Developing Irrigation Zones from a Field Scale Crop Water Productivity Map.

Water is an essential resource to the productivity of agriculture, particularly in arid or semi-arid regions. Water scarcity threatens sustainable food production and population expansion in regions dependent upon irrigation. Variable rate irrigation (VRI) is a promising technology capable of increasing water productivity by adjusting the irrigation rate to match field variability in soil properties, yield potential, topography, etc. However, grower adoption of VRI is limited from lack of research and decision support systems. A field scale crop water productivity (CWP) analysis in a 22 ha field with winter wheat (Triticum sp.) was evaluated under three grower determined irrigation zones (0%, 90% and full irrigation) near Grace, ID, USA in 2016. Crop water productivity was calculated as yield over seasonal evapotranspiration (ET), where ET was determined using a water balance method including seasonal precipitation, irrigation, and pre- and post-season soil moisture at 80 sampling locations. The spatial variation of CWP ranged from 4.6-25.6 kg ha^-1 mm^-1 with an average of 16.8 kg ha^-1 mm^-1. The average for the 90% zone was 17.9 kg ha^-1 mm^-1 and the 0% irrigation zone overlaid exposed bedrock so there was no yield data; but the VRI system allowed 100% water savings in this zone. The CWP layer identified spatial variation in crop water demand and potential management locations where irrigation was over or under applied. The average CWP for the 90% zone was greater than the field average, but the current VRI zones had little influence on CWP spatial variability, indicating underutilization of the VRI system. Using the CWP analysis with yield history and grower experience, the 2017 irrigation zones were altered attempting to conserve water by maximizing the crop produced per unit of applied water. Growers standard test strips overlay the altered irrigation zones to investigate the effectiveness of this irrigation strategy.