Water storage is a term used within agriculture to define locations where water is stored for later use. These range from natural water stores, such as groundwater aquifers, soil water and natural wetlands to small artificial ponds, tanks and reservoirs behind major dams.
Types of water storage
Groundwater is water 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: confined and unconfined. 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 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 ay 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 waters in the river and its floodplains.
Ponds and tanks
These can be defined as community-built or 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, the can promote vector-borne diseases such as malaria or schistosomiasis.
Dams and reservoirs
In the past, large dams have often been the focus of water storage efforts. Many large dams 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, nutrients are no longer delivered in large quantities to the Nile floodplain. This has reduced soil fertility and increased the need for fertilizer.
Water storage: a weapon against climate variability
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 successful farming more difficult. Providing farmers with access to a range of water stores could help them 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. And in Niger, they have led to three or fourfold increases in millet yields.
- Johnson, R. and McCartney, M. 2010 Inventory of water storage types in the Blue Nile and Volta River Basins, Working Paper 140, [IWMI]
- Finding ways to boost productivity and reduce poverty through better water management in Africa Success Stories, Issue 4, 2010, IWMI.
- The Wetlands and Poverty Reduction Project, Wetlands International
- McCartney, M. and Smakhtin, V. Blue Paper. Water storage in an era of climate change: Addressing the challenge of increasing rainfall variability, IWMI
- Diverse water sources key to food security: report, Reuters, September 5, 2010