EVALUATING THE SPATIAL DISTRIBUTION OF EVAPORATION

Authors: Moran, Mary; Kustas, William - Bill; NOROMAN, JOHN - UNIV OF WISCONSIN

Submitted to: Handbook of Weather Climate and Water
Publication Type: Book / Chapter
Publication Acceptance Date: 1/15/2000
Publication Date: N/A
Citation: N/A

Interpretive Summary: Evaporation of water from soil and plant surfaces influences the large- scale circulation of the planetary atmosphere and affects water use by agricultural crops and natural vegetation at the field level. The vast range encompassed by the process of evaporation and its role in weather, hydrology and plant growth make this phenomenon of vital environmental interest. There have been many excellent reviews of theoretical, modeling, and experimental approaches for evaluating evaporation from uniformly-vegetated regions. However, there have been few attempts to review the approaches for mapping evaporation from highly diverse landscapes at the global scale. This chapter presents a brief discussion of the physics of evaporation and the conventional methods of evaluating evaporation rates and then focuses on the use of satellite imagery for mapping regional and global evaporation rates. Several approaches were identified with high potential for operational implementation for agricultural and rangeland applications, and related to such implementation, a set of eight research and development issues were addressed. This synthesis should provide farmers, rangeland managers and research scientists with a focus for continuing research toward the ultimate goal of estimating regional daily evaporation rates with information from orbiting satellite sensors.

Technical Abstract: This chapter provides an overview of techniques for evaluating evaporation from local to regional scales. Remote sensing data are seen as having great potential for providing key surface boundary information in a spatially distributed manner and at regional scales, including surface radiation fluxes, vegetation cover, surface temperature, and surface moisture conditions. Many of the early approaches made no distinction between the vegetation and soil/substrate and used the instantaneous remotely sensed observations (primarily surface temperature) to directly compute instantaneous and daily evaporation rates. More recently, attempts have been made to develop physically-based models that explicitly treat the soil and vegetation interactions with the atmosphere. However, many of these approaches also require daily remotely sensed data as well as meteorological data for computing the surface fluxes over the course of a day. This has led to the development of soil-vegetation-atmosphere (SVAT) schemes that are linked to atmospheric models which in principle do not require remote sensing or meteorological inputs at every time step. These have the greatest potential for operational and regional applications. However, with optical data, atmospheric attenuation and clouds present difficult problems for frequent observations of the region of interest. In contrast, microwave observations have atmospheric and cloud penetration, high spatial resolution with radar, and day/night acquisitions. It has been shown that microwave observations combined with other remotely sensed observations (for example, two sensor view angles of the surface temperature) can reduce the need for simultaneous ground-based meteorological observations at each time step.