Introduction

Although it is widely recognised that many aspects of land-atmosphere interactions greatly affect the forecasting capabilities of both general and regional circulation models, comprehension of the landscape patchiness of

Remote Sensing and Climate Modeling: Synergies and Limitations, 307-327. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.

water and heat exchange processes at the land surface is limited by the inadequate amount of data generally available in rugged mountainous terrain. The land surface characteristics in Alpine regions present complex interaction and high variability on short intervals of space and time, giving rise to difficult problems in the modelling ofrelated processes.

According to the International Convention on the Protection of the Alps (CIPRA, 1991), the Alpine environment is one of the most sensitive European terrestrial ecosystems. Aggressive development of tourism, long-term effects arising from changes in climate and loss of a protective forest layer could lead to an increase in the frequency and severity ofnatural disasters. Under such conditions the Convention demands immediate comprehensive counter-measures.

The processes which require more thorogh invesitgation within Alpine environments, at different spatial and temporal scales, have been identified as a result of the experience gained over the last few decades in modelling the evolution of the geosphere/biosphere system and its links to climate (Rott andRast, 1999).

Monitoring of complex areas by remotely-sensed imagery and ground data may provide an important contribution to develop and test new techniques for land surface parameterisations, for the evaluation of soil-vegetation-atmosphere-transfer (SVAT) schemes, and for biogeochemical (BGC) remote sensing (RS) driven model input (Olioso et al., 1999; Running et al., 1999). Many biophysical and biochemical parameters can be realistically derived from remotely-sensed data and employed as input in distributed SVAT and BGC models for determining the energy balance components and estimating the actual evapotranspiration.

Evapotranspiration is generally estimated by conventional ground-based methods, such as the Bowen ratio coupled with a net radiometer (Spittelehouse and Blac, 1980) or eddy covariance technique measurements (Baldocchi et al., 1988). However, although these techniques do provide accurate measurements over an homogeneous area surrounding the meteorological station, the results are not directly applicable to large areas or to natural heterogeneous surfaces. Satellite RS can offer the way for deriving biophysical and biochemical parameters without the loss of accuracy associated with spatial interpolation techniques among point measurements.

Several researchers have applied RS data for estimating evapotranspiration by using different modelling techniques (Moran et al., 1989; Goodin, 1995; Pegrum and Bastiaanssen, 1996; Kustas et al., 1989; Anderson et al., 1996; Hall et al., 1991), with successful results over semiarid rangeland basins (Chehbouni et al., 1997; Hurtado et al., 1997; Kustas et al.1994), and over nearly full agricultural canopy covers (Hurtado et al., 1994). However, few experiments have been made in highly forested complex terrain (Kaneko and Hino, 1996; Vidal et al., 1994; Wigmosta et al. 1994) and in heterogeneous Alpine environments (Kaimal and Finnigan, 1994; Menzel and Lang, 1998).

In this study, one such experiment conducted in a mountainous forested environment is presented, and related problems are analysed. Instantaneous latent heat fluxes were mapped for all vegetated surfaces of the Valmasino catchment in the Italian Alps by integrating RS data and ground measurements in a single-layer resistance model.

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