The concept of water resources is multidimensional. It is not limited only to its physical measure (hydrological and hydrogeological), the ‘flows and stocks’, but encompasses other more qualitative, environmental and socio-economic dimensions. However, this report focuses on the physical and quantitative assessment of the resource.
This report focuses on renewable water resources. These are defined as the average manual flow of rivers and recharge of aquifers generated from precipitation. It distinguishes between the natural situation (natural renewable resources), which corresponds to a situation without human influence, and the current or actual situation (Figure 1). The computation of the actual renewable water resources of a country takes account of possible reductions in flow resulting from the abstraction of water in upstream countries. The study also reviews available information on exploitable water resources, i.e. the part of the actual water resources available for beneficial use.
The focus is on freshwater resources. The data collected do not distinguish between different water qualities. Brackish, saline and non-conventional water sources are not accounted for.
In computing water resources on a country basis, a distinction is to be made between renewable and non-renewable water resources.
Renewable water resources are computed on the basis of the water cycle. In this report, they represent the long-term average annual flow of rivers (surface water) and groundwater.
Non-renewable water resources are groundwater bodies (deep aquifers) that have a negligible rate of recharge on the human time-scale and thus can be considered non-renewable.
Natural renewable water resources are the total amount of a country’s water resources (internal and external resources), both surface water and groundwater, which is generated through the hydrological cycle. The amount is computed on a yearly basis.
This report also considers actual renewable water resources. These are defined as the sum of internal renewable resources (IRWR) and external renewable resources (ERWR), taking into consideration the quantity of flow reserved to upstream and downstream countries through formal or informal agreements or treaties and possible reduction of external flow due to upstream water abstraction. Unlike natural renewable water resources, actual renewable water resources vary with time and consumption patterns and, therefore, must be associated to a specific year.
FIGURE 1: From hydrological concepts to water resources concepts
Notes:
(1) Actual resources: when taking into account the resources shared with neighbouring countries (geopolitical constraints)
(2) Exploitable resources: according to socio-economic and environmental criteria.
(3) A country may have to reserve for downstream a part of its external resources (e.g. Sudan and Syria).
NB: Where one takes into account both the geopolitical constraints and the socio-economic and environmental criteria, one should speak of ‘actual and manageable water resources’.
Source: FAO/BRGM, 1996.
Not all natural freshwater, surface water or groundwater, is accessible for use. In this report, exploitable water resources (manageable water resources or water development potential) considers factors such as: the economic and environmental feasibility of storing floodwater behind dams or extracting groundwater; the physical possibility of catching water which naturally flows out to the sea; and the minimum flow requirements for navigation, environmental services, aquatic life, etc. (Figure 2).
FIGURE 2: Water: natural resources and technically exploitable resources
Notes:
1. According to the sum of the capacities of the reservoirs and the variability of the inflow.
2. Flow evaporated from reservoirs and dams (according to their surface).
3. According to the variability of the inflow.
4. According to the intake capacity of the aquifers.
* Reserved flow not included.
Source: FAO/BRGM, 1996.
This concept varies according to:
natural conditions that may affect the development of water resources (regularity of the water regime, fragmentation of the hydrographic or hydrogeological systems, convenience of the sites for dams, and water quality);
the importance of demand for water, which will determine the acceptability of internal and external costs of water development and management; it also involves arbitration for allocation between in situ use (reservation) and ex situ use or abstraction.
As it depends on the choice of a set of criteria (physical, socio-economic, environmental, etc.), this concept varies from country to country. It can also evolve according to demand pressures. However, it represents a realistic vision of the renewable resources available for use in a given situation and period.
In general, exploitable resources are significantly smaller than the natural resources (Box 1). Preferably, national data on exploitable water resources should be completed by an indication of the set of criteria considered.
|
BOX 1 - EXPLOITABLE WATER RESOURCES IN LEBANON The case of Lebanon illustrates the difference between water resources and exploitable water resources. A large part of the country’s water resources is hardly exploitable. The groundwater losses to the sea that are accounted for in the assessment of potential yield (about 0.7 km3/year) come out as submarine springs. These resources are difficult to mobilize as the karstic channels in which the water flows are subject to mixing with saline water. Similarly, the floodwater running from the small watersheds of the coastal mountains is lost to the sea with little possibility of putting it to beneficial use. Thus, out of total water resources estimated at 4.8 km3/year, exploitable water resources represent about 2.2-2.5 km3/year. |
Internal renewable water resources (IRWR) is that part of the water resources (surface water and groundwater) generated from endogenous precipitation (Figure 3). The IRWR figures are the only water resources figures that can be added up for regional assessment and they are used for this purpose.
