Water Resources Research Act Program

Details for Project ID 2015TN110B

Measuring evapotranspiration and soil moisture to close the hydrologic budget under different land-uses in Tennessee

Institute: Tennessee
Year Established: 2015 Start Date: 2015-03-01 End Date: 2016-02-28
Total Federal Funds: $15,000 Total Non-Federal Funds: $31,440

Principal Investigators: Thanos Papanicolaou, Christopher Wilson

Abstract: Evapotranspiration (ET) is the collective water loss from soil or water bodies by evaporation, and from vegetation by transpiration. It is a major component of the land surface water cycle, as it directly affects the amount of water available for surface water runoff and groundwater recharge, and hence human consumption. Despite the relative importance of ET to the hydrologic cycle, especially in the U.S. Southeast which has some of the highest mean annual ET in the country, it is one of the least systematically measured parameters. The focus on this study is Eastern Tennessee, which has recently experienced gradual urbanization at the interface of existing agricultural areas leading to highly variable land covers and soil conditions in terms of water holding capacity and emissivity. Two critical gaps in our current ET monitoring and modeling capabilities that must be addressed to enable effective water resources decision-making are the following: (1) the lack of understanding about the role of soil moisture and pedology at different spatial and temporal resolutions sufficient to quantify ET, especially in regions exhibiting high heterogeneity in landscape characteristics, and (2) a limited ability to transfer this information and resulting model predictions from relatively small scales to scales of societal importance. We propose the development of a mobile array of state-of-the-art sensors capable of measuring not only the rate of evapotranspiration under multiple Land-Use/ Land Covers (LULCs) throughout Eastern Tennessee, but also the resulting change in soil moisture. Instrumented, experimental hillslopes or farms will allow for controlled, spatially distributed measurements of ET to isolate the dominant hydropedologic factors affecting it and to inform better the parameterization of existing ET models. Past research has used meteorological properties (such as solar radiation, air temperature, relative humidity, and wind speed/ direction) and reference crop corrections to determine potential ET, but neglected the role of soil moisture at the soil boundary surface (i.e., top 30 cm) on actual ET. Key science questions that remain open are how, under different temperature gradients, do ambient soil moisture conditions and vegetation coverage, characterized by different Leaf Area Index (LAI) values, affect the onset and amount of ET, and what are those threshold values for soil moisture and vegetation cover that can trigger high ET losses? By addressing these questions, this research will provide improved predictive ability of ET from the soil surface in response to changing weather conditions by incorporating the memory effects of soil. The proposed spatially detailed measurements will also provide ground-truthing of satellite-derived estimates of ET. Scaling up using the remotely sensed data and Geographical Information Systems (GIS) software with kriging interpolation techniques based on in-situ property cross correlation functions from the matrix of the proposed controlled measurements will offer data to water resources decision- makers at meaningful scales. Overall, this research will lead to a robust method to estimate ET at a higher spatial resolution by considering for the first time the effects of soil moisture on ET under different LULCs and climates throughout mixed agricultural-urban watersheds in Tennessee. The improved monitoring will provide more accurate quantification of the partitioning of precipitation to surface runoff and groundwater recharge. The resulting mobile array of state-of- the-art sensors to measure ET, LAI, and soil moisture changes can provide essential but missing data for a GIS Data Management System for water resources research in Tennessee, as well as ground-truthing data for satellite-based estimates of ET and soil moisture to develop regional scale water budgets for long-term water resources planning, management and risk analysis.