Ponds to establish micro catchments, harvest runoff water and reduce soil erosion. (南非)

Semi-circular ponds are depressions constructed in the soil to harvest water, reduce runoff, increase infiltration, break soil sealing and crusting, capture organic material and seed where sheet erosion is predominant in degraded, low gradient hillslopes. The opening of the semi-circular pond is up-slope for runoff water to flow into the pond.

Ponds are applied to disturbed and degraded natural and agricultural areas that have undergone sheet erosion, sealing and crusting on slopes not exceeding 10 degrees. Ponds are constructed of semi-circular hollows dug out of the soil. Ponds are constructed either by hand (micro-ponds) or mechanically using a backhoe-loader or excavator (macro-ponds). Micro ponds are typically 1m wide, 1m long and 30cm deep at its centre, with a water holding capacity of 131 litres. Macro-ponds are typically 3m wide, 3m long and 50cm deep at its centre, with a water holding capacity of approximately 600 litres. The function of ponds is primarily to reduce surface or small rill erosions, reduce water velocity down slope and to harvest water to increase infiltration. Additionally, ponds reduce down-slope siltation of streams, wetlands, dams or reservoirs and even damage to infrastructure like low water bridges, capture seed and organic matter and provide a preferable environment for plant recruitment, particularly in arid areas. Micro-ponds are constructed by pick, to loosen the soil. Loosened soil is then removed by spade and deposited on the downward slope of the pond to form a semi-circular wall similar to that of a dam. The pond wall should be compacted with a spade to reduce breakage during flooding events. Approximately 15% of ponds exhibit broken “dam walls” after two years in extremely arid environments and are greatly dependant on vegetative recruitment. This happened mainly with micro-ponds. Follow-up repairs are advised during the first year, but not thereafter as ponds are also prone to silt up. In arid environments, ponds that do not adequately develop above ground biomass initially are prone to silting closed within three years. Siltation is also dependant on ponding density, spacing and soil type for both micro and macro ponds. The reason for this is they are bigger in size and from our experience, more effective in this arid landscape. Macro-ponds exhibit an increased rate of vegetative recruitment and biomass production, presumably due to less erosion of surface soil and soil nutrients, increased soil moisture content and water retention capacity compared to micro-ponds. Micro-ponds are typically constructed at an average rate of four (4) ponds per hour and 19 ponds per day per person. Production rates are typically 34% lower in hot summer months (2.39 ponds/hour). Macro-ponds are constructed at a rate of approximately 4.43 pond’s per machine hour. Volumetric costs are approximately 31% less expensive when constructed mechanically per litre of water holding capacity. However, large machinery can only be used in severely degraded areas and where access to the project site is available. Ponding density and spacing for micro as well as macro ponds depend on the topography of the landscape to be rehabilitated, the budged and resources available. The more dense, the more effective but also the more expensive the total operation. Some of the threats associated to constructing ponds are soil loss when pond walls break and in the case of steeper slopes may lead to the start of erosion head-cuts if not constructed appropriately. Pond inlets should be constructed at as low gradient decline as possible to reduce the probability of head-cut incision. Ponding at steeper gradients nearing 10-degree slope require particular consideration regarding spacing and density to avoid causing erosion associated to pond breakage. Staggered rows should be considered along the contour. In severely degraded areas, ponds are applied at approximately 400 micro-ponds or 100 mega-ponds per hectare. In severely degraded sites, micro-ponds are applied at approximately 250 ponds per hectare and 160 ponds per hectare in moderately degraded locations. Application rates are greatly determined by soil type and structure, slope, costs and local legislation regarding soil disturbance. Local South African environmental legislation require an Impact Assessment to be done if a certain cubic metres of soil is disturbed, even for rehabilitation purposes. In order to avoid this very costly and tedious approval process, consideration should be given to the amount of soil disturbed or moved. Caution must be taken in severely dispersive soils with regards to soil loss when flooded. Duplex soils are prone to “undercutting” and ponding on these kind of soils should be avoided. Pond depth should be considered with regards to soil water infiltration rates and saturation rate.
The effect of plant growth in and around ponds could also be accelerated or improved by brush packing if material is available in the area. If not, the cost become to high to transport material. Brush packing in the ponds further help to create a micro environment for plant growth by providing a bit of shade, nutrients from the decaying branches and also protection against grazing of newly sprouted plants within the ponds by wildlife occurring naturally in the area. Ponding in semi-arid environments are only used on natural areas, mostly used for grazing of wildlife and livestock. Ponding are usually not used on cultivated fields. Other measures like contours and waterways, vegetative strips are rather used than ponding. There are also very little cultivation happening in semi-arid areas, this project area in Baviaanskloof specifically due to low and unreliable rainfall. Since ponding provide a favourable micro environment for plant growth, grass, shrub and tree species, adaptable and endemic to the area, can be manually planted in the ponds to accelerate vegetative recovery and also to improve the biomass production and species composition in the area. This technology is applied by farmers and land rehabilitation projects. Considering the cost of ponding, it is mainly used in development and restoration projects with donor or government support. In south Africa, this technology is preferred in donor or government funded development project for job creation purposes. Lastly, ponding help reduce soil erosion, increase water infiltration and eventually working towards improve vegetation cover by establishing favourable micro habitats in and around the ponds for vegetation growth. Increased vegetation growth, especially Spekboom, reduced erosion all help to store soil organic carbon and is therefor a carbon mitigation measure as well. Ideally the whole area is withdrawn from grazing for at least 3 years to allow revegation to happen.

地点: Baviaanskloof, Eastern Cape Province, 南非

分析的技术场所数量: 10-100个场所

技术传播: 均匀地分布在一个区域 (6.0 km²)

在永久保护区？: 否

实施日期: 2019; 不到10年前（最近）

• 通过土地使用者的创新
• 作为传统系统的一部分（> 50 年）
• 在实验/研究期间
• 通过项目/外部干预
• Existing technology