Project Number: 6209-13000-013-12
Project Type: Nonfunded Cooperative Agreement

Start Date: Apr 15, 2007
End Date: Mar 31, 2010

Objective:
1. Calibrate the Large Aperture Scintillometer (LAS) using lysimetric data (dry and irrigated crop land) for estimating latent heat flux as the residual of the land surface energy balance. 2. Use of the scintillometer over cropped and natural landscapes to evaluate the accuracy of remote sensing based regional evapotranspiration maps for the Texas Panhandle.

Approach:
The proposed study concentrates on the region in and around the USDA-ARS Conservation and Production Research Laboratory (CPRL), at Bushland, TX. The CPRL is located 1170 m above mean sea level at 350 11' N, 1020 06' W. Objective 1: Three Large Aperture Scintillometers (LAS) will be deployed at the USDA-ARS-CPRL, Bushland, Tx: one across two dry land lysimeter fields; one across two irrigated lysimeter fields; and one over rangeland. These instruments are scheduled to be installed beginning in April 2007 for 12-24 months. The orientation of each LAS was selected to have the scintillometer's path perpendicular to the wind direction and to avoid direct sunlight on the lenses. Refractive index of air measurements will be made on a continuous time scale at 1 Hz and averaged at 15-min intervals, synchronized with the weather station and lysimeter measurement averaging period. In addition, net radiometer and soil heat flux plates will be installed in each field depending upon the crop type to measure net radiation and soil heat flux over 15-min intervals. Crop heights and leaf area indexes will be measured once every 8 days to coincide with Landsat 5 and ASTER satellite overpasses. The height of each LAS will be adjusted to keep its height above the crop surface throughout the monitoring period. For estimating sensible heat flux (H) from the refractive index of air, air temperature and wind speed data are required. These data will be obtained from the grass reference weather station located near the irrigated lysimeter fields and from sensors on each lysimeter. The LAS data will be tested for accuracy and erroneous data will be removed before being used for estimating H. Finally, predicted evapotranspiration (ET) rates will be compared with area-weighted lysimeter data at 1-h, daily, monthly, and seasonal time steps to evaluate the ability of LAS to estimate ET. After calibration, the ET data derived from the scintillometers will be used to evaluate ET maps developed from ASTER, TM, and MODIS images using a flux source area (footprint) function. Objective 2: As part of the ongoing groundwater modeling project, there is a need for an ET map for a 4-county area (Dalham, Sherman, Hartley, and Moore) in the Texas Panhandle. For this purpose, we are planning to acquire two Landsat Thematic Mapper (TM) 5 scenes during the 2007 crop season. However, there are no lysimeter eddy covariance or Bowen ratio systems in that area for evaluating the accuracy of the remote sensing based ET maps. Existing weather stations will supply the needed wind, solar radiation, air temperature, and air moisture data. We believe that a calibrated LAS will provide necessary data for such assessment. We will deploy LAS, heat flux plates, and net radiometers across a 2x2-mile section in two different locations on the day of satellite overpass to estimate area-weighted ET over non-homogenous land surface. These data will be used to evaluate the accuracy of the ET map obtained using both METRIC and TSEB models.