Rainwater harvesting ex situ

Technology group

Most precipitation that falls on human settlements is lost to the atmosphere through evapo-transporation (evaporation plus transpiration of water taken up by plants), or runs into rivers away from settlements before it can be used. In some water-rich regions, particularly wealthy regions with centralized water infrastructure, these losses may not be a major concern. However, in many water-poor areas, small-scale collection infrastructure can contribute greatly to the volume of freshwater available for human use. This is especially true in arid and semi-arid regions, where the little rainfall received is usually very intense and often seasonal. Because of this, runoff and river flows can be abundant for brief periods and non-existent throughout the rest of the year (Pacey and Cullis, 1986; Liebe et al., 2007).

Relevant CTCN Technical Assistance


Rainwater harvesting for storage, also known as ex situ water harvesting, is a practice in which rainwater is collected and stored for productive use, for example drinking water, agriculture, sanitation and more. The rainwater can be directly captured in open storage systems, but can also be collected from roofs, soil surfaces or roads. The most common storage devices for harvesting rainwater are tanks. The tanks are commonly plastic, but also can be made from fiberglass, wood, metal, concrete, or stone and plastered tires, depending on resource availability and costs. Simple tanks are commonly used for water capture and storage at the household level, while large capture ponds built into the ground are often for community initiatives. Unpredictable weather patterns and seasonal water resource scarcity make this technology relevant in assuring rainwater lasts for longer time spans. It is particularly important in arid and semi-arid areas that may experience extended periods without rain mixed with periods of intense precipitation. These challenges are further exacerbated by changing climate.

The two broad categories covered in this chapter are:

  1. Collecting rainfall from ground surfaces utilizing “micro-catchments” to divert or slow runoff so that it can be stored before it can evaporate or enter watercourses; and
  2. Collecting flows from a river, stream or other natural watercourse (sometimes called floodwater harvesting). This technique often includes an earthen or other structure to dam the watercourse and form “small reservoirs.”

Micro-catchments are often used to “store” water as soil moisture for agriculture. Small reservoirs are typically used in areas with seasonal rainfall to ensure that adequate water is available during the dry season. This broad categorization can provide a basic framework for defining which strategies may be appropriate in a given setting. Detailed discussion of the technical definitions of these two categories can be found elsewhere (Pacey and Cullis, 1986; Critchley and Siegert; 1991); the technical distinctions are not essential to understanding and are beyond the scope of this article.

Collection and storage infrastructure can be natural or constructed and can take many forms. These include:

  • Below ground tanks (i.e. cisterns) and excavations (either lined for waterproofing or unlined) into which rainwater is directed from the ground surface. Volumes of these are typically small (a few m3 or less) and they are usually used by one household or institution (e.g. a school or health clinic).
  • Small reservoirs with earthen bunds or embankments to contain runoff or river flow (“dugouts” in northern Ghana, “village tanks” in South Asia). The earthen bunds or embankments are typically built from soil excavated from within the reservoir to increase storage capacity. A spillway or weir allows controlled overflow when storage capacity is exceeded. Surveys of small reservoirs in Ghana and Sri Lanka revealed a wide range of surface areas and volumes; median surface areas for Ghana and Sri Lanka were 5 ha and 12 ha, respectively. The mean storage volume in Ghana was roughly 50,000 m3 (Liebe et al., 2007; Critchley and Siegert, 1991).
  • Groundwater aquifers can be recharged by directing water down an unlined well. Groundwater recharge is also an added benefit of unlined reservoirs; stored water will infiltrate permeable soils during storage and eventually reach the groundwater table. Successful examples of groundwater recharge through rainwater collection are included in the references (Stiefel et al. 2009; Sayana et al., 2010).
  • As soil moisture for agriculture. Many runoff control methods for irrigation incorporate inundation or extended contact time with soils to increase topsoil moisture. Traditional methods were often developed in response to local conditions and have been practiced for centuries. Examples of these practices include variations of contour farming, which is broadly defined as ploughing or digging trenches perpendicular to the direction of runoff flow; this slows rainfall, decreasing erosion and increasing infiltration. Numerous other examples are described in detail in the references (Pacey and Cullis, 1986; Critchley and Siegert, 1991).

