal ETref and all the meteorological variables on a large scale showed positive correlation with vapor pressure deficit (VPD), incoming solar radiation (Rs), and wind speed (u) with an R² of 0.85, 0.43, and 0.11, respectively; and inverse relationship with relative humidity (RH) (R² = 0.51). The authors found that the statewide decreasing trend in T_avg, T_min, u, and Rs collectively resulted in the overall reduction in ETref over the study area. Time series plots of CWUE and ETWUE were developed to observe potential spatial and temporal variability in CWUE and ETWUE. Counties with a relatively low mean CWUE in combination with low year-to-year consistency have been identified as areas with highest potential for improving CWUE. On a statewide scale, average increases of 34 and 32% were observed in irrigated and rainfed maize CWUE, respectively, and a 34% increase in maize ETWUE. Irrigated and rainfed soybean CWUE have increased by 29 and 33% from 1986 to 2009, respectively, whereas soybean ETWUE increased by 32%. The authors presented detailed spatio-temporal analyses of all the climatic variables that are used to evaluate CWP, which can be valuable to state and federal water management agencies and practitioners to evaluate long-term productivity indices of crop production and associated water productivity indices on large scales for various policy and management decisions as well as future planning and forecasting of crop productivity versus water demand and use, and associated analyses.