Water storage

Types

Groundwater

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.

Aquifers receive water through two ways, one from precipitation that flows through the unsaturated zone of the soil profile, and two from lakes and rivers. When a water table reaches capacity, or all soil is completely saturated, the water table meets the surface of the ground where water discharges in the form of springs or seeps.

It is also possible to artificially recharge aquifers (using wells), for example through the use of Aquifer storage and recovery (ASR).

Soil moisture

Groundwater is stored in two zones, one being the saturated zone, or Aquifer, the other is the pore space of unsaturated soil immediately below the ground surface. 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

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.

Wetlands are basically sponges that capture and slowly release large amounts of rain, snowmelt, groundwater and floodwater. Trees and other wetlands vegetation slow the speed of flood water and more evenly distribute it across the wetland. The combination of increased water storage and flood water hindrances lower flood heights and reduce erosion.

Ponds and tanks

Detention basins 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.

Detention basins are designed for temporary capture of flood waters and do not allow for permanent pooling of water and therefore do not make viable or reliable sources of water storage. Retention basins are similar to detention basins for flood control management, but are built for permanent pooling to control sediment and pollutants in the flood water.

Dams and reservoirs

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. Water stored in dams and reservoirs can be treated for drinking water, but in the past due to poor taxation and high water prices in the US, water supply dams are unable to reach their intended levels of operation. Due to the increased surface area of water that dams create, huge amounts of water is lost to evaporation, much more so than what would have been lost from the river that flowed in its place.