Water returned to the atmosphere as evapotranspiration (ET) is approximately 1.6x global river discharge and has wide-reaching impacts on groundwater and streamflow. In the U.S. Midwest, widespread land conversion from prairie to pasture to cropland has altered spatiotemporal patterns of ET, yet there is not consensus on the direction of change or the mechanisms controlling changes. We measured ET at three locations within the Long-Term Agroecosystem Research 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 (22% and 5%) higher, respectively, caused primarily by higher ET during springtime when fields are fallow. At the southern location, the Central Mississippi River Basin (CMRB), the prairie has 69 mm/year (11%) higher ET, primarily due to a longer growing season. Differences in climate and that the CMRB prairie is remnant native prairie, while the UMRB and KBS prairies are restored, make it challenging to attribute differences to specific mechanisms. To accomplish this, 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 insight into impacts of warm season grasses on the hydrology of the U.S. Corn Belt by providing a mechanistic understanding of how land use change affects the water budget.

中文翻译：

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表面阻力控制美国玉米种植带农田和草原之间蒸散量的差异

以蒸散量 ( ET )形式返回大气的水量约为全球河流流量的 1.6 倍，对地下水和溪流具有广泛影响。在美国中西部，从草原到牧场再到农田的广泛土地转变改变了蒸散的时空格局，但对于变化的方向或控制变化的机制尚未达成共识。我们在长期农业生态系统研究网络内的三个地点沿着纬度梯度测量了蒸散量，每个地点都有配对的雨养农田和草原点。在北部地区，密西西比河流域上游 (UMRB) 和凯洛格生物站 (KBS)，农田的年蒸散量分别高出 84 毫米和 29 毫米（22% 和 5%），这主要是由于较高的蒸散量造成的春天田地休耕时。在南部密西西比河流域中部 (CMRB)，大草原的ET升高 69 毫米/年 (11%) ，这主要是由于生长季节较长。气候差异以及 CMRB 草原是残余的原生草原，而 UMRB 和 KBS 草原已恢复，因此很难将差异归因于特定机制。为了实现这一目标，我们使用双电阻法 (TRM) 检查能量平衡。 TRM 的结果表明，与草原相比，农田地表电导率较高是导致农田春季蒸散发较高的主要因素。这项研究的结果通过提供对土地利用变化如何影响水预算的机制理解，深入了解暖季草对美国玉米带水文的影响。