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Highlights
Hydrology
Venkat Lakshmi

Land surface hydrology is an accounting of water received, water lost, and the gain in water storage, which is measured by the rise in level of inland water bodies, increase in soil moisture, and water table rise. Water is received by the land through precipitation and lost through evaporation and runoff from land to streams and oceans. This accounting is based on a simple water balance principle: conservation of mass. However, as evapotranspiration-the loss of water due to evaporation from the land surface and loss of water from the plant stomata due to transpiration-involves energy, the hydrological budget involves water and energy balance.

Quantification of the hydrological budget is extremely difficult over large spatial domains and long periods. Direct observations as in-situ observations are labor intensive as well as expensive. Satellite remote sensing provides an easy way to overcome these issues with broad spatial coverage and repeat temporal coverage.

A wealth of satellite data at various spatial scales and different temporal resolutions can be used to put together a complete picture of the land surface hydrological cycle. Leaf area index, soil moisture, surface temperature, surface air temperature, and precipitation are measured by a number of sensors at various spatial and temporal resolutions. The figure illustrates both the wealth of data and the dilemma facing its users. Exact estimation of water available to the user community is the challenge faced by our society and this can be answered by combining hydrological modeling and remote sensing. Reconciling these issues and using these data in the most synergistic methodology possible remain challenges for the scientific community.

Land surface hydrology can be represented by a simple two-layer model. More layers can be used for more a more complete profile of the soil moisture.

Vegetation is characterized using the Normalized Difference Vegetation Index (NDVI), a ratio of the difference and sum of the visible (0.55-0.68 m) and near-infrared (0.725-1.10 m) radiances measured by the Advanced Very High Resolution Radiometer (AVHRR). Currently, the vegetation index is measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on the TERRA and AQUA. The terra satellite has instruments that characterize the land surface and AQUA measures the water in the land/atmosphere. The NDVI can be converted into leaf area index (LAI) using Beer's Law for exponential decay. MODIS has many more channels and much higher spectral and spatial resolution than AVHRR (Justice et al., Remote Sensing of the Environment, v. 83 (1-2), p. 3). Vegetation plays a role in land surface hydrology indirectly by providing a shadow effect on the ground for direct solar radiation, by intercepting precipitation, and through transpiration loss. Vegetation also changes the roughness and aerodynamic resistance of the surface for latent heat fluxes and those that can be sensed.

In the case of soil moisture, microwave frequencies respond best to variations in moisture content due to the polar nature of the water molecule. With the launch of the AQUA satellite in June 2002 and ADEOS II in December 2002, data from the Advanced Microwave Scanning Radiometer (AMSR) are becoming available at two equatorial overpass times per day (Njoku et al., IEEE Transactions on Geoscience and Remote Sensing, v. 41(2), p. 215). The C-band of the AMSR (6.9 GHz) has better sensitivity than the 19.4-GHz channels of the Special Sensor Microwave Imager (SSM/I) for retrieving soil moisture. In the past, soil moisture has been measured from space using the SSM/I with the 19-, 37-, and 85-GHz channels. This sensitivity is highest at lower frequencies (L-band: 1.4 GHz and C-band: 6.9 GHz) and decreases as the frequency of observation increases due to increased contribution from the atmosphere and vegetation.

Surface temperature has been derived using the AVHRR, the Television InfraRed Observation Satellite (TIROS) Operational Vertical Sounder (TOVS), Advanced Infra Red Sounder (AIRS) (Susskind et al., IEEE Transactions on Geoscience and Remote Sensing, v. 41(2), p. 390), Moderate Resolution Imaging Spectroradiometer (MODIS) (Justice et al., Remote Sensing of the Environment, v. 83 (1-2), p. 3), and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). The energy emitted in the thermal channel, which is observed by these sensors, is directly related to the surface temperature.

Surface air temperature can be derived from the TOVS and AVHRR. More recent data are available from AIRS (Susskind et al., IEEE Transactions on Geoscience and Remote Sensing, v. 41(2), p. 390) as a Level-2 core product.

Precipitation is measured in the microwave region by the SSM/I, AMSR (Wilheit et al., IEEE Transactions on Geoscience and Remote Sensing, v. 41(2), p. 204), and Tropical Rainfall Measuring Mission Microwave Imager. The microwave frequency responds to the falling hydrometeors, and this response can be translated into a rainfall rate.

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Lakshmi is an associate professor in the Department of Geological Sciences at the University of South Carolina, Columbia. E-mail: vlakshmi@geol.sc.edu.

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