- Road change the surface hydrology in semi-arid areas, now often causing extensive erosion, flooding and sedimentation with considerable damage to road bodies themselves too
- Because roads acts as barriers and drains for rain run-off, they can also be used for water harvesting at large scale
- There are several techniques to systematically divert or retain water in semi-arid areas by making use of the road infrastructure, such as flood water spreaders, flow dividers at culverts, road drifts (Ch 8) or road embankment acting as storage reservoirs
- It is useful to connect road drainage to water storage, such as infiltration trenches, converted borrow puts (Ch 7) or farm ponds (Ch 10)
Semiarid areas cover 15.2 percent of the world and are home to 1.07 billion people (14.1 percent of the global population). They are defined by an aridity index of 0.20 to 0.50, meaning that actual precipitation is between 20 and 50 percent of evapotranspiration. Because of the sheer population size and their climate being “on the edge,” semiarid areas are most vulnerable to droughts.
There is enormous potential to connect road building and water harvesting in these areas. Semiarid areas are not only characterized by relatively low overall annual rainfall (typically less than 600 mm), but also by rain being concentrated in part of the year, usually in one or sometimes two rainy seasons. The retention of rainfall in semiarid areas is of vital importance: it assures the availability of water and moisture for productive and consumptive use in dry periods. Moreover, especially if done intensively, the impact goes beyond providing more water for agriculture. Having more moisture in the landscape, for instance, also improves soil fertility because nitrogen fixation is accelerated in moist soil conditions. It also improves the landscape microclimate: more moisture in the soil will affect soil temperature and thus the area’s ability to deal with temperature shocks.
This chapter advocates for a systematic optimization of the effect of roads on moisture and water in semiarid areas and, as much as possible, the combination of water harvesting and road building. The sheer magnitude of road programs makes them a powerful asset for retaining water in semiarid areas. Moreover, as discussed in the first chapter, there is a triple win, because beneficial road water management in semiarid areas can also contribute to reduced damage to the roads.
If one were to take a soil moisture map of an area and overlay it with a road map, one would likely see strong correlations. As roads influence surface and subsurface flows, the moisture in a landscape can be significantly affected by where the roads are located and how they are constructed. Roads can block surface and subsurface flows, creating moist stretches upstream of the road and dry patches downstream. Road drifts in riverbeds, if properly constructed, may retain subsurface flows and spread floods, again enriching the moisture in the area. On the other hand, erosion from culverts and road drainage may create gullies that deplete the moisture around them.
Road infrastructure itself can be used to harvest water and redistribute runoff to areas where it is beneficial. Roads either act as an embankment that guides water or as a drain that channels rainwater. This can be used in a systematic manner. The amount of water that can be harvested depends on the rainfall pattern, the catchment area as defined by the road, the rainfall patterns, and the land use and soil characteristics within the catchment area. For a road to act as water harvesting mechanism, the road drainage mechanism needs to be well developed by having the road on an elevated embankment, having a system of side drains or cross drains, or having drainage structures such as water bars and rolling dips integrated into the road surface (the latter particularly for unpaved roads; see Chapter 9).
Table 2.1 presents the order of magnitude of the amount of water that can be harvested from 1 km of road equipped with drainage facilities.
Table 2.1: Volume of water (m3) that can be harvested from 1km of road1
The challenge is not only to capture the rainfall runoff, but also to store it for later use. Runoff in the landscape that is guided by road infrastructure can be stored in three different ways:
- In surface storage structures, such as ponds and converted borrow pits;
- Spread over land areas and used to replenish soil moisture, e.g., for rainfed cultivation or for rangeland improvement, retained by bunds, terraces, and micro-basins; and
- Routed to recharge areas where it will replenish shallow aquifers. Water can be lifted and pumped up from shallow aquifers.
Therefore, there are three types of storage: (i) surface reservoirs, (ii) soil profiles, and (ii) shallow aquifers.
- In the case of surface reservoirs such as ponds and borrow pits, the total storage capacity is limited but the water is readily available. On the downside, water from surface storage is reduced by evaporation. Chapters 7 and 10 describe the development of borrow pits and ponds.
- In contrast, great quantities of water can be stored in the soil and shallow aquifers, provided that the geology of the area allows this. The capacity of the soil to store water differs with the soil texture. Table 2.2 compares the storage capacity of different soils.
