Surface resistance controls differences in evapotranspiration between
croplands and prairies in U.S. Corn Belt sites
Abstract
Water returned to the atmosphere as evapotranspiration (ET) is
approximately 1.6x greater than global river discharge and has
wide-reaching impacts on groundwater and streamflow. In the U.S.
Midwest, widespread land conversion from prairie to cropland has altered
spatiotemporal patterns of ET, yet there is no consensus on the
direction of change in ET or the mechanisms controlling changes. We
aimed to harmonize findings about how land use change affects ET in the
Midwest. We measured ET at three locations within the Long-Term
Agroecosystem Research (LTAR) network along a latitudinal gradient with
paired rainfed cropland and prairie sites at each location. At the
northern locations, the Upper Mississippi River Basin (UMRB) and Kellogg
Biological Station (KBS), the cropland has annual ET that is 84 and 29
mm/year higher, respectively, caused primarily by higher ET, likely from
soil evaporation during springtime when agricultural fields are fallow.
At the southern location, the Central Mississippi River Basin (CMRB),
the prairie has 69 mm/year higher ET, primarily due to a longer growing
season. To attribute differences in springtime ET to specific
mechanisms, we examine the energy balance using the Two-Resistance
Method (TRM). Results from the TRM demonstrate that higher surface
conductance in croplands is the primary factor leading to higher
springtime ET from croplands, relative to prairies. Results from this
study provide critical insight into the impact of land use change on the
hydrology of the U.S. Corn Belt by providing a mechanistic understanding
of how land use change affects the water budget.