Irrigated agriculture represents the primary consumer of global freshwater resources (90% during last century). However, quantifying the amount of water consumed is difficult because all irrigation wells are not metered. Estimating actual ET (AET), which represents crop water consumption, from satellite data provides a regional approach for quantifying water consumption by irrigated agriculture. Such data are essential for management of water resources. The, Bushland Evapotranspiration and Agricultural Remote Sensing Experiment (BEAREX), is a multi-institutional (USDA ARS, Univ. Texas, and Utah State Univ.) research effort that was initiated in 2007 and continued through 2008 (Figure 1). BEAREX was conducted in the Texas High Plains, at Bushland near Amarillo.
The satellite portion of the program involves application of SEBAL, Metric, TSM, and SSEB to MODIS, LANDSAT, and ASTER data. The different satellites provide data at varying spatial (15 m - 1 km) and temporal (1-16 d) scales. Including all the different satellites will allow us to determine if a combination of data from different satellites can be used to provide ET at high spatial and temporal resolution. The inverse relationship between spatial and temporal resolutions of satellite data make it difficult to optimize for both space and time.
The airborne portion of the program includes multiple flyovers during the 2007 and 2008 crop growing seasons. These data are also being analyzed using the various models SEBAL, Metric, TSM, and SSEB and will provide ET estimates at 0.5 to 1.8 m resolution.
The ground-based program includes Large Aperture Scintillometers (3), Eddy Covariance stations (EC, 2), Bowen Ratio Energy Budget (BREB, 2) and weighing lysimeters (4). The LAS may prove to be the most appropriate technique to ground reference satellite based ET because it provides averaged ET data for up to km scales, which are appropriate scales for satellite data. In contrast to traditional micrometeorology approaches, such as BREB and EC, LAS systems are relatively easy to operate and data processing is not very time consuming.
The objectives of the experiment were to:
- Optimize ET estimates from satellite data by combining data from multiple satellites to enhance the spatial and temporal resolution of ET estimates
- Evaluate scaling issues related to ET estimates from satellites by estimating ET using airborne data (0.5 & 1 m resolution) and ground-based large aperture scintillometers (LAS)
- Ground reference the satellite-based ET estimates using large aperture scintillometers (Figure 2), weighing lysimeters, eddy covariance stations, and Bowen ratio stations. BEAREX includes MODIS, LANDSAT, and ASTER satellite data, airborne multispectral digital data (0.5 to 1.8 m resolution), 3 large aperture scintillometers, 4 weighing lysimeters, 2 eddy covariance stations and 2 Bowen Ratio stations.
The airborne data include multiple flyovers during the 2007 and 2008 crop growing seasons. The satellites provide temperature data at varying spatial resolutions (1 km MODIS, 30 m Landsat, and x m Aster) and temporal resolutions (daily MODIS, ~ bimonthly Landsat, and occasional ASTER). The inverse relationship between spatial and temporal resolutions makes it difficult to optimize ET estimates for both space and time.
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Figure 1. Schematic of the different components of the BEAREX program. |
This study attempts to provide optimal spatial and temporal AET estimates by combining data from multiple satellites.
Evaluating scaling issues is important for any satellite data. Airborne approaches provide temperature data at 0.5 to 1.8 m resolution at regional scales that can be compared with satellite based estimates. Although airborne data are expensive to obtain and data processing is very time consuming, they provide highly accurate information for validating satellite based estimates.
Ground-referencing satellite-based ET estimates is essential to provide confidence in the estimates and to increase the reliability of the estimates.
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| Figure 2. Large aperture scintillometer for monitoring ET installed in a crop field in the Texas High Plains. Photo shows composited image of transmitter and receiver, 500 m apart. |
Traditional micrometeorological approaches such as Eddy Covariance and Bowen Ratio systems have inherent limitations related to footprint area, energy closure problems (= 30%), and extremely time consuming data processing. Large aperture scintillometers can be used to estimate sensible heat flux and ultimately ET using the energy balance equation at spatial scales up to several km which are appropriate for ground referencing the satellite based estimates. The LAS systems also have the advantage that they are easy to operate and data processing is not time consuming. Weighing lysimeters provide the most reliable ET estimates on the ground at a point scale and are being compared with the LAS estimates.
Results from BEAREX will greatly improve our understanding of ET estimation using satellite data and appropriate ground referencing. The approach of combining satellite data and large aperture scintillometers may serve as a template for ET estimation in other regions.
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