Variability in Freshwater Availability

Although available freshwater constitutes only a minute proportion of the total water on the planet, there is still much more renewable freshwater (i.e. precipitation falling on land as a global annual average) that would be needed to support human needs. The spatial variation of annual average precipitation, however, ranges from <100 to >2000 mm.

In Figure 1, this variability is shown on a map that is faithful to land surface area (unlike the more commonly-used Mercator projection). The Eckert projection in Figure 1 emphasizes the huge landmass of the African continent, as well as the extreme variability in the annual average precipitation that it receives. Even in regions where the local precipitation is not dramatically different from the world average, strong temporal variability in rainfall can lead to extended dry periods.

Figure 1. Mean annual precipitation (1981-2010) shown in a projection (Eckert IV) that accurately reflects land surface area. Source: GPCC Full Data Reanalysis Version 6.0 at 0.5o, monthly land-surface precipitation from rain-gauges built on GTS-based and historical data

Rivers naturally supply lowland areas in catchments with water derived from precipitation or from the melting of snow and ice in upland areas. Withdrawal from lakes or groundwater can also offset deficiencies in water availability associated with variability in precipitation. Both natural conveyance and storage, as the end results of processes that occurred over geologic time, are spatially differentiated.

The focus on storage and conveyance throughout history reflects the vulnerability of societies to the spatial and temporal variability of precipitation and of natural storage and conveyance. Artificial storage (reservoirs) and conveyance (aqueducts) date back to antiquity in Egypt, Persia, and the Roman Empire.

The reduction of societal risks by water infrastructure motivated past investments made in industrialized countries and supports arguments for increased infrastructure investment in low- and middle-income countries (LMICs). The medium term needs for investment for water infrastructure (which can be exacerbated by a culture of poor maintenance) are estimated at more than $1.2 trillion annually.

Conflicting Uses and Competing Demands. The societal benefits of infrastructure projects have also been accompanied by harm, most often to the environment but also to people, especially those who have been relocated. Many of the environmental impacts of past infrastructure projects (e.g. loss of biodiversity) are no longer considered acceptable in industrialized countries, which have consequently invested in restoration and expressed objections to infrastructure projects in LMICs.

These objections are reinforced by concerns that climate change introduces unacceptable levels of uncertainty into infrastructure planning and might compromise the expected benefits of infrastructure investment.

Direct human uses of water often involve the extraction or diversion of (blue) water from the environment. In addition, societies are dependent on ecosystem services provided by the aquatic environment, including the support of terrestrial ecosystems and rainfed agriculture (green water).

These uses and services are illustrated schematically in Figure 2, which highlights that blue and green water are intimately linked and that good ecosystem function is vital for both. The use of water for dilution or transport of waste is sometimes referred to as gray water, though this can be difficult to quantify since it depends on the standards applied for acceptable water quality.

Figure 2. Direct human uses (blue) and indirect ecosystem services (green) provided by the water environment

It is undeniable that direct human uses of water can be incompatible with each other or with the maintenance of ecosystem function and the continued provision of ecosystem services. Perhaps the most dramatic example of such conflicts resulted from the diversion of flows to the Aral Sea for cotton irrigation. Over four decades, the volume of the Aral Sea decreased by >80% and nearly all its native species were extirpated.

The management of transboundary waters must explicitly address the competing interests of different national (or sometimes sub-national) parties. Diversion of surface flows (or groundwater) by upstream (or upgradient) users can have obvious negative impacts on downstream (or downgradient) users. Despite such conflicting interests, transboundary water resources have generally inspired more cooperation than open conflict.

The United Nations Convention of the Protection and Use of Transboundary Watercourses and International Lakes was adopted in 1992 by European countries and was amended in 2003 to allow adoption by all UN member countries. The Water Convention, which addresses water pollution, ecologically sound water management, equitable use of transboundary waters, and ecosystem conservation and restoration serves as the basis for the concept of “hydro-diplomacy" or “blue peace".

An additional source of international competition arises from the transfer of virtual water in international trade (especially of food). Nearly 70% of the use of blue water is associated with irrigated agriculture and some countries (particularly in the Middle East) rely on food importation to offset deficiencies in local water availability.

Export-oriented agricultural production may, however, come at the expense of land use for local needs. Some water-sufficient countries (e.g. Switzerland) are heavily dependent on imported food though the biggest environmental impacts abroad are, for Switzerland, associated with imports of cocoa, coffee, and palm oil for food processing.

Further development of large-scale water infrastructure (i.e. for storage and conveyance) in LMICs must balance needs for improved water security with impacts on ecosystems, particularly in the context of climate change. It is, however, neither ethical nor realistic to expect that LMICs will forego the improvement in water (and associated energy) security that infrastructure can provide and from which societies in industrialized countries have long benefited. A substantial expansion in hydropower production is anticipated from the planned or current construction of over 3700 major dams having capacities of at least 1 MW, most of which will be sited in LMICs.

The multiple uses of water and multiple demands to which aquatic ecosystems are subject clearly pose challenges. This has led to polarization that tends to obscure possible common interests and/or opportunities for collaborative solutions. Often such polarization is embedded in laws, regulations, and administrative structures, which obscure the linkages between uses of waters in different sectors.

Table 1. Dichotomies in perception and implementation that impede sustainable water management

Some perceived dichotomies that arise in the management of water supply, water resources, and aquatic ecosystems are listed in Table 1. Some of these perceived dichotomies (e.g. between surface and groundwater) clearly defy physical reality and all of them stand in the way of an integrated approach to SWM.