FIGURE 3: Surface water and groundwater concepts
Source: FAO/BRGM, 1996
Although the hydrological cycle links all waters, surface water and groundwater are usually studied separately and represent different development opportunities. Surface water is the water of rivers and lakes; groundwater is the water captured in underground reservoirs.
Surface water flows can contribute to groundwater replenishment through seepage in the river bed. Aquifers can discharge into rivers and contribute their base flow, the sole source of river flow during dry periods. Therefore, the respective flows of both systems are not wholly additive. This report uses the concept of overlap to define the part of the country’s water resources that is common to rivers and to aquifers.
This study defines external water resources as the part of a country’s renewable water resources that enter from upstream countries through rivers (external surface water) or aquifers (external groundwater resources). The total external resources are the inflow from neighbouring countries (transboundary flow) and a part of the resources of shared lakes or border rivers, defined for the purposes of this study through an arbitrary rule (unless defined by an agreement or treaty).
Most of the inflow consists of river runoff, but it can also consist of groundwater transfer between countries (e.g. between Belgium and France, Bulgaria and Romania, or Sudan and Egypt). However, groundwater transfers are rarely known and their assessment requires a good knowledge of the piezometry of the aquifers at the border. In arid areas, they may be important in comparison with surface flow.
In assessing the external flow of a country, this report distinguishes between natural incoming flow and actual incoming flow.
Natural inflow is the average annual amount of water that would flow into the country in natural conditions, i.e. without human influence.
Actual inflow is the average annual quantity of water entering the country, taking into account that part of the flow which is secured through treaties or agreements and possible water abstraction in upstream countries.
Outflow is the flow of water leaving a country to the sea or to neighbouring countries. Part of the outflow to neighbouring countries may be subject to reservation where a treaty or an agreement allocates a certain flow to a downstream country. This is reflected in the computation of actual water resources by subtracting the allocation for the country’s water resources.
Differences in water quality may be significant locally but are difficult to aggregate in a meaningful way at national level. In addition, water quality must be expressed not only in terms of physical, biological and chemical variables, but also according to quality standards that vary according to use. Therefore, the evaluation of water quality requires the use of a water quality grid, defining quality classes according to several criteria and variables.
This report considers only inland freshwater and does not distinguish categories according to water quality levels, owing to the unavailability of reliable information. Therefore, it assumes that the water resources it computes are of sufficient quality for beneficial use for agricultural, domestic or industrial purposes.
In countries where freshwater resources are scarce and under pressure, the assessment of freshwater resources is often complemented by an assessment of the available brackish water that can be used for specific purposes (desalination, some types of agricultural production).
With increasing pressure on natural freshwater in parts of the world, other sources of water are growing in importance. These non-conventional sources of water represent complementary supply sources that may be substantial in regions affected by extreme scarcity of renewable water resources. Such sources are accounted for separately from natural renewable water resources. They include:
the production of freshwater by desalination of brackish or saltwater (mostly for domestic purposes);
the reuse of urban or industrial wastewaters (with or without treatment), which increases the overall efficiency of use of water (extracted from primary sources), mostly in agriculture, but increasingly in industrial and domestic sectors. This category also includes agricultural drainage water.
This study does not consider non-conventional water sources in the computation of freshwater resources. Where available, the tables give an indication of the water produced by desalination (FAO, 1997b; Gleick, 2000). Moreover, this report concentrates on ‘blue water’ and does not consider what is often termed ‘green water’ (Box 2).
|
BOX 2 - BLUE WATER AND GREEN WATER Although this study does not address these concepts, it is useful to clarify them in order to avoid misinterpreting some of the information presented in the study. Rainfall may either flow on the surface or underground. It may finally reach the sea or it may return to the atmosphere, evaporated or consumed by the vegetation (two universal paths of the water cycle). In general, only the first type is considered ‘water resources’ offered by nature to humans. This is especially the point of view of hydrologists, who measure or assess them, and of developers. They consider evaporation as ‘losses’ (there are only lost for runoff). The use of words such as efficient rainfall or useful rainfall are significant in this regard. However, from an ecological point of view, it is excessive to judge such ‘water resources’ as useless because they maintain soil moisture and nourish natural and cultivated vegetation in rainfed systems. Both hydrologists and agronomists distinguish two types of water: blue water and green water. They cannot be summed but contribute to the water potentialities of a country:
In theory, green water can be assessed as it corresponds (as a maximum) to the volume of the actual evapotranspiration or to the runoff deficits of each catchment. However, such a global calculation is meaningless for local flows that cannot be aggregated by groups of surface units as can be done for blue water. Comparing the difference between the green water and the theoretical needs of the crops is an average indicator for the irrigation needs. |
){kind=link}
){kind=link}