Remote sensing is a valuable tool to provide reconnaissance information on parameters of interest to hydrology, including water storage, evapotranspiration, river flows etc. Remote sensing is particularly useful in developing countries where access to field sites is difficult and available information from previous studies is limited. Ground referencing remote sensing is critical.
Our group focuses on use of the Gravity Recovery and Climate Experiment (GRACE) satellite data to monitor changes in water storage and we collaborate with Dr. Prasanna Gowda at USDA on remote sensing estimation of evapotranspiration.
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Figure 1. GRACE consists of 2 satellites, called Tom and Jerry, 220 km apart and about 500 km above the land surface. |
Water scarcity is a critical issue in semiarid regions; however, regional groundwater monitoring is extremely limited. This study evaluates the ability of the GRACE satellite to monitor groundwater storage in the semiarid High Plains aquifer, USA (450,000 km2 area), which is subjected to intense irrigation. The GRACE satellite was launched in 2002 and provides monthly data on gravity at ~ 200,000 km2 resolution. GRACE-derived terrestrial water storage (TWS) is highly correlated with the sum of soil moisture (SM) and groundwater storage (GWS) (r = 0.90 for in-situ measured SM [90 stations], r = 0.91 for simulated SM with a land surface model) in the High Plains. Correlation between GWS changes calculated as GRACE TWS-SM and measured GWS [~1000 wells] is also high (r = 0.74 for in-situ SM, r = 0.79 for simulated SM). Variability in SM is mostly restricted to the upper 2 m of the soil. Monitored SM compared favorably with simulated SM (r = 0.82). Study results show the potential for using GRACE gravity measurements to monitor TWS and GWS over large semiarid regions subjected to intense irrigation.
Current studies by Laurent Longuevergne using GRACE are focusing on application of GRACE data to assessment of the 10 d solution for GRACE and comparison with the monthly solution in the Baltic Sea area where dynamic changes are occurring, application of GRACE to monitor declines in groundwater storage in the North China Plain, and evaluation of assimilation of GRACE data to water balance models using data from the Amazon. These studies are funded by NASA with supplemental funding from the Jackson School.
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| Figure 2. High correlations between GRACE Terrestrial Water Storage (TWS) and (a) in situ monitored soil moisture + groundwater storage (r=0.96) and (b) simulated soil moisture + groundwater storage (r=0.95). (Strassberg et al., 2009) |
Evapotranspiration is one of the primary parameters in the water budget and is extremely useful for monitoring water consumption by different types of vegetation. Accurate information on water consumption can be used to manage water resources. Our group is collaborating with Dr. Prasanna Gowda and others at USDA on research on ET estimation from satellite data. Our focus is on ground referencing satellite estimates of actual ET using large aperture scintillometers (LAS). LAS systems are used to estimate sensible heat flux and by monitoring the other components of the energy budget (ground heat flux and net radiation) actual ET can be calculated. Distances between transmitter and receiver coils can be up to several meters. We have deployed x scintillometers in the Texas High Plains to ground reference satellite estimates.
Bushland Evapotranspiration and Agricultural Remote Sensing EXperiment (BEAREX)
The BEAREX program is the most comprehensive assessment of remote sensing based ET data and includes satellite, airborne, and ground-based measurements of ET. The program was conducted during the summer of 2007 and 2008 and involves a collaboration among scientists from Univ. of Texas, USDA, and Utah State Univ.
The satellite portion of the program involves application of SEBAL, Metric, TSM, and SSEB to MODIS and Landsat data. The different satellites provide data at varying spatial (30m - 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 airborne portion of the program includes 12 days of flights in 2007 and 11 days in 2008 during the 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 and 1 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.
To learn more about BEAREX, click here.
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Figure 3. Schematic of all instruments deployed under BEAREX 2007 and 2008 programs. |
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