Spatial and temporal changes in maize and soybean grain yield, precipitation use efficiency, and crop water productivity in the U.S. great plains

Sustainable agricultural utilization of the limited water resources demands improvements in understanding the changes in crop water productivity (CWP) in space and time, which is often presented as a potential solution to relieve the growing pressure on fresh water resources. In addition, crop yield needs to be studied in relation to precipitation received annually and during the growing season for its contribution to reduce irrigation water requirements, which is quantified through precipitation use efficiency (PUE). Hence, systematic quantifications, mapping, and analyses of large-scale CWP and PUE levels are needed. This study aims to quantify long-term (1982-2013) information on grain yield, PUE, and CWP for maize and soybean in the U.S. Great Plains counties and to map and analyze them. Multiple public data sources were used, including weather, satellite, and yield datasets for the 834 counties over a 32-year period. Long-term average maize grain yield ranged from 1.56 to 12.81 t ha-1 with a regional average of 6.66 t ha-1. Long-term average soybean grain yield ranged from 0.47 to 3.46 t ha-1 with an average of 2.17 t ha-1. About 87% and 89% of the counties in the region showed increasing trends in grain yield for maize and soybean, respectively, with regional average increasing trends for maize and soybean yield of 0.1014 and 0.0328 t ha-1 year-1, respectively. The regional annual PUE (ANNPUE) and growing season PUE (GRSPUE) were 1.09 and 1.90 kg m-3, respectively, for maize and 0.32 and 0.55 kg m-3, respectively, for soybean. In addition, the regional average increasing trends in maize ANNPUE (exhibited by 88% of counties) and GRSPUE (exhibited by 85% of counties) were 0.0174 and 0.0316 kg m-3 year-1. For soybean, regional average increasing trends in ANNPUE (exhibited by 91% of counties) and GRSPUE (exhibited by 87% of counties) were 0.0048 and 0.0081 kg m-3 year-1. The magnitude of maize CWP varied from 0.30 to 2.97 kg m-3 with a regional average of 1.08 kg m-3, and soybean CWP varied from 0.15 to 0.67 kg m-3 with a regional average of 0.40 kg m-3. It was found that 79% and 86% of the counties showed positive trends in maize and soybean CWP, respectively, and the increasing trend magnitudes were 0.0144 and 0.0047 kg m-3 year-1. Pooled data from all counties and growing seasons were used to develop frequency distribution histograms to quantify the inter-annual variation and distribution characteristics. The level of CWP variability represented via maps revealed regions where opportunity exists for improvements in production system efficiency. A comprehensive understanding of the spatial and temporal patterns in these efficiency indices will provide a basis for decision-making in resource assessments, planning, evaluation, and investment by state and federal agencies and stakeholders.