Water and Soil Conservation
Water and soil conservation in drylands are intricately connected: to a large extent, plant and animal life determine water use efficiency. Even though water is considered the main factor limiting productivity in drylands, the potential of dryland ecosystems is mostly determined by a range of other limiting factors. The significantly reduced biological productivity in degraded areas results in exposed soil, increased water runoff and enhanced erosion. This induces a cycle of increasing soil degradation, which ultimately leads to the complete loss of fertility and biological productivity. This chapter documents methods for the rapid restoration of degraded drylands to high productivity and resilience. Topographical remodeling for water harvesting and watershed protection can contribute to controlling soil erosion. This, however, requires support through dense tree planting and the recovery of annual herbaceous vegetation or perennial shrub patches. Only dense vegetation and plant litter can induce the necessary improvement of compacted soils for enhancing infiltration and reducing water runoff. The planting of nitrogen-fixing, drought-resistant species such as Acacia, Albizia or carob is especially helpful for this purpose. These species can rapidly create thick leaf litter layers and restore soil nutrients and soil organic matter, while reducing water evaporation and runoff. Project Wadi Attir is demonstrating these approaches and mechanisms to great effect, continuously monitoring all progress and relevant parameters such as soil nutrients, soil moisture, infiltration and biological productivity.
Soil Erosion and Desertification in Drylands
Somewhat paradoxically, it is excess water that causes the most damage to degraded drylands. Conserved dryland areas have healthy covers of trees, shrub patches or litter layers that reduce water runoff, and, in exposed soil areas, a stable soil crust composed of cyanobacteria or other microorganisms, stabilizing the soil surface and reducing soil erosion. In contrast, degraded dryland areas, after significant loss of plant cover and biological productivity due to overgrazing or soil tilling, have large areas of non-vegetated, exposed soil surfaces that facilitate intensive water runoff, causing extreme soil erosion and gully formation (Fig. 1).
Left untreated, such erosion will result in complete topsoil loss and the creation of rocky badlands that can never be restored to their original productive rangeland state. Soil erosion and degradation are thus the causal origins of desertification: the poor soil that remains is no longer capable of holding sufficient water, nutrients and seeds to support adequate germination and plant life. Thus, in drylands, more easily than in humid areas, degradation becomes irreversible because the small amounts of rainfall are not sufficient to restore perennial plant cover, plant litter cover, and soil organic matter once these are lost.
Fig. 1: Loess soil collapse due to massive soil erosion at Project Wadi Attir: Water runoff from a rocky, de-vegetated slope created cracks in exposed loess soil, that further downstream results in total collapse and the creation of meters-deep erosion formations. This has caused the loss of hundreds of cubic meters of valuable soil in only the last few years.
Increasing vegetation cover, water retention and biological productivity is therefore essential to ecosystem restoration in degraded drylands, whereby tree planting and increased water infiltration can enhance water retention and control water runoff. Ideally, a combination of these steps is required in profoundly degraded locations such as Project Wadi Attir in order to achieve rapid success.
A common technology for desertification control is tree planting using the so-called contour trenching approach. However, this technology can have serious drawbacks as observed in the nearby Chiran forest, where steep, de-vegetated slopes are incapable of adequately preventing water runoff (Fig. 2 a), and the planted eucalyptus trees suppress establishment of annual vegetation. Thus, massive volumes of water runoff continue to further degrade the large exposed soil surfaces (Fig. 2 B), and the location has been determined to be the most degraded of several soil management regimes analyzed (Helmann et al 2013, Mussery et al 2013).
Fig. 2: Degraded soil cannot be saved by the contour trenching method alone, as the dams and terraces built are regularly damaged and overpowered by intensive rain events. The situation is worsened by the application of herbicides, or continued overgrazing, both of which cause depleted crusted soils with reduced water infiltration, facilitating rapid water runoff and continued soil erosion (Chiran forest, Negev).
