Hydrologic controls on temperature extremes in managed landscapes

该项目研究植被，特别是农业的变化，如何影响农田的土壤湿度和极端温度。一项初步研究表明，在农作物产量显著增加的美国中西部，在作物生长季节的高峰期，最热的温度也随之下降。这表明，植物叶片蒸发潜力的增加导致了气候变冷，改善了农业条件。考虑到这一点，本研究考察了以下问题：1)向高产作物的过渡增加了用水的潜力；2)历史上最热生长季节温度的冷却确实是由农业用水变化引起的；3)目前的模型充分代表了土壤湿度、温度和用水之间的关系？前两个问题将通过土壤水分传感器、测量大气水通量的塔以及对温度和蒸发的卫星估计来解决。为了确定中西部地区土壤湿度、作物用水和温度之间的关系是否可以推广，将分析全球气象站数据以及农业普查报告中的作物面积、生产力和灌溉数据集。问题3将使用与温度和蒸发有关的模型来解决，包括这种关系如何随着作物生长而变化。农作物、用水和温度之间关系的发展也将有助于重建上个世纪水资源可用性的变化。总而言之，这项工作将提高对农业地区土壤水分可得性和利用的认识，以及土壤水分、极端温度和农业之间的关系。本研究的目的是探讨管理植被的变化是否显著改变了农业地表与低层大气边界层之间的水文耦合。初步分析表明，温度分布的大规模变化与农业强度的变化有关。具体来说，美国中西部农业区夏季最高气温在生长季节显示出显著的降温，这与区域农业生物质产量的增加成正比。据推测，农业强度的增加导致潜在蒸散量的增加，从而改善了最热生长季节的温度。为了测试支撑作物生长、蒸散和温度之间联系的水文机制，建议利用原位土壤湿度测量、涡旋通量测量和卫星观测来评估季节性和历史作物发育、水分利用和蒸散之间的关系。关于全球作物面积、产量和物候的新数据集也将用于评估农业集约化、蒸散和极端温度之间诊断关系的普遍性。此外，将评估蒸散发模型在农田环境中的准确性，以及利用早期可用的粗输入数据归纳农田蒸散发和土壤湿度推断的能力。这项工作还将允许测试几个子假设：(i)高产作物通常具有更高的蒸散量，（ii）中西部缺乏夏季变暖是作物集约化的结果，以及（iii）干旱使密集种植的生态系统恢复到历史温度条件。这些调查路线对于理解土壤湿度和极端温度的变化具有广泛的相关性，这些变化是由向更集约化的农业管理过渡造成的。

This project examines how vegetation, particularly changes in agriculture, influence soil moisture and extreme temperatures in croplands. A preliminary study indicates that in the US Midwest, where crop production has increased significantly, there was an accompanying decline in the hottest temperatures at the height of the growing season. This suggests that increased potential for evaporation from plant leaves has led to cooling and improved conditions for agriculture. With this in mind, this research examines if: 1) transitions to higher productivity crops increase the potential for water use, 2) historical cooling of the hottest growing season temperatures is indeed caused by agricultural changes in water use, and 3) current models adequately represent the relationship between soil moisture, temperature, and water use? The first 2 questions will be addressed using soil water sensors, towers that measure atmospheric water flux, and satellite estimates of temperature and evaporation. To see if the relationships between soil moisture, crop water use, and temperature identified in the Midwest can be generalized, global weather station data as well as crop area, productivity, and irrigation datasets from agricultural census reports will be analyzed. Question 3 will be addressed using models that relate temperature and evaporation and that include how this relationship changes with crop development. Development of the relationships between crops, water use, and temperature will additionally help in reconstructing changes in water availability over the last century. All told, this work will improve understanding of the availability and use of soil moisture in agricultural regions, as well as the relationships between soil moisture, extreme temperatures, and agriculture. The purpose of this research is to explore whether changes to managed vegetation have significantly altered the hydrologic coupling between agricultural land surfaces and lower atmospheric boundary layers. A preliminary analysis indicated large-scale changes in the distribution of temperature related to changes in agricultural intensity. Specifically, the highest summer temperatures in US Midwest agricultural areas show significant cooling during the growing season that is proportional to increases in regional agricultural biomass production. It is hypothesized that increased agricultural intensity causes increased potential evapotranspiration and, thereby, amelioration of the hottest growing season temperatures. To test the hydrologic mechanisms that would underlie such connections between crop growth, evapotranspiration, and temperature, it is proposed to evaluate relationships between seasonal and historical crop development, water use, and evapotranspiration using in-situ soil moisture measurements, eddy flux measurements, and satellite observations. New datasets on global crop areas, yield, and phenology will also be used to assess the generality of the diagnosed relationships between agricultural intensification, evapotranspiration, and temperature extremes. Further, evapotranspiration models will be evaluated for accuracy in cropland environments, as will the ability to generalize inferences of cropland evapotranspiration and soil moisture using the coarse input data available in earlier periods. This work will also permit testing several sub-hypotheses: (i) that higher yielding crops generally have higher evapotranspiration, (ii) that lack of summer warming in the Midwest is a result of crop intensification, and (iii) that drought reverts intensely cultivated ecosystems back toward historical temperature conditions. These lines of investigation have broad relevance for understanding changes in soil moisture and temperature extremes resulting from transitions toward more intensive agricultural management.