Mitigation and Intervention Techniques Related to Micro-hydro Power (MHP) Projects

The identification of appropriate mitigation and intervention techniques on the basis of the Environment Assessment is crucial for the implementation of a successful Watershed Action Plan in order to achieve a sustainable use of the catchment area and thus the MHP project. Biophysical as well as political mitigation and intervention techniques are listed in the table below:

 

BIOPHYSICAL MEASURES
Gully control	Stone Checkdams Objectives: A stone checkdam is a structure across the bottom of a gully or a small stream, which reduces the velocity of runoff and prevents the deepening and widening of the gully. Sediments accumulated behind a checkdam could be planted with crops or trees/shrubs grass and thus provide additional income to the farmer. Brushwood Checkdams Objectives: Brushwood checkdams are vegetative measures constructed with vegetative materials, branches, poles/posts and twigs. Plant species which can easily grow vegetatively through shoot cuttings are ideal for this purpose. The objective of BWs is to retain sediments and slowdown runoff, and enhance the revegetation of gully areas. They are constructed either in single or double row. Some of the vegetation can be used for fodder. BWs are also ideal to stabilize conservation structures bunds, SS bunds, check dams, bench terraces, road sides, etc. Gully Resheping, Filling and Revegetation Objectives: Reshaping & Filling is an operation meant to decrease the gully erosion angle of incidence, create planting areas and encourage revegetation & stabilization, usually in small gullies or in other medium sized gullies from which most runoff has been diverted into a stable waterway or drainage line. When these gullies are shaped and smoothed, vegetation can be established over the levelled gullies. Some of these areas can be used for farming purposes. Revegetation of a gully is the plantation of the reshaped gully sides and bottom with multipurpose species so that it reduces runoff and control erosion and make the land productive. Gully revegetation implies reaching a dense vegetation cover over the entire gully surface, i.e. both sides and bottom by planting grasses, legumes and a variety of trees and shrubs. Sediment Storage and Overflow Earth Dams (SS Dams) for Productive Gulley Control Objectives: SS dams are water harvesting and conservation systems that convert unproductive large and active gullies into productive areas (fertile cultivated or fodder producing areas, mixed plantations, and fruit tree orchards). SS dams are stone-faced earth dams constructed across medium/large size gullies to trap sediments, collect water and divert excess runoff. SS dams accommodate the runoff generated by the catchment located above the gully. The structures are often constructed in series along the gully. It is just like creating a land that does not exist. Contribute significantly to protect cultivated lands, arrest gully expansion and recharge water tables. Huge potential in Ethiopia - can provide tens of thousand Km of gullies to poor households (small land holders and landless). Sediment Storage and Overflow Soil Bunds (SS Bunds) Objectives: SS bunds are large and strongly built soil embankments, constructed across gullies, often in series. As for SS dams, their purpose is to create a new field for cultivation by allowing and helping the sedimentation (filling) of the space behind each bund. After rains, the new fields fill up and retain enough soil and moisture to grow crops. The excess runoff move to the next structure downstream through a spillway.
Agro-forestry, Forage Development and Forestry	Area Closure Objectives: Strip cropping is a cropping practice where strips of two or more crops are alternately established on the contour or, it is a system of establishing more than one crop in alternate strips following a contour pattern for the purpose of erosion control, crop diversification, and decrease the risks associated to the use of single crops only. This cropping system is designed as a defence mechanism against soil erosion in areas where the cropping system is dominated by row/sparsely growing crops that exposes the ground to erosive forces. For instance, crops like sorghum and maize are susceptible to erosion and need to be grown alternately with soil conserving crops. Microbasins (MBs) Objectives: MBs are small circular & stone faced (occasionally sodded) structures for tree planting. Are suitable for medium and slightly low rainfall areas, stony areas and shallow soils. Based upon experience they are not very effective in low rainfall areas (where trenches, eyebrows, etc. are preferred). Eyebrow Basins (EBs) Objectives: EB are larger circular and stone faced (occasionally sodded) structures for tree and other species planting. Based upon experience they are effective in low rainfall areas to grow trees and harvest moisture. Can be constructed in slopes above 50% for spot planting. Controls runoff and contribute to recharge of water tables. Herring bones (HBs) Objectives: HBs are small trapezoidal structures (called also A structures) for tree and other species planting. HBs are suitable for both dry and medium rainfall areas, and medium soil depth. Based upon experience HBs are most effective in medium/low rainfall areas (500-900 mm). Can be constructed only on slopes < 5% and soils > 50 cm depth Micro-trenches (MTRs) Objectives: MTRs are rectangular and deep pits constructed along the contours - main purpose and effects are same as for trenches. Can support the growth of trees, shrubs, and cash crops. Can be constructed on slopes 3-30% max. gradient and soils at least 50 cm depth. Trenches Objectives: Trenches are large and deep pits constructed along the contours with the main purpose of collecting & storing rainfall water to support the growth of trees, shrubs, cash crops and grass or various combination of those species in moisture stressed areas (350-900 mm rainfall). Trenches can have FLEXIBLE DESIGN, to accommodate the requirements