Combining water harvesting and tree planting

turbulence in the planting caused by the presence of irregularities such as branches, leaves, and complex leaf structures. The more irregularities the structure contains, the more dust and pollutants will be trapped. In comparison, almost all dust will “leap” over solid barriers, and little will be intercepted. Porous plantings allow a large part of the airflow to traverse the planting. Dust will be trapped better because there is more contact with the leaves of the trees and shrubs. To achieve a good degree of porosity, plantings should be approximately 5–20 meters wide and consist of tall trees with a bush layer underneath.

Hagen and Skidmore (1971) investigate the effectiveness of windbreaks and establish that the porosity of a windbreak should be less than 35 percent in order for the windbreak (including roadside planting) to have a significant effect on wind speeds. This effect is best achieved with double rows of trees and bushes. Double rows would balance both effects: trapping dust and other pollutants and bringing down wind velocity on the leeward side.

COMBINING WATER HARVESTING AND TREE PLANTING

as discussed in previous chapters, opportunities to harvest water along roads are plentiful, and this activity can be combined with tree planting. Small diversion channels can be constructed to slowly divert surface flow from roadside drainage systems toward the tree seedlings. These diversion structures can be combined with small storage structures around the trees to retain this water for the tree. Smaller bushes and grasses can also complement water harvesting by slowing down the flow of drainage. Grasses will allow water infiltration and trap sediments, thus restoring soils, reducing erosion, and improving hydrological soil conditions.

Network of shallow trenches

a network of shallow trenches can be made along the road to route road runoff over a large area, water trees, and even create small roadside forests (photo 12.4). Trenches together with single-line roadside tree plantings can sequester carbon and offset some of the negative effects of road development.

Micro-catchment water harvesting

Micro-catchment water harvesting is one method used to collect surface runoff from a small catchment area into the root zone of an adjacent infiltration basin. This basin can be used for plants. Micro-catchments are alterations of the topography that direct rainfall runoff to plants. They are simple and inexpensive and provide many advantages over alternative irrigation schemes. Micro-catchment techniques are more effective on slopes not exceeding a 7–8 percent gradient. The optimal size of the micro-catchment depends on the site characteristics and the size of the seedlings. a semicircular bund (also known as an eyebrow or demilune) is a stone structure that contains a water-soaking pit and a planting pit (photo 12.5 and figure 12.3). Semicircular bunds are commonly used on steep land (such as grades greater than 50 percent).

PHOTO 12.4

Network of trenches for roadside tree planting, Uzbekistan

Source: © Jonathan Demenge. Used with the permission of Jonathan Demenge. Further permission required for reuse.

Infiltration trenches

Infiltration trenches are large, deep pits that protect cultivated land from flooding and erosion while recharging groundwater (figure 12.4). Road runoff can be channeled to infiltration trenches where water will percolate. These infiltration trenches will increase the soil moisture of the adjacent area. Trees can be planted next to the trenches such that the roots of the seedlings benefit from the increased soil moisture. Grass strips can be planted at the edges to protect the holes of these infiltration trenches.

Swales

a swale is a broad, shallow channel (natural or man-made) designed to promote infiltration and reduce the flow velocity of runoff (photo 12.6). Its main purpose is to slow, spread, and infiltrate runoff. Swales are appropriate for harvesting road runoff and intercepting silty or contaminated runoff. Swales are never compacted or sealed because their main purpose is groundwater recharge. In fact, swale soils can be loosened to increase recharge and infiltration rates. They also provide an opportunity for tree planting because of the soil moisture increase along and around the swales.

Contour stone bunds

Contour bunds (figure 12.5) are designed to collect enough water to recharge soils and provide water for plant growth during the dry season. They also protect soils from erosion during peak rain events. Design criteria for determining the