Table 2.2 Available water in different soils in Yemen’s Abyan Delta
- The infiltration characteristics and the capacity of shallow aquifers to store water differs with the type of geological formation (see Table 2.2), the soil crust, and the type of rainfall (heavier rainfall means more infiltration).
- The shallow aquifer’s recharge capacity can be enhanced by techniques that accelerate the infiltration of runoff into shallow aquifers, such as infiltration trenches or infiltration ponds, tube recharge (called bhungroo in India), or well recharge.
- Several techniques, such as mulching or deep plowing, may be used to preserve moisture in the soil profile and ensure its availability in the growing season (van Steenbergen et al. 2010).
- The disadvantage of shallow aquifer storage is that the water must be pumped up, but many low-cost solutions are available. The advantage is that water will be available for a long time and can be accessed on demand, making it suitable for precision uses. Very shallow groundwater is particularly important, because up to a suction depth of 10 m it is possible to lift groundwater with low-cost centrifugal pumps or solar pumps, making smallholder irrigation possible as a route from poverty to prosperity.
- Runoff generally carries sediment. In the case of surface storage, the reservoirs gradually fill up with this sediment and need to be cleaned. This is not the case in soil moisture storage or shallow groundwater recharge. In fact, silt often improves the soil structure and is a rich source of micronutrients. Therefore, while in surface storage, sediment is a problem, but in the case of soil moisture storage it can be an asset. In the case of groundwater recharge, fine sediment such as clay may also be problematic. However, it may seal the soil surface and reduce the infiltration capacity of the underlying shallow aquifer. This sealing may be prevented by regular plowing or by the action of soil (rain worms, sow bugs, or termites) that tends to take fine sediment down from the surface and mix it with lower soil layers.
Figure 2.1 Runoff and sediments
The different storage methods are contrasted here. But in reality, for road water harvesting in semiarid areas, there is usually no “either/or,” and all three storage methods can be used simultaneously. In many cases, road-water harvesting can be part of larger watershed improvement programs that deploy a broad range of methods to capture and store runoff, with road-water management being a part of this (see Box 2.1).
Box 2.1. Road water harvesting campaigns in Ethiopia: mobilizing millions to increase resilience
Every year, millions of people are engaged in Ethiopia to work on soil and water conservation and water harvesting during the Mass Mobilization. The 2016-2017 road and hillside water harvesting campaign in the Amhara Region involved 1,450,000 persons and benefitted 751,000. In Tigray, 1,306,000 persons were involved and 409,000 directly benefitted.
The main goal is to reverse severe land degradation and work on retaining runoff in the landscape. Men, women, and youth contribute 20 to 30 days of free labor during the February to April slack labor season. The approach involves organizing land users in development teams of 20 to 30 members, further divided into work teams of five members. Activity planning is done locally and includes the collection of field measurements. Women and men participate equally in the work groups and as team leaders. Activities undertaken during the Mass Mobilization are mostly carried out on cultivated lands. The regional Woreda (district) and Kebele (sub-district) administrators, specialists, and development agents coordinate the implementation and planning of the approach. Planning and measurement are conducted by land users themselves. The target area is defined by administrative and as well as watershed boundaries. The implementation plans are later discussed with the communities. At the end of each day the work group evaluates its activities and discusses the plan for the coming days.
Since 2015, the Mass Mobilization has placed special focus on road-water harvesting. A wide array of road-related water harvesting measures have been implemented to protect roads and increase farm productivity: floodwater spreaders, roadside infiltration trenches, water diverters from culverts, road-water storage ponds, and converted borrow pits.
The hydrological and socioeconomic impact of these technologies has been measured since 2015. Monitoring data have shown an increase of 1.2 to 2 m in groundwater levels during the dry period. Soil moisture next to the road has increased up to 100 percent in some cases and farm productivity has risen by 35 percent on average. Moreover, there is the added value of protecting the roads from erosion, flooding and sedimentation, and the drastically reduced damage to the landscape. The costs and benefits were calculated: against an average investment of US$1,800 in the road-water harvesting measures there was a benefit of … due to reduced downtime and road maintenance, reduced damage from erosion, flooding and waterlogging, and of course the beneficial use of the water harvested.