Water storage is a broad term referring to storage of both potable water for consumption, and non potable water for use in agriculture. In both developing countries and some developed countries found in tropical climates, there is a need to store potable drinking water during the dry season. In agriculture water storage, water is stored for later use in natural water sources, such as groundwater aquifers, soil water, natural wetlands, and small artificial ponds, tanks and reservoirs behind major dams. Storing water invites a host of potential issues regardless of that waters intended purpose, including contamination through organic and inorganic means.
Groundwater is located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock become completely saturated with water is called the water table. There are two broad types of aquifers: An unconfined aquifer is where the surface is not restricted by impervious rocks, so the water table is at atmospheric pressure. In a confined aquifer, the upper surface of water is overlain by a layer of impervious rock, so the groundwater is stored under pressure.
Soil moisture is the water held between soil particles in the root zone (rhizosphere) of plants, generally in the top 200 cm of soil. Water storage in the soil profile is extremely important for agriculture, especially in locations that rely on rainfall for cultivating plants. For example, in Africa rain-fed agriculture accounts for 95% of farmed land.
Wetlands span the surface/sub-surface interface, storing water at various times as groundwater, soil moisture and surface water. They are vital ecosystems that support wildlife and perform valuable ecosystem services, such as flood protection and water cleansing. They also provide livelihoods for millions of people who live within and around them. For example, the Inner Niger River Delta in the Western Sahel zone supports more than a million people who make their living as fishermen, cattle breeders or farmers, using the annual rise and fall of the river waters and its floodplains.
Ponds and water tanks can be defined as community-built and household water stores, filled by rainwater, groundwater infiltration or surface runoff. They are usually open, and therefore exposed to high levels of evaporation. They can be a great help to farmers in helping them overcome dry spells. However, they can promote vector-borne diseases such as malaria or schistosomiasis.
In the past, large dams have often been the focus of water storage efforts. Many large dams and their reservoirs have brought significant social and economic benefits. For example, Egypt's Aswan High Dam, built in the 1960s, has protected the nation from drought and floods and supplies water used to irrigate some 15 million hectares. However, dams can also have great negative impacts. Because sediment is trapped by the Aswan High Dam, the Nile no longer delivers nutrients in large quantities to the floodplain. This has reduced soil fertility and increased the need for fertilizer.
Rainfed agriculture constitutes 80% of global agriculture. Many of the 852 million poor people in the world live in parts of Asia and Africa that depend on rainfall to cultivate food crops. As the global population swells, more food will be needed, but climate variability is likely to make farming more difficult. A range of water stores could help farmers overcome dry spells that would otherwise cause their crops to fail. Field studies have shown the effectiveness of small-scale water storage. For example, using small planting basins to 'harvest' water in Zimbabwe have been shown to boost maize yields, whether rainfall is abundant or scarce. In Niger, they have led to three or fourfold increases in millet yields.
As of 2010, it was reported that nearly half of the global population depends on in-home water storage due to a lack of adequate water supply networks. Many of the in-home solutions have improvised from available materials. It has been suggested that the lack of proper tools and equipment for construction, leads to a system more likely to contain breaches, making them more susceptible to contamination from the environment and users.