Indices of water availability assessment
Introduction: Water demand has a continued growth, but not its availability. To achieve a regional balanced development of the activities that demand water as a basic input, it is necessary that water demands are in agreement with water availability. To balance both components is not always an easy task, because of the multiple water uses (agriculture, domestic use, aquaculture, industry, energy generation, etc.), and because frequently the users do not have a registration or official assignment with the correct physical location and the volumes to use; on the other hand, the offer is also frequently only supposed, since measurements are not made, neither controlled due to, among other factors, the lack of inadequacy of the hydrometric network, the lack of economic resources to assist this activity, and the rural and isolated control points, etc.
Nevertheless, the environmental sustainability, considering the water as the main axis, depends on the knowledge about the certainty of the availability of the resource, as well as its demand, in order to achieve an exploitation and rational use without causing unnecessary water stress and without committing the stability and future development.
Technical and institutional basis: Estimating the available water volumes in Mexico on an annual basis is basedon an official standard, relative to the water conservation, whichestablishes the specifications and the methodology for determining the annualaverage availability of the national waters on a hydrological basin as a unit ofanalysis.Briefly, the method is based in estimating the balance between demand andsupply, when both components are calculated separately, with the considerationsfor each case. For the supply, available hydrometric information is used,registered as monthly runoff volumes measured in specific places, usually at thebasin or sub basin exit, where there is a reservoir, a diversion dam or ahydrometric station. To these volumes are added the upstream used volumeswithin the basin, to obtain a total runoff volume. On the other hand, the demandis considered from the last or the lowest delivery point toward upstream, in orderto accumulate the partial volumes and then to obtain the whole demand to thehydrological system.
Reserved volumes estimation within the sub basin: The downstream reserved volume, RXY, for a given sub basin X, is the fraction ofthe runoff that comes out at the end of the area, such that it contributes to satisfythe extractions and demands of the next downstream sub basin Y; and reservedvolume for sub basin X, RXX, is that which contributes to the satisfaction ofdemands inside the same sub basin X. The demand estimation is carried out fromdownstream toward upstream.
Available volumes in each sub basin: Water volumes available at the end of a sub basin (DXY), can be estimated as thedifference of ABi, the runoff downstream, minus the reserved volumes, RXY, thosevolumes which sub basin X contributes to satisfy the demands of sub basin Y.This way, the available volumes from sub basin X to sub basin Y are:
DXY= ABX – RXY
Similarly, the remainder water volumes, available for sub basin X itself are:
In order to classify the sub basins according to water available, the term relative availability coefficient (Dri) is used, which is expressed by the equation:
Application case: The case here is presented is related to the San Pedro River Basin, located in theCenter-West of Mexico, that includes approximately 28,563 km2, andis formed by 11 well defined sub basins, two of which are closed, with noconnection or exit, and the other nine are connected to each other downstream,having a final point at the discharge to the Pacific Ocean.
Although a defined hydrographic network exists, which includes main and tributaries streams in all the sub basins that discharge to San Pedro River, which finally ends in the sea, the hydrometric information is not as complete as would be desirable, neither has it the record longitude and homogeneity since the dynamics of the basin has changed now that reservoirs, diversion works, and other works have been built which alter the old measurements sites, or they have disappeared or been cancelled, which all results in that the data cannot be used efficiently as a direct information source.
In those cases, it is necessary to use the indirect method to estimate the surface water contributions; it consists basically on transporting the runoff coefficients, CE, from physiographical neighbor and similar sub basins, which have enough hydrometric records.