Consequently, efficient restoration of degraded soils, erosion control and watershed protection must rely on detailed scientific understanding of both physical and biological processes in soil and soil surfaces. Any topographical changes must be performed with great care and minimal disturbance to the remaining intact soil structure and vegetation, while strictly avoiding the use of herbicides.
Erosion Control by Vegetation
While water runoff can be reduced in eroding gullies by the establishment of small dams or terraces, the major restoration focus must be directed towards enhancing vegetation cover and plant litter layers that enhance soil water infiltration and reduce water runoff. The principal interactions of vegetation with water runoff are described in Figure 3. Vegetation creates plant litter layers or patches that collect rainwater and facilitate its infiltration into the soil, thus reducing water runoff. Large perennials and trees also provide shade, avoiding evaporation and thus maintaining soil water content. In contrast, large patches of bare soil allow water to run off and gain destructive levels of energy, resulting in the erosion phenomena pictured above.
Fig. 3: Dense vegetation is the best way to control water runoff and thus soil erosion or desertification. Dry, dense woodlands (Mussery et al 2013) are the most efficient tool, as they also create a continuous layer of leaf litter that prevents runoff and promotes the growth of annual vegetation and soil rehabilitation. Dense and diverse shrublands are equally effective in runoff control and can be more biologically diverse (Leu et al 2014), though biological productivity is lower due to reduced water utilization efficiency (Fig. 4). Exposed large soil patches give rise to excessive water runoff, resulting in the erosion phenomena pictured in Fig. 2.
Trees for Watershed Protection and Erosion Control
Dense vegetation is the most effective means to maximize water use efficiency and soil reclamation, not only in drylands, but in many types of ecosystems worldwide. Tree planting has therefore been widely applied for this purpose, with pine trees having been successfully applied to restore large stretches of Israel’s dry, hilly badlands.
In the Negev, the mostly eucalyptus species that have been planted suppress other vegetation and have not been successful in erosion control. However, many other dryland trees such as the acacia, have also been used and are far more suitable for this purpose. Acacia victoria has been identified by our scientists as the most effective tool to rapidly reduce excessive runoff. Its rich litter, composed of dry leaves and seeds, provides soil cover and decomposes to a rich compost layer that facilitates the growth of annual vegetation. This, in turn, results in higher soil organic matter, higher soil water content and higher productivity of annual vegetation resulting in enhanced soil water infiltration and soil organic matter.
A densely planted Acacia victoria woodland near ‘Project Wadi Attir’ has been determined to be the most highly productive ecosystem in the area, with biological productivity 10 times higher than in the surrounding degraded rangeland. The acacia trees enhance soil water content (Fig. 5 left) and moderate the micro-climate to the extent that olive seedlings successfully took root and established themselves in their plot after only four ten liter portions of water (Fig. 5 right), while identical seedlings planted in exposed areas outside the project site all perished.
Fig. 4: A. victoria is a tree that transforms its immediate environment into an island of fertility. The large amounts of leaves and seedpods create a thick compost layer that enhances soil water content, fertility and annual plant growth, as seen in the dense vegetation patch surrounding these trees.
Fig. 5: Soil water content during 2009 (left), within the A. victoria woodland (dark green line) is significantly higher than in conserved (brown) or overgrazed (bright green) shrubland. This significantly enhanced soil moisture content allowed the olive seedlings planted inside this woodland (shown to the right) to survive the summer with minimal irrigation.
Limans, Water Harvesting and Erosion Control at Project Wadi Attir
The Project Wadi Attir site was heavily degraded, featuring many meter-deep erosion gullies, very low biological productivity and no perennial plant cover. Based on the above results, the restoration program was designed based on minimal soil movement, filling of major erosion structures, reducing water runoff by terracing all runoff channels, and rapid establishment of dense perennial and annual vegetation. Fig, 6 shows the major erosion hotspots before (2011) and after (2014) soil conservation work was performed and trees were planted.
