Water for people.pdf - WHO Thailand Digital Repository

T H E N A T U R A L W A T E R C Y C L E / 7 5exists and more are being developed including: rainfall-runoffmodels, aquifer models, ecosystem models and catchment models,many including monitoring of water quality. There are processmodels, hydroecological models and management models backed upby decision support systems and expert systems. There arestochastic and deterministic models with a complexity that rangesfrom simple lumped and black-box models to very sophisticatedphysically-based models with a high resolution of the land surface,including the surface-soil-vegetation-atmosphere interface and theprocesses operating there. Satellite data are being employed in anumber of different types of models and they are proving valuablein assessing the water quality of large basins.However, there are also studies which demonstrate that thesophistication and likeness to reality of a model are no guide to itspredictive success (Naef, 1981). The additional problem of scale existswhen the results of a limited experiment carried out over distances oftens of metres have to be extrapolated to kilometres by modelling.Scale is also a problem that has to be addressed when different typesof models are to be coupled, meteorological and hydrological modelsfor example, but the increase in computing power is one of the factorseasing this difficulty. Some of these techniques have been employedto estimate water resources on a global or continental scale grid,producing maps showing variations with time (McKinney et al., 1998).Improving knowledge of hydrological processes is essential tothe understanding that permits water resources to be safeguardedand managed. The physical processes operating at the surface ofthe ground where the atmosphere, soil and vegetation meet areimportant to runoff generation and infiltration and also to theclimate models developed for atmospheric studies, such as for workon climate change. Likewise, studies of the interaction of water withthe biotic environment are necessary for a number of practicalapplications, the control of algal blooms and the maintenance offish stocks, for example. Hydrological processes operating in bodiesof surface water are a major factor in the complex and seasonallydynamic groundwater flow fields associated with them;consequently the representation of these processes has to beadequately captured in models which aim to portray these systems.Global Hydrology and Water ResourcesVariability over a wide range of scales in space and time is the mostobvious feature of the global pattern of the hydrological cycle and ofits component parts which determine water resources (see map 4.1).Map 4.1: The long-term average water resources according to drainage basins(in mm/year)0 10 50 100 200 300 500 1,000 [max 6,160]The long-term average of water resources by drainage basin is used as an indicator of water available to the populations in the basin. The use of the drainage basin as thebasic unit sharpens the contrast between adjacent water-rich and water-poor countries, compared to map 4.4, based on a grid scale.Source: Map prepared for the World Water Assessment Programme (WWAP) by the Centre for Environmental Research, University of Kassel, based on Water Gap Version 2.1.D, 2002.