| 英文摘要: | 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. |