Distributed variability of Earth's water resources

The basic measurements:
assessment methodologies

The quantitative characteristics of renewable water resources of a region or river basin can be determined by two approaches: by using meteorological data or by using river runoff observations.

The first approach is widely applied where there are insufficient hydrological observations but where the meteorological data is both significantly detailed and comprehensive. In practice, it is usually achieved by using the simplest equation for the long-term water balance of a territory. According to this equation, the size or extent of renewable water resources, averaged over a long period of time, is determined by the difference between precipitation and evaporation from land. Precipitation is calculated from observational data and evaporation by various estimation formulae. This estimation technique is quite simple and has been widely used in many countries since the beginning of this century when the hydrological network was insufficiently developed and meteorological data were much more widely available. The technique is still applied quite frequently; for instance, an estimation of water resources for the Latin-American countries was made in this way.

Estimating renewable water resources by meteorological data, though very simple, has a number of serious disadvantages.

It cannot be recommended for use in detailed calculations, especially in countries and regions with limited water resources. First, because of low accuracy, this technique is not applicable to arid and semi-arid regions where river runoff is very small since absolute values will be close to any errors in the determination of evaporation and precipitation. Second, it is impossible to estimate water resources reliably by this method for individual years or, moreover, for a specific season or month. Third, this technique is inapplicable for water resources estimation in countries and regions located in international river basis because a larger volume of river runoff comes from outside than is generated within the territory in question.

In this connection, the second approach has been applied to estimate global water resources for continents, regions and even countries located within various different physiographic conditions. This is based on observations from the world's hydrological network, and the meteorological information serves a subsidiary role. The approach was successfully applied as early as the 1960-70s in the preparation of the monograph World Water Balance and Water Resources of the Earth referred to earlier. By now, an even longer series of hydrological observations has been collected and it was possible to obtain data, previously inaccessible, for many regions in Africa, Asia and Latin America poorly covered with hydrological information. There is, therefore, considerably more justification to apply this latter approach.