Water harvesting is when rainwater or stormwater is collected and stored for productive use later. It can be used for agriculture, drinking and more. Historically, rainwater harvesting is a common practice and has been used by many communities to support agriculture in sensitive and variable climates.
As a result of its widespread and historical use, many varieties of harvesting water exist and depend on the area available for catchment as well as the intended post-collection use. Water harvesting techniques can be divided in two main types: in situ and ex situ.
In situ rainwater harvesting is a technique that increases the capacity of the soil to store water, thus collecting water where it lands. In situ collection ensures that rainwater remains where it falls with little distance between capture and usage areas. Some examples of in situ water harvesting include terracing, pitting and conservation tillage practices. These measures are also used for for soil conservation (UNEP and SEI 2009).
Ex situ water harvesting is a technique where water is collected in an area external to where it falls and is stored for later use. Ex situ water harvesting is often used in urban areas natural soil surfaces or rooftops, roads and pavements in urban areas. Examples include capturing and storing water in dams, wells, ponds, cisterns, etc. (UNEP and SEI 2009).
Benefits & Co-Benefits
In situ water harvesting has multiple benefits. It allows for the collection of water in soil, thus increasing water infiltration and holding capacity which results in improved soil fertility for agriculture and/or biodiversity. Other benefits can include reduced runoff from slopes and facilitates groundwater recharge (Agriwaterpedia 2014).
Ex situ benefits are usually related to storing excess water, particularly stormwater runoff for productive use later. In urban areas, the reduced stormwater runoff volumes also contribute to minimizing the amount of pollutant loads entering stormwater collection systems, helping to prevent potentially damaging effects on water quality (EPA 2013). In addition, it contributes to water conservation, reducing the pressure on surface water sources and groundwater. When used for irrigation purposes in households, the harvested water also enhances groundwater recharge. In urban areas, reduced energy requirements for water treatment and transport can contribute to better air quality, and reduced CO2 emissions from local power plants. Even if treated for potable use, rainwater, in most cases, requires less energy than conventional water treatment and distribution.
Costs
Water harvesting measures vary in cost depending on type, design and scale. For example, in situ solutions in rural areas using traditional methods may be low cost and only incur the cost of labor and time. For ex situ methods, the building of storage tanks, cisterns, pumps, etc., will incur costs of its own. The scale is also a factor.
The scale of water harvesting methods can also influence the hydrological regime of a river, particularly if its large-scale. For example, water harvesting that significantly reduces surface runoff may increase groundwater recharge and evaporation losses. This may negatively impact downstream water users, including ecosystems. When multiple users are involved and scale is significant, it is important to undertake comprehensive planning and with proper knowledge of the hydrological system in question.