Subsurface dams are another form of collection/storage infrastructure that can be used to address these same problems. However, they do not fit strictly within the scope of this article, and are included only for comparison and to make the reader aware of another technical option. These dams do not technically collect rain from the ground, but they serve the same purpose as the above technologies and are discussed briefly here. Subsurface dams are typically used in arid and semi-arid areas where riverbeds are often dry for a portion of the year; they consist of a low-permeability barrier (e.g. concrete) inserted into the ground across a riverbed, blocking the direction of flow. Though a seasonal riverbed may be dry at the surface, subsurface flow often continues throughout the year. Drilling a well on the upstream side of the subsurface dam enables access to water year-round. Subsurface dams cannot be applied everywhere and will only work when the stream is underlain by a shallow impermeable layer such as bedrock or clay. However, they have the following advantages over conventional dams: less evaporative loss, superior water quality, and less vector/parasite breeding (WaterAid, 2006; Foster and Tuinof, 2004).

Groundwater is generally of superior microbial and aesthetic quality when compared to surface water. Therefore, groundwater recharge is often used to replenish aquifers that provide high quality drinking water. Rainwater collected from the ground surface is typically used for non-potable purposes, including irrigation, general domestic use, and livestock. However, in some regions with seasonal rainfall small reservoirs are commonly used for drinking water supply during the dry season, despite the high turbidity and poor bacteriological quality of the water (Cobbina, 2010).


Calculations based on historic records of rainfall intensity and seasonal distribution help inform the  necessary and feasible storage capacity of the harvesting structures. Structure examples include catchment dams, rooftop water harvesting tanks and courtyard water harvesting systems. In most cases construction would require regular maintenance to ensure optimal functioning and prevent health hazards, for example conducting necessary water treatment prior to use in households, securing structures against proliferation of water borne disease carrying organisms, and cleaning and maintenance of tanks.

Advantages of the technology

Climate change is projected to increase the variability and intensity of rainfall. Variability is of particular concern close to the equator, where most developing countries are located (IPCC, 2007; Nicholls and Wong, 1990). Groundwater depletion due to excessive abstraction, land use change and population growth is likely to be exacerbated by these changing precipitation patterns.

Collection and storage of rainwater can provide a convenient and reliable water supply during seasonal dry periods and droughts. Additionally, widespread rainwater storage capacity can greatly reduce land erosion and flood inflow to major rivers (Liebe et al., 2005; Udawattage, 1985). Rainwater collection can also contribute greatly to the stabilization of declining groundwater tables (Stiefel et al., 2009; Sayana et al., 2010).

Lack of adequate water supply during drought and seasonal dry periods can halt economic development and hinder human health and well-being (Gleick, 2002). Access to a convenient supply of stored rainwater can decrease travel time to remote water sources, increase agricultural productivity and reduce depletion of groundwater resources. Increasing the availability of irrigation water during the dry season and even during short dry spells has been shown to yield large increases in agricultural production (Foster and Tuinof, 2004; Rockstrom et al., 2002; Ngigi, 2003).

Environmental benefits

- Contributes to groundwater recharge (when utilizing stored rainwater for irrigation), and removes pressures on other depleted water sources. Assists in diversification of water sources.

- Reduces the risk of soil erosion and soil loss from runoff, especially if substantial amounts of rainwater are collected, for example, in large community capture ponds. This has a positive effect on the surrounding vegetation and cropland. 


Socio-economic benefits

- Helps reduce volumes of storm water runoff, and therefore also reduces the number of pollutants entering water collection systems in urban settings. 

- Decreases pressures on water drainage and sewage systems and reduces the risk of sewage water overflows, as well as costs and energy needed for water treatment and transport.

