Evapotranspiration from Corn, Soybean, and Prairie Grasses using the METRIC Model

Land use in the US Upper Midwest has evolved from small grains and prairie systems to corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. This change has been assumed to cause a dramatic decrease in landscape evapotranspiration (ET) and consequently a large increase in streamflow. This study assessed ET from corn, soybean, and native prairie grasses using the satellite image based surface-energy balance model named METRIC in west-central Minnesota. The METRIC model uses properties of satellite images along with daily weather to calculate landscape ET. The land covers analyzed were irrigated and non-irrigated corn, non-irrigated soybean, native prairie grasses under different grazing and burn regimes, and wetlands. For 8 June through 22 Oct. 2015, estimated ET followed the trend: wetlands (717 mm) > non-irrigated corn (662 mm) > irrigated corn (647 mm) > non-irrigated soybean (554 mm) > previously burned prairie (537 mm) > recently burned prairie (406 mm). For 1 May to 22 Oct. 2015, the estimated ET of wetlands, previously burned and recently burned prairie grasses were 884, 666, and 511 mm, respectively. These model estimates showed that ET of native prairie grasses are similar to slightly higher than that of corn and soybean. Considering soybean also replaced low ET small grains suggests that changes in landscape ET from a changeover of vegetative cover starting in 1940s are likely minimal. This supports our earlier findings that recently increased streamflows in the Upper Midwest are primarily from increased precipitation and not due to land cover changes.