Using High-Resolution Remote Sensing, Lysimetry, and Big Leaf Modeling to Infer Crop Water Use in the Wisconsin Central Sands.

Rapid land use conversion to irrigated agriculture in the Wisconsin Central Sands (WCS) has led to a unique water management dilemma that challenges the spatiotemporal limits of evapotranspiration (ET) measurement and modeling.  The WCS is a small (6400 km²) ecological region where the seemingly abundant groundwater supply has made Wisconsin a top five producer of potatoes, sweet corn, beans, and peas. However, land use conversion to irrigated agriculture in the WCS has coincided with lower lake levels, stream flows, and overall surface water degradation, which are particularly observable during warm, dry years. We estimate that an increase in ET of 50 mm yr^-1 due to consumptive groundwater use is enough to explain observed impacts on surface waters; however, the water budget in these humid agroecosystems is difficult to predict within 50 mm yr^-1 of certainty because of high interannual climate variability. This uncertainty in the water budget exacerbates tension between stakeholders and hinders management, as symptoms of surface water scarcity are not always observable. Stakeholders require a greater understanding of where and when water can be conserved for effective management. Our goal was to identify trends in the spatiotemporal variability and uncertainty of ET and its drivers from irrigated cropping systems in the WCS during the 2014-2016 growing seasons. First, we collected thermal imagery via aircraft, which we used with the High-Resolution Mapping of Evapotranspiration (HRMET) energy balance model to map ET from four commercial fields in the WCS. Simultaneously with aircraft missions, we collected gas exchange measurements and used a big leaf model (Shuttleworth-Wallace) to estimate instantaneous ET within the four fields. Finally, we made weekly estimates of ET using 16 passive capillary lysimeters in the same four fields. Preliminary results reveal future opportunities for conservation such as irrigation scheduling, precision irrigation, and crop rotational strategies.