Fig. 6: Erosion control and watershed protection at Project Wadi Attir: the two strongly eroding main gullies (left, in 2011) were partly filled and terraced, and densely planted with a wide variety of agroforestry trees (right, 2013) to stop soil erosion and water runoff, while establishing productive agricultural activities. Smaller gullies (bottom left) were terraced by building small dams in order to collect runoff for agroforestry purposes. A rocky slope was planted with about 300 olive trees (center right) that protects soil and prevents runoff while producing valuable oil.
In order to achieve maximal speed of soil and watershed protection, and to reap the maximal benefits of the planted trees early on, such as fruit, fodder, litter production, wood or a windbreak effect, the trees were densely planted and well-watered. Indeed, within one and a half years, the newly planted seedlings succeeded in establishing significant vegetation cover. Falling leaves and chopped weeds are left on the ground, which helps in recycling nutrients and enhancing water retention and infiltration capacities (Fig. 7).
Fig. 7: Watershed protection in record time: A small liman planted in 2013 with A. victoria and other tree species already provides full protection in 2014, thanks to rapid tree growth, litter production and soil improvement. The chopped weeds together with leaf litter already provide almost closed soil cover, enhancing water retention, infiltration, and nutrient availability and allowing for continued, self-sustained growing productivity in the absence of further irrigation.
Restoration of Degraded Slopes
Degraded slopes along the various side valleys are the source of significant runoff that threatens the whole water and soil conservation system in the event of sudden flash floods. Re-vegetation of such crusted bare soil areas was thus initiated using A. victoriae and Caparis seedlings, known for their ability to grow quickly under extremely dry conditions. The purpose was to provide 20 – 30% soil and plant litter cover within a few years, thereby enhancing water retention. So far, many of the seedlings established well without further irrigation, and thanks to good rainfall, we expect to achieve the above goals in many of the critical locations. Interesting and highly promising is the fact that A. victoriae has been able to establish in newly developing erosion gullies by exploiting the higher water availability, thus stabilizing them (Fig. 8).
Fig. 8: A. victoriae and Caparis spinosa are highly drought tolerant species that can rapidly cover large areas of degraded slopes, reducing runoff and restoring vegetation and soil quality.
Fig. 9: A victoria is successfully applied to stabilize emerging erosion gullies, and form roots strong enough to immediately arrest gully progression and further erosion.
Protection of Natural Shrubland Vegetation for Reducing Water Runoff
Well-conserved dryland shrublands function well in reducing water runoff and erosion. Project Wadi Attir features a number of diverse, partly degraded shrubland reserves (Fig. 10) that will develop a high level of erosion control once sufficiently restored. Current levels of protection from grazing together with adequate precipitation for the last two years have induced significant vegetation recovery and increased animal life, which will rapidly reduce water runoff and protect soils, while establishing ecologically diverse reserve areas (Fig. 11).
Fig. 10: One of the conserved shrubland areas at Project Wadi Attir. Recovery of shrub patches and soil fertility to a fully functional state will contribute significantly to reducing water runoff, and increasing erosion control, biodiversity and productivity.
Fig. 11: Natural mechanisms, such as recovering patches of perennial vegetation like Asphodelus ramosus, and digging animals and insects such as harvester ants, loosen and improve soil and create litter and soil dumps which enhance water infiltration and growth of annual plant life.
Ecosystem function in drylands is determined by a well-developed perennial plant cover and fertile soil, which in turn facilitates water infiltration and the growth of annual vegetation. Restoration of degraded drylands requires erosion control by water harvesting, reducing runoff for the regeneration of plant growth, as well as restoring perennial and annual vegetation to improve soil quality and water infiltration. Project Wadi Attir has managed to demonstrate these principles in different ways:
- Small terraces have been rapidly vegetated by agroforestry trees, providing maximum erosion control within two years;
- Shrubland areas, subject to strict protection, have been recovering at high speeds to provide enhanced runoff control;
- Degraded slopes are being protected by emerging dry woodland patches in order to enhance productivity and soil conservation.
This first experience demonstrates that three years of well-planned project implementation suffice to dramatically reduce erosion while restoring water retention and biological productivity.