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Methodology and definitions used
Results
Breakdown of water resources by basic unit
Assessment of water resources can only be done at basin level. At country level it is possible to assess that part of the water resources which is generated inside the borders of the country. However, exchanges of water through international rivers represent a significant part of the water balance for several countries. In extreme cases, an arid country may depend almost entirely on water produced outside its borders. This explains the necessity to compute irrigation potential on the basis of river basins rather than countries.
In 1995 FAO conducted a review of the water resources of the African countries, considering internally as well as globally produced renewable water resources [21]. The survey was principally based on information produced by countries or regional and international organizations, completed with information gathered from previous studies. A summary of the review is given below.
Methodology and definitions used
Potential yield
Surface water and groundwater
Internally and globally produced renewable water resources
Periods of reference
Evaporation from wetlands and lakes
Potential yield is defined here as the global amount of water resources, be it surface water or groundwater, which is generated on a yearly basis in a given area.
The most widely used approach to computing water resources at national level is to study surface water and groundwater resources separately. One of the major risks in assessing them separately lies in the possible double counting of part of the resources. For example, in countries where part of the river flow is generated by discharges from the upper aquifers, mostly in humid areas, this figure includes a part of the water resources which can be considered as groundwater and could in fact be developed through wells. On the other hand, in arid areas, the river system usually acts as a preferential source for groundwater recharge and shows very limited base flow. The river runoff typically occurs in flash floods of high intensity and short duration. In this survey special care was taken with the computation methods, so that possible overlaps could be detected and removed from the accounting of water resources.
Internally and globally produced renewable water resources
When computing water resources on a country basis, it is important to make a distinction between internally and globally produced renewable water resources1. Internally produced renewable water resources (IRWR) refer to the water resources resulting from rain falling within the borders of the country and are a combination of surface water and groundwater resources. Globally produced renewable water resources (GRWR) are obtained by adding incoming surface water and groundwater flows to the internally produced renewable water resources.
1The term 'renewable' here is used as opposed to fossil waters, which have a negligible rate of recharge on the human scale and can thus be considered 'non-renewable'. Non-renewable resources are usually expressed either in terms of volumes or extractable flow, while renewable resources are always a measure of flow, usually presented on a yearly basis.
The internal water resources figures are the only quantities that can be added together for regional or continental assessment. The computation of global renewable water resources requires the assessment of surface water and groundwater flowing from neighbouring countries and between neighbouring countries (rivers that form the border between countries). Rules have been established for these computations, which are explained in detail in [21]. By definition, global water resources are not additive at the scale of international river basins. The definition implies that unused water, accounted for as a resource in upstream countries, is again counted as a resource in downstream countries.
The review concentrated on long-term averages and did not consider seasonal or inter-annual variations. However, it should be stressed that the review is based on information available from a multitude of sources and that no consistency in the choice of the period of reference can be expected. Some examples of rather important differences that might occur in average flow estimates, depending on the period of reference, are given in Chapter 6 for specific basins. Chapter 6 also gives more details on each of the 25 major basin groups, including information on their rivers, discharges and seasonal flow variations.
Evaporation from wetlands and lakes
In humid regions, the internally generated water resources of a country can be calculated by comparing incoming and outgoing flows and taking into account withdrawals inside the country. In arid regions, however, this method leads to important underestimates and even negative values for internally produced water resources. This situation occurs, for example, in Sudan, Mali and Botswana, where the quantity of water leaving the country is inferior to the quantity of water flowing into the country. In such countries losses by evaporation play a major role and a countrywide approach is not feasible. In arid regions, groundwater recharge also plays a major role in the assessment of water resources. In such situations, it is necessary to review in detail all possible sources of water and assess the quantity of water which would be available before being lost by evaporation.
In all climate situations it is difficult to account for evaporation from large lakes in the water balance of a country. This uncertainty may greatly impair the reliability of a country estimate. This typically the case of Lake Victoria for which no consistent water balance can be established.
In the present review, no systematic approach could be taken towards evaporation from lakes or other water bodies. Sometimes the resources were calculated without removing evaporation losses, as was the case for Mali, Uganda and Egypt1. For Sudan, evaporation in wetlands was subtracted from the total to obtain internal water resources.
1The external incoming water resources of Egypt are estimated at 65.5 km3/year. However, the evaporation from the Aswan reservoir, just downstream of the border with Sudan, is estimated at 10 km /year, so the flow at the outlet of the reservoir is in fact only 55.5 km3/year.
Table 6 and Figure 6 present the results of the review in terms of water resources by country. Surface water and groundwater resources have been presented in a non-additive way, that is to say that the base flow appears in both columns. This 'overlap' represents the part of water resources which is common to surface water and groundwater. The reason for presenting these figures in such a way is that this is how the water resources are usually presented in country studies and that there is no objective reason for subtracting the common part from one or other category.
The total for internally produced renewable water resources in column 4 is found by adding the surface water and groundwater resources and then subtracting the overlap (base flow) to avoid double counting.
Global renewable water resources are the sum of internal renewable water resources and incoming water. In an attempt to make a distinction between flow entering a country and border rivers, these two components have been presented in two separate columns (5 and 6).
In order to complete the picture on water resources, non-conventional sources of water, including potential development of fossil resources and desalination, have been added to the table. This may be of particular relevance to arid regions. However, it must be stressed that the figures in these columns are indicative and should be subjected to more detailed study. Moreover, technological advances can also lead to different estimates of the potential use of desalinated or fossil water.