Water Resources Research Act Program

Details for Project ID 2013SC90B

Enhancing Sattelite Measurements of Water Surface Temperature Using a Thermal Model of the Lake Surface for Improved Evaporation Estimates

Institute: South Carolina
Year Established: 2013 Start Date: 2013-03-01 End Date: 2014-02-28
Total Federal Funds: $18,000 Total Non-Federal Funds: $36,000

Principal Investigators: John Saylor, Nigel Kaye

Abstract: 9. Abstract Problem: Predictions and simulations of water availability in a lake, reservoir or water basin require detailed knowledge of inflows and outflows. These include dam outflows, stream inflows, seepage, precipitation and evaporation, among others. Of these, evaporation is, surprisingly, one of the least understood and most poorly quantified. This stems primarily from the way in which it is measured, which is almost always by using an evaporation pan on a shore adjacent to the lake or reservoir of interest. These measurements are flawed for several reasons including the fact that the pan water heats up (and cools down) much more rapidly than the lake water; the flow of air over the lip of the pan is different from that of an extended lake fetch; the relative humidity and air temperature need not be the same over the pan and over the lake, and others. An alternative approach to pan evaporation measurements is to use physicsbased parameterizations of evaporation. These parameterizations require measurements of air temperature Ta, relative humidity ind speed U, and water surface temperature Ts. With the exception of Ts, these measurements are all readily available from several sources (e.g. NWS stations). However, accurate water surface temperature measurements are challenging to obtain. Radiometric methods are required, since it is very difficult to position even a very fine thermistor right at the water surface in a field application. However, radiometric measurements are both expensive and technically difficult, since the angle of incidence of the sensor can significantly affect the measurement. Methods: The launch of the MODIS instrument on the Aqua and Terra satellites opened the door to obtaining accurate, synoptic measurements of Ts on lakes and reservoirs. MODIS has IR measurement capabilities that are used to obtain water surface temperatures. This enables the measurement of evaporation using a (now) complete set of inputs (Ta, Ts, ) for a physics based parameterization of the evaporation rate E. Using both Aqua and Terra, a total of four Ts measurements can be obtained daily for every location on Earth, allowing for four measurements of evaporation each day. We are currently using these evaporation measurements to better understand the quantity and variability of evaporative loss from reservoirs in the Savannah River Basin (SRB). However, this work is complicated by the fact that four measurements of Ts do not allow us to identify the maximum and minimum temperatures (and concomitantly evaporation rates) during the course of one day, resulting in significant uncertainty in the evaporation rate. Because the other relevant variables (Ts, ) are known at higher temporal resolution (hourly or better), this four measurements/day restriction makes Ts the limitation in our evaporation measurement approach. This limitation could be eliminated or at least significantly improved with a model of the diurnal variation in lake surface temperature. This is the focus of this proposed one year study. Objective: The objective of this work is to develop a thermal model of the diurnal variation of lake surface temperature for a holomictic lake through an energy balance of radiation, evaporation, and convection (to/from the air and to/from the surface to the bulk water). We do not propose the development of a model that would actually predict Ts based solely on measured environmental conditions; this would be quite challenging for a one year effort. Rather, our goal is to develop an accurate functional form for Ts (actually a family of functions, as is described herein). Once this functional form is obtained, the four daily Aqua/Terra values will be used as fitting parameters to provide a daily function, which can then be used to evaluate Ts at the times during which the more temporally-resolved values for Ta, d U are available. This will enable estimates of E at hourly or better intervals. These estimates will be used to answer several questions about evaporative loss from reservoirs in the SRB. Among these are: exactly what is the evaporative loss from these reservoirs? Where/when does the majority of this loss occur? How much variability is there in E on an hourly, daily, monthly and yearly time scale?