Producing high resolution maps of water use efficiency (crop yield per unit of water consumption; WUE) for precision crop management is limited by our ability to readily produce maps of soil moisture content. On-the-go grain yield monitors or biomass scans can provide a spatial measure of crop productivity, the numerator of a WUE ratio but water use, the denominator, is limited by physical practicalities to a few single-point measures. Volumetric moisture content inferred from an EM38 electromagnetic induction (EMI) survey, and biomass evolution (Z31-43) derived from optical reflectance measurements were combined for a wheat crop in order to generate a map of water use efficiency (t/ha/mm). Taken over the entire field, the change in soil moisture (mm) was found to explain 38% of the variance in the change in biomass (t/ha). The implications for using multi-temporal EMI surveys in combination with yield maps to produce a spatial measure of water use efficiency are discussed.