Constraining Regional Evapotranspiration in the Upper Midwestern United States Using In Situ Observations and Numerical Modeling

Abstract

Evapotranspiration (ET) is a critical component in the global water cycle and water resource management. The Upper Midwestern United States (US), a major agricultural production region with large areas of lake and cropland, is facing challenges related to extreme variations in precipitation, increasing irrigation water usage, and large fluctuations in water budgets. Lake evaporation and cropland ET represent two significant components of the regional water budget. However, regional ET estimates from these two sources contain large uncertainties due to their complex interactions with atmospheric conditions (e.g. precipitation) and land surface processes (e.g. ice/plant phenology, atmospheric demand), affected by climate change and anthropogenic activities. This dissertation combines in situ observations and modeling to better constrain the regional ET, focusing on lake evaporation, local water recycling, and cropland ET in the Upper Midwest US.Evaporation from a temperate closed-basin lake, White Bear Lake (WBL), was estimated using the eddy covariance method and an optimized lake model CLM4-LISSS. The annual evaporation totals from 2014 to 2016 were 559 ± 22 mm, 779 ± 81 mm, and 766 ± 11 mm, respectively. The combined effects of smaller average daily evaporation and a shorter ice-free season caused lower evaporation in 2014. Retrospective analyses indicated that WBL evaporation increased by 3.8 mm/year during 1979–2016, which was driven by increased wind speed and lake-surface vapor pressure gradient. Lake evaporation is expected to increase by 1.4 mm/year from 2017 to 2100 under the business-as-usual greenhouse gas emission scenario, largely driven by extended ice-free periods. These results imply that the water level of WBL is closely coupled to evaporation and consequently impacted by the large-scale synoptic and climatic conditions. The contribution of ET to regional precipitation, known as “local water recycling”, is a key process in the water cycle. An idealized two-layer equilibrium planetary boundary layer model was coupled with a stable isotope module that included HDO and H218O in water to constrain the local water recycling ratio (LRR) by isotope observations. The regional value of the summer LRR was estimated to be 0.29 ± 0.12. The summer LRR values for the years 2006–2010 varied between 0.17 and 0.36. The smallest value of LRR was in 2008 which corresponded to a drought year. Cropland has likely changed the regional LRR by −7.6 to 19.5% under different pre-agriculture land cover scenarios. The model also implies that local water recycling is expected to be weakened under drought conditions, but it will be enhanced if irrigation is applied more intensely. In humid continental climates, forecasting cropland ET is challenging due to the variable precipitation and plant phenology. An ET forecast system (ETool) was built upon the Weather Research and Forecasting (WRF) model with the Noah land surface scheme to forecast the weekly ET at 3-km resolution in Minnesota, US. The near real-time leaf area index (LAI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) product was used in ETool to improve the representation of plant phenology. At a cropland site, the LAI improvement led to a 17.7% reduction in the weekly ET forecast bias, with an R2 value of 0.82 and a root-mean-square error of 0.64 mm/day. Using the predicted difference between precipitation and ET, ETool can inform irrigation scheduling to balance the tradeoff between safeguarding yields and conserving water usage. Collectively, this dissertation also revealed the feedback processes between ET and climate under the influence of anthropogenic activities in the Upper Midwest US. As the climate continues to warm, the regional lake evaporation is expected to increase with lengthening ice-free periods. The regional cropland ET and local water recycling are expected to be weakened due to an increased likelihood of drought events. However, irrigation is anticipated to increase in response to the more frequent drought events, which will conversely enhance ET and local water recycling. Furthermore, more intense groundwater usage and greater fluctuations in lake water levels within the region are expected with increased use of irrigation.