This section discusses the most common techniques for harvesting water on roads in semiarid areas. Topography, climate, and economic land use differ from place to place: different road-water harvesting techniques suit different conditions. In sloped areas, for instance, it is easier to collect and store water by making use of the natural topography and ability for land to be drained. In flat areas, water harvesting is different: there are more opportunities to spread water over large areas but waterlogging and sedimentation are major issues.
However, there are a number of general principles. The first is that runoff is preferably managed intensively throughout the entire water catchment. By developing different water conservation techniques (retention ponds, soak pits, infiltration galleries, terraces, eyebrows throughout the watershed, a large proportion of the runoff in a catchment is retained. In untreated areas, approximately 8 to 12 percent of the runoff is retained, but when intensive water harvesting is practiced this proportion can double or triple, exceeding 30 percent of the runoff. With this, the volume of potentially destructive storm water in the lower part of the watershed can also be reduced. This intensive approach ensures that a large amount of water is stored, creating a system change in water availability for crops, soil processes that accelerate natural fertilization, and more conducive and better buffered micro-climates. Moreover, with intensive catchment treatment, including the highly systematic use of all road-water harvesting opportunities, sedimentation can be brought under control.
The second principle for water harvesting is the “slowdown” of runoff, achieved by guiding water to level land and spreading it. As such, water runoff loses its erosive nature and sediments settle. If the speed of runoff is reduced, more water will infiltrate. This can be done by providing check dams, guiding water from steep slopes, building terraces and furrow ditches, and more. As more storm water infiltrates the soil, less water must be managed as surface runoff.
Third, it is important to understand the needs of the users of harvested water. This should be the central concern. During the planning, design, and implementation stages, the priorities of roadside users need to be discussed, taking into consideration all possible alternative water harvesting mechanisms and the purposes to which the water is placed. Gender is an important consideration and sometimes a divider: in Ethiopia it was found that women in poor female-headed households are less equipped to prepare their land for road-water harvesting, for instance, because they lack access to animal traction (Demenge et al. 2015). Different livelihood systems have different water harvesting demands. Smallholder/household-scale irrigation in many semiarid areas supplements rainfed systems. If rainfall is scarce or not timely enough, water harvesting from roads could help during periods of scarcity or water can be added to the buffer capacity. Shallow groundwater extraction and small storage structures could serve this purpose. The needs of pastoralist communities are different: their interest is in grazing lands. In this case, water harvesting techniques that spread flows as sheetflow over extended areas is the preferred option. Commercial farming usually comes at the expense of high water demands. Medium- to large-scale storage of runoff water is the preferred water-harvesting option, including borrow pits, earth dams, and ponds.
The sustainability of road-water harvesting structures needs to be ensured. Some earthwork structures need regular repairs. The water users will need a routine to inspect the water management systems periodically (after each rainy season) and modify or improve the systems as required to address any impacts caused, such as erosion, overflowing, health, safety, and environmental issues. In this regard, attention should be paid to the risk of mosquito breeding and waterborne diseases in standing water, and measures should be taken to prevent them (see Chapter 6).