- Builds climate resilience.


Financial requirements and costs

Implementation of large-scale rainwater collection programs should include a survey of current reservoir capacity and location. Satellite-based methods for tracking surface water, including radar and other methods that are not hindered by cloud cover, can reduce the costs of the survey (Critchley and Siegert, 1991; Alsdorf et al., 228; Alsdorf and Lettenmaier, 2003).

It is difficult to find specific data on the construction and implementation costs of rainwater collection projects. Many factors, including the scale of the project, location, etc. will strongly affect costs. The program costs of foreign-funded rehabilitation and development of small reservoirs (locally known as “village tanks”) in Tamil Nadu, India have been reported. The village tanks in that program were relatively large (40 ha or greater) and the average cost for each project was about $50,000 (Gupta, 2006).

Institutional and organisational requirements

Rainwater collection projects can have adverse hydrological impacts on communities downstream if too much water is stored or diverted. Local governments must have the technical ability to assess these impacts if they are to prevent major externalities and resolve conflicts. Knowledge of geographic information systems (GIS) and remote sensing/satellite imagery software and other tools are necessary to determine small reservoir storage capacity (Munamati et al., 2010).

Policies, legislation and institutional capacity are needed to address conflicts and externalities that can result from to rainwater collection. Conflicts between small-scale farmers competing for limited runoff have been reported in Kenya. Additionally, as storage infrastructure grows larger it has the potential to reduce flows and negatively impact communities downstream. One state government irrigation department in India destroyed a communal reservoir for fear of negative hydrological consequences for communities downstream (Ngigi, 2003).

Small reservoir projects are likely to fail if communities do not identify the need for rainwater storage and have a choice in the technology (Ngigi, 2003; Nunamati and Senzaje, 2007; Gupta, 2006). Management strategies of communally owned storage and irrigation infrastructure are likely to be subject to the same determinants that govern the success of small drinking water systems. The “demand-driven, community-managed” model that has worked for small drinking water supplies is likely to work for these systems as well Whittington et al., 2009).

Barriers to implementation

  • Barriers include the potential for adverse hydrological impacts downstream and the need for adequate capacity to assess these impacts. However, the environmental and hydrological impacts of small reservoirs have been reported to be minor (Van de Giesen et al., 2002).
  • Additionally, surface storage can lead to parasite/vector breeding, algal blooms and poor water quality, particularly in small reservoirs fed by agricultural runoff. Despite the poor aesthetic and microbial quality of such waters, they are often used for drinking when other water points are distant or expensive (Cobbina et al., 2010). Implementation of household water treatment technologies may be able to address this problem.
  • Large water harvesting systems can have negative effects on runoff and groundwater levels, which in turn can affect ecosystem dynamics and downstream users of the water source
  • Still-sitting water in the storage tanks can create habitats for carriers (e.g. mosquitos) of vector-borne diseases, and therefore preventative measures are often required, particularly in warmer climates
  • Collected water may require treatment before direct use, particularly in urban settings (e.g. pollutants collected from roof surfaces)
  • Because of the largely local impact, initial investments in the technology often need to be borne by the households. Alternatives may be programmatic approaches financed by local governments, or other projects 
  • Some areas may have local restrictions on rainwater harvesting

Opportunities for implementation

  • Low cost technology
  • Flexible infrastructure: it can be built from a number of different materials and in various storage capacities
  • Directly benefits local communities 
  • Barriers:Increased agricultural productivity, the potential for year-round water supply, and decreased time spent collecting water provide strong incentives to landowners or communities considering rainwater collection. Increased opportunities for ground-level rainwater collection should arise when rainfall is highly variable or seasonal, agricultural productivity is clearly hindered by dry periods, and alternative water supplies are distant.
  • Opportunities for small reservoir capacity development may arise where water availability is inadequate but environmental, social or legal concerns preclude the development of large reservoirs.


  • UNEP-DHI Partnership: Rainwater harvesting ex-situ
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