Recent efforts have concentrated on developing models that couple the hydrologic cycle with climate because evaporated water (latent heat flux) from the land surface and oceans has a significant impact in regulating the energy balance of the atmosphere, which in turn feedbacks to drive atmospheric circulation and weather. The coupling of the hydrologic cycle and climate occurs at different time and spatial scales and is not well understood. For example, evaporation from the land-surface and atmospheric boundary layer development interact at time scales of hours whereas changes in root zone soil moisture and synoptic weather patterns occur over time scales of days to weeks. Moreover, it is difficult to validate model simulations at regional and certainly at the global circulation model (GCM) scale. On the other hand, it is imperative that we validate GCM output because unless these models can reliably simulate the observed water and energy cycles in the present climate, future predictions of climate change are rather tenuous (Kinter and Shukla, 1990).
To improve the parameterization of land-surface processes and atmospheric exchanges, the international community has coordinated multidisciplinary field experiments for collecting hydrologic, atmospheric and remote sensing data over a range of spatial and temporal scales. Efforts include the Hydrological Atmospheric Pilot Experiment (HAPEX) whose priority is in developing models that can link the hydrologic cycle to atmospheric processes at the GCM-scale and the International Satellite Land-Surface Climatology Project (ISLSCP) whose goal is in testing the utility of remote sensing data for determining land-surface parameters and fluxes (Shuttleworth, 1991a). The objectives of both of these programs are now combined under the Global Energy and Water Cycle Experiment (GEWEX), which aims to evaluate the global distribution of water and energy fluxes from observations. The GEWEX Continental-Scale International Project (GCIP) initiative will use the Mississippi River basin as its first major project, and will attempt to validate macroscale hydrologic-atmospheric models over a continental scale (Leese, 1993). Another program developed by the Commission of the European Communities and members of the European Research Community also serves to unite these objectives. The European International Project on Climatic and Hydrological Interactions between the Vegetation and the Atmosphere and Landsurface (ECHIVAL) plans to address research issues related to the interaction of vegetation-soil profile and overlying atmosphere at scales compatible with climate model grid domains, namely 104 km2.
These projects are indeed a major undertaking, especially given the wide range in spatial and temporal scales that hydrologic and atmospheric processes operate. For example, the variation in root zone and surface soil moisture has horizontal scales on the order of 0.1 km, but time scales of days. In contrast, the surface energy fluxes and interactions with atmospheric processes cover a range of spatial scales from 0.1 km to 10 km, but the times scales are on the order of half an hour. This means that to adequately observe both hydrological and atmospheric processes both long term (i.e., seasonal and yearly) and local to regional scale (i.e., 0.1 to 100 km) measurements of the energy and water budget are necessary.
This paper reviews some of the research results over the last several years coming from multidisciplinary experiments on measurement and modeling of the energy and water balance over large areas. In addition, several large scale field experiments recently completed or that are being conducted will be briefly discussed. This article is not intended to be an exhaustive overview of all the experiments that have taken place. The reader is referred to Shuttleworth (1991a; 1991b) who provides an overview of many of these experiments.