appendix a: definitions of water resources
This appendix defines a series of basic concepts concerning water resources that have been used in this report. They have been mainly adapted from the publication "Introduction à l'Économie Générale de l'Eau" by Erhard-Cassegrain and Magret (1983).
1) Inflows, Outflows and Reserves
The inflows to a territory can come from two sources:
effective precipitation obtained by subtracting actual evapotranspiration from the total precipitation, which gives rise to rapid runoff in the rivers and recharge into the aquifers and
external contributions of water that may enter the system through the rivers or connected aquifers outside of the area.
The outflow of a territory is defined as the total yield and is the sum of the discharges flowing through the surface water courses and which leave the territory concerned and the groundwater flow that leaves a territory through existing aquifers.
When considering the total yield, in certain cases it is advisable to distinguish between the part that flows into the sea from the part exported to adjacent territories.
The reserves are the natural water volumes that exist in a particular territory on a specific date, and these include both surface volumes (rivers, lakes, snow, etc.), and groundwater (aquifers). The reserves vary in time, as a function of the differences between the territory inflows and outflows. The reserves that exist in a system throughout a period that is sufficiently long to be considered representative, are taken to be the average reserves. Such a period must fulfil the requirement that the average inflows and outflows are roughly the same, thereby ensuring a balance in the system.
The association between the flows and reserves that exist in a territory, allows the renewable and non-renewable water resources to be defined for that territory.
2) Renewable and non-renewable water resources
The renewable water resources in a territory are equivalent to the total yield. An evaluation thereof must make an explicit reference to whether or not the external contributions are included and it is advisable to provide a breakdown of the exports to adjacent territories and water that flows directly into the sea, especially in the case of international frontiers. In this case, the total yield for a territory minus the external contributions, is referred to as internal renewable water resources.
In physical terms, the non-renewable water resources, refer to the amount of water obtained by a decrement of the reserves in the surface or groundwater systems. Non-renewable resources are characterised by the fact that they can only be used once during the period considered.
It is customary to associate the water resources concept with the aforementioned renewable water resources concept, not considering the non-renewable resources because they are reserves, and thus the existence of a time limit for their use. In this sense, the presence of considerable non-renewable resources, would allow for greater flexibility in resource management, but would not increase such resources.
3) Potential and exploitable water resources
The potential water resources are that part of the water resources which constitute the potential offer of the territory under consideration, bearing in mind the restrictions existing in areas greater than the territory itself.
These restrictions, which are usually expressed in political terms, are:
Water Quality restrictions: keeping minimum quality standards in river reaches and aquifers.
Ecological restrictions: keeping minimum surface or groundwater flows to the sea, in order to maintain the rich and diverse biological habitats, to prevent marine intrusion, ensuring that aquifers do not fall below a minimum level, etc.
Geopolitical restrictions: the need to make sure that the flow does not fall below a minimum and has an acceptable quality when the water crosses a frontier.
The exploitable water resource is that part of the potential water resource that can be used under given technical and economic conditions, these being determined by the characteristics of the demand to be met. The relative nature of this definition is evident, given that the degree of technical and/or economic validity of certain systems of use, varies as a function of several factors: the technical and economic potential of the country, economic importance of the demand, etc.