All measures on this page target water retention as one of their main aims. These measures hence could result in an extra source of water available for agriculture in areas characterized by erratic rains, long periods of drought and a limited soil moisture holding capacity. Most measures are based on existing projects performed in areas where water scarcity already had a great impact on households and their livelihoods.
Runoff water diversion to infiltration basins
A system of hand dug canals (main diversion canal and secondary and/or tertiary canals) conveys and distributes the captured water from ephemeral rivers, roads and hillsides to the cultivated fields. Agriculture under this system would thus receive runoff water besides rainwater to supply its water requirements. The runoff harvested, could be stored for later use or could be lead to the field immediately. The measure provides water to fulfill crop water requirements, increases soil moisture and recharges the ground water level. Besides, it supports the control of runoff water (UNEP-DHI Partnership, UNEP-DTU, & CTCN, 2017).
To avoid water loss, the receiving fields could be divided into rectangular basins bordered by ridges. This also minimizes erosion risk. Another option would be to lead the water to infiltration pits. Centering crops around such pits would further improve irrigation efficiency.
This practice needs preparedness for immediate action by the farmers: When high rainfall is expected and the in the catchment, farmers should be available to start erecting the embankment across the bed of the stream. Similarly, each farmer starts to maintain the canal which leads water to his field. A schedule should be prepared collectively, defining the date and time each farmer is allocated his turn to irrigate. When the water reaches the field, it could be spread either through flooding or distributed in furrows which are opened and closed using a local tool.
While the diversion canals and basins for tree planting are permanent structures under proper maintenance, basins for annual crops are seasonal. Maintenance is needed every season before the onset of rains. Soil fertility (and hence water retention) could be improved by additional measures such as composting and mulching. (WOCAT SLM Technologies, 2010; WOCAT SLM Technologies, 2011)
Runoff water harvesting from external catchment
This measure consists of non-graded, hand dug ditches that collect and retain runoff water from roads and/or built-up areas outside the target catchment in which agriculture is performed. These ditches hence provide an extra source of water that could be used for irrigation, on top of the water that is available in the catchment itself. Water is lead from the road to the ditch through a hand dug diversion channel. Besides serving as a source of irrigation water, this measure increases soil moisture and recharges the ground water level. The ditches are accompanied by a soil bund to reduce erosion. (WOCAT SLM Technologies, 2011b)
On-farm pond/percolation pit
Digging a pond on the lowest site on his/her field, a farmer gets access to groundwater as well as the runoff water that is captured. Ponds in the pathway of water runoff intercept the downflow of water and sediments, and thereby reduce erosion. The water that is not used for irrigation purposes, can infiltrate in the soil to retain the aquifer.
Bigger ponds could be built with an inlet to collect and dispose excess runoff. To stabilize the pond, one could plant local grass species in it. To keep the pond operational, it must be desilted on a regular basis. Main benefit of this measure is that it is localized and it does not have to consume much land. (WOCAT SLM Technologies, 2011c; WOCAT SLM Technologies, 2011d; WOCAT SLM Technologies, 2016; WOCAT SLM Technologies, 2017)
The gley method can be used to line ponds and canals. By adding organic materials like banana leaves, grass and manure to the bottom soil of the pond and treating it with cardboard and/or soil, an impermeable layer will be formed as the result of anaerobic decay (FAO, n.d.). This layer prevents leaking and hence results in a higher irrigation efficiency. Especially for farmers without access to groundwater, this technique could be used to retain rainfall and runoff water at the surface. Note that the impermeable layer results in a hampered aquifer recharge.
The rock catchment system is a large scale wate rharvesting system constructed on a gently sloping hillside. The (bare rock) hillside is the surface from which rainwater is harvested b y a concrete dam. To withstand the force of the collected water, this concrete dam should be reinforced by iron bars, A weir in this dam creates the opportunity to lead water through a metal piping system towards downstream storage reservoirs (masonry tanks). From the tanks, communities can draw water at ‘kiosks’ where the storage level is being gauged.
The length, height and thickness of a weir varies with the size and the slope of the rock catchment area. On avarage, a weir will be 10 meters long, 2 meters high and 0.5 meters thick. (WOCAT SLM Technologies, 2017b)
In the cistern system, runoff water is retained and stored in underground storage tanks (cisterns). These tanks are lined with gypsum or concrete to avoid vertical and lateral infiltration, covered by a stone construction and can be of individual or communal use. Each cistern system consists of three parts. The impluvium is the sloping catchment area bordered by a diversion channel. The water from this area is concentrated in the sediment settle basin, and then lead to the storage reservoir.
The size of cistern systems vary from 5-200 m3 (private) up to 70,000 m3. It is estimated that a tank with a capacity of 35 m3 can meet the annual domestic water needs of a family and its livestock (Ennabli, 1993). (WOCAT SLM Technologies, 2011e)
Rooftop rainwater harvesting
The rooftop rainwater harvesting system consists of a catchment roof, conveyance pipes, and a storage jar. The storage tank can be made out of cement for it to be well isolated. To avoid losses through evaporation from the storage tank, it must be placed in the shadow of the house and covered by a (tin) lid. The lid should be large enough to fit a person for occasional maintenance and cleaning. Tanks can have a storage capacity up to 2000 litres. To lead rainwater from the roof gutter (made from longitudinally split pipes) to the tank, conveyance pipes should be installed. These can be made from polythene, plant material or plastic bottles (image).
A tap should be installed around 20 cm above the ground. This height allows for water to be collected in the typical 15 litre water vessels and could avoids collection of too much water in bigger vessels as well as minimising the dead storage of water (Nakarmi et al. 2003). Trained masons can easily install the entire system. (WOCAT SLM Technologies, 2018; WOCAT SLM Technologies, 2019)
Underground rainwater harvestingWhere the roof is catching the rain as in the previous measure, this measure stores the water in an underground tank. Besides storing water to be used in dry spells without rain, it offers a way in which water is kept cool in a warm environment. The harvested water runs from the roof to the tank through gutters and pipes. (WOCAT SLM Technologies, 2017c; WOCAT SLM Technologies, 2019b)
A hole (3 m wide, 2 m deep) is excavated to build the tank. The walls are constructed with concrete bricks and mortar, followed by a mortar lining. The base is also lined with mortar. To reduce losses through evaporation, the tank is sealed – except for the lid. Again, the lid should be large enough to fit a person for occasional maintenance and cleaning.
Woollen water retention in root area
Planting trees (or crops) in a bed of wool prolongs the source of soil moisture in the root zone of the tree. Subsequently, this could support the tree to survive longer periods of drought while staying productive. Combined with precision irrigation, it helps to increase irrigation efficiency since less water is lost through evaporation or leakage to deeper aquifers. A plastic pipe could be installed leading water directly from irrigation to the wool.
It is estimated that 10 kilograms of wool, enough for one tree to be based in, retains around 8 litres of water. A substitute for wool (if not locally available) would be hay or pressed sawdust. (WOCAT SLM Technologies, 2017h)
Applying a layer of plastic under fresh, uncontaminated soil destined for crop production is a method to reduce both water and fertilizer leakage. It could work as a support to other measures on the farm, reducing the need of investment in large scaled equipment and labour (for example to excavate a larger pond). It is estimated that leakage of water and fertilizer may be reduced with 85% using this method