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Water is essential to life, therefore, it is important that adequate supplies of water are available. However, water supplies should be developed in such a way that ecosystem functioning and the hydrological cycle are not negatively affected. Within this context, water harvesting has received increasing attention worldwide. The harvesting of rainwater refers to the collection of water from surfaces on which rain falls, and subsequently storing this water for later use. For as long as humans have occupied and cultivated dry lands, water harvesting has been practiced. It has been the basis of living and has allowed the establishment of civilizations in dry lands. Water harvesting has provided for drinking, domestic needs, livestock, crops, pastures, and trees, and a way to replenish groundwater levels. There is a range of rainwater harvesting systems, operating at large and small scales. The choice of system depends on physical and human considerations, and matching the needs of the farmers with environmental, economic, and political conditions.
Water harvesting can occur in multiple contexts and at a variety of scales. The collection of rainwater from the rooftops of buildings is suitable for urban areas and requires little investment or technology, as only roof gutters and storage tanks are needed to capture rainwater. Measures may be needed to keep insects away from the stored water, to avoid increases in waterborne diseases such as malaria. The water harvested from rooftops is suitable for nonpotable purposes, such as watering plants, washing clothes, and flushing the toilet. It is also possible to achieve drinking water standards through additional treatments, such as filtration. In many regions, however, rooftop water harvesting is problematic due to the type of building materials customarily used (such as thatched roofs), and due to the lack of storage facilities.
Water harvesting for agriculture aims to secure the water supply for human use, irrigation, and livestock without tapping surface or groundwater sources. In the past, rainwater harvesting was the backbone of agriculture in dry lands. Thousands of years ago, indigenous populations in dry lands throughout the world developed water-harvesting systems that sustained large civilizations (such as the Nabataeans in the Negev). After being neglected for some time, these systems have received new attention, particularly for small-scale agricultural development in the dry lands of developing countries for poverty reduction efforts. Throughout the developing world, food insecurity and low agricultural productivity continue to be a concern. In many water-scarce areas, rainwater harvesting offers a viable option, as it increases the availability of water for irrigation and consequently reduces the risk of crop failure. Moreover, low-cost technologies are becoming available that particularly benefit resource-poor farmers. Water harvesting will continue to play an important role in supplementing rain-fed agriculture in areas where rainfall is variable, as well as in working toward poverty and food security targets.
Micro-catchment rainwater harvesting systems collect runoff from a small catchment and store the water in an adjacent infiltration basin. This infiltration basin is usually planted with a single tree, bush, or annual crop. The system is simple to design, and cheap. Soil and water conservation measures are often carried out on cultivated lands to increase onsite water harvesting. Micro-systems are suitable for any terrain. Medium-sized rainwater harvesting for human and animal consumption and small-scale irrigation can be conducted by redirecting runoff into storage facilities. The catchment generally has a size of up to 200 hectares, depending on local conditions. The collected rainwater is stored in ponds or cisterns, then used for supplemental irrigation. The cultivated areas are located outside of the catchment. The cropping area is often terraced or located in level terrain and may have a slope of five to 50 percent. These systems are often used in combination with water conservation measures, such as water-efficient drip irrigation systems.
Larger scale rainwater harvesting includes catchments with a size of many square kilometers, from which runoff flows through a riverbed of an ephemeral stream. It requires more complex structures of storage dams and water distribution networks. The two most common systems are floodwater harvesting and floodwater diversion. The former blocks the runoff to inundate the valley bottom of the whole flood plain, forcing the water to infiltrate and using the wetted area for crop production or pasture. The latter system diverts floodwater from the ephemeral stream to adjacent agricultural fields. Both systems were commonly used in ancient cultures in the Negev, China, and Pakistan.
In the United States the ability to harvest rainwater varies from state-to-state based on water law. In the state of Colorado, for example, harvesting of rainwater by homeowners is illegal under prior appropriation law, since the water technically belongs to a downstream user with an earlier right to the water. In other states such harvesting is legal, as long as the water is not used for agricultural purposes. This variability in the legal status of harvested water, along with the potential restrictions it imposes, creates a disincentive for employing this ancient and common-sense approach to water conservation and management.
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