As land pressure rises, more and more marginal areas in the world are being used for agriculture. Much of this land is located in the arid or semi-arid belts where rain falls irregularly and much of the precious water is soon lost as surface runoff. Recent droughts have highlighted the risks to human beings and livestock, which occur when rains falter or fail. While irrigation may be the most obvious response to drought, it has proved costly and can only benefit a fortunate few. There is now increasing interest in a low cost alternative - generally referred to as "water harvesting".
The aim of water harvesting is to collect and store runoff or groundwater from areas with a surplus or in areas where these resources are not used to make water available during shortages. This results in an increase in water availability by either impeding and trapping surface runoff and maximizing water runoff storage or trapping and harvesting sub-surface water. Water harvesting makes more water available for domestic, livestock and agricultural use by buffering and bridging drought spells and dry seasons through storage. Instead of runoff being left to cause erosion, it is harvested and utilized.
Rain fed areas are also confronted with problems of land degradation through soil erosion. Watershed management interventions through water harvesting are often synonymous with soil and water conservation. They act both to harvest rainfall and to conserve soil and water, as a mean of increasing farm productivity. Water harvesting can be considered as a rudimentary form of irrigation. The difference is that with water harvesting the farmer has no control over timing. Runoff can only be harvested when it rains. In regions where crops are entirely rain fed, a reduction of 50 % in the seasonal rainfall, for example, may result in a total crop failure. However, if the available rain can be concentrated on a smaller area, reasonable yields will still be received. Of course, in a year of severe drought, there may be no runoff to collect, but an efficient water harvesting system will improve plant growth in the majority of years.
(FAO 1991, Mekdaschi & Liniger 2013, UNEP 2009)