Water Supply Sustainability Revisited: Assessment Methodology for Multiple Water Resources

The adequacy of existing water supply depends on a number of factors. These factors include water demand and its variations over time, the amount of water supplied on average, and the nature and types of the water supply alternatives available. A significant question with implications for all of these is whether the existing supply is sustainable in the long term. Answering this question becomes more important when demands are increasing to cater to rising human and animal populations and supply sources are changing due to increasing urbanization and a changing climate. This study presents a new basis for assessing water supply sustainability for regions that depend on multiple sources of supply. This new basis is defined using a multivariate water supply index attempting to join the various sources of supply into one. Water demand is ascertained not based on human needs but the water needs of growing vegetation. Use is made of traditional methods for assessing the adequacy of existing water storage using this multivariate supply and the associated demand, with the added consideration of any systematic decrease in the water supply that may be observed over time. This study uses a partly engineered semiarid basin to illustrate the proposed methodology. Upstream subbasins in this basin are anthropogenically unaffected and have natural vegetation demand and supply dependent on a single source (precipitation). Downstream subbasins, though, have demand dependent on the agricultural needs of the region and supply that is both natural and engineered through diversions from elsewhere. The water supply sustainability of the basin is shown to be dependent on the nature of demand and supply in each of the subbasins in focus. In general, the upstream basins are found to be sustainable according to the proposed index, whereas downstream subbasins are markedly unsustainable, requiring the introduction of a new water supply source or a reduction in the water demand for the region. The proposed method requires knowledge of all water sources applicable in a region, which include naturally occurring and engineered sources (such as diversions or groundwater pumping), while relying on remotely sensed vegetation estimates alone to quantify the applicable demand. As a result, it is possible to use this approach in most regions of the world that use water from a combination of naturally occurring and engineered water supply options and where water use is reflected through the vegetation sustained.