of different species. Therefore they can suit what the farmer want to grow. Trenches collect and store considerable amount of runoff water, thus vegetation grows faster and vigorous. Trenches protect cultivated fields located downstream from flood and erosion. Part of the water captured by the trenches reaches the underground aquifer. Therefore, water tables are recharged and supply springs and wells with good quality water and for a long period of time. Improved Pits (IP) Objectives: IPs are square shaped water collection pits constructed along the contours with a plantation pit in front of the main water storage pit - main purpose similar as micro-trenches. IP support the growth of trees and fodder shrubs, and can be used for cash crops like coffee. They are effective in medium and low-medium rainfall areas (above 600-700 mm). Multistorey Gardening Objectives: Multi-storey gardening is a way of planting a mixture of crops, shrubs and trees of different heights and different uses: food crops, cash crops, fruit trees, woody perennials, and forage plants. It makes the land more productive and improves soil fertility, reduces temperature, provides shade, and increase family income, particularly during a period of drought.Fields close or within homesteads can be converted into diversified productive units (decreasing risk factors, increased income, etc.). Such systems have higher resistance to pests and insects. A broad network of such homesteads can extend into open fields and constitute large erosion-protected zones or a “web” of green fences and mutually supporting units. Seed Collection Objectives: Seed collection is divided into two:(1) Seed collection of tree species: mostly for indigenous trees to grow in nurseries for specific and multipurpose uses. (2) Legume shrubs and grass seeds: mainly legume shrubs seeds and grass/plant species that can be used for stabilisation, homestead plantations, grazing lands improvement, support to nurseries, fencing, gully control. This activity is especially valuable for grass seeds collected outside the nursery areas and seed multiplication centers, particularly native grasses of particular value for their palatability and adaptation to local conditions. The collection of local seeds is an effective way to protect valuable planting materials from extinction and to replenish depleted areas with materials collected from other areas.
Soil Fertility Management and Biological Soil Conservation	Compost Making Objectives: To promote compost making at household level for cost-effective soil fertility improvement and to support local level compost entrepreneurship linked to natural resources management activities at watershed level. CM will improve soil fertility, increase water storage within the soil profile and reduce surface runoff, thus reduce soil erosion. It is one of the best “hidden” water harvesting methods available (compost absorbs water 4-7 times its own weight). CM can be undertaken at very large scale and linked to area closure management and the upgrading of terraces into “cash crop production belts”. CM can become a business: groups of households (landless, etc.) can decide to become compost makers and provide “fertilization” services to other farmers. Fertilization and Manuring Objectives: The current cropping systems in Ethiopia are highly extractive as chemical fertilizers are expensive and not always available, manure is used in most areas as fuel or income source, due to open grazing manure not readily available as compared to tethered animals, mulching is not practiced due to use of crop residues for forage. Levels of organic matter are low leading to reduced soil stability, infiltration, water holding capacity and increasing erosion. Application of fertilizers will increase yields, while manure and mulching will improve longterm soil quality and nutrient levels. Live Checkdams Objectives: Live check-dams are established by planting/seeding or plugging of cuttings in gully bottoms to replace or reinforce physical check-dams. The main objective of the establishment of live check-dams is to reduce the cost of establishing physical check-dams. Live check-dams can be applied by individual farmers and does not need the organization of a large group of people to undertake physical structures. Mulching and Crop Residues Management Objectives: Mulching is the covering of the soil with crop residues such as straws, maize/sorghum stalks or standing stubble. The cover protects the soil from raindrops, drastically reduces splash erosion and velocity of runoff. It then minimises erosion, increases soil infiltration and permeability, prevents the formation of hard crusts and contribute to improve fertility. The second major advantage of mulching/crop residue maintenance, is its potential for sustaining productivity. Mulching, in addition to its positive effects on soil structure also helps in reducing evaporation and maintaining soil moisture. The improved soil structure also will have an effect on moisture retention and consequently higher water budgets for the growing crop. Grass Strips along the Contours Objectives: Grass strips are vegetative barriers made out of grass planted in narrow strips of 0.5 to 1.5 m width laid out along the contour. Grass strips control erosion rather effectively in gentle slopes but above 5-8% slope their effect decreases. While contributing to protect soils against erosion they also provide valuable biomass meant to increase animal feed or used for different purposes (eg. roofing, etc.). Grass strips cause less interference than other measures as they can easily be crossed by oxen and plough. Moreover, grass strips take out little amount of arable land. Cost of construction is much lower than physical structures. Stabilization of physical Structures and Farm Boundaries Objectives: Stabilization refers to the planting of crops, grass, shrubs and trees in different combinations in order to strengthen the resistance and stability of physical structures such as bunds, trenches, checkdams, SS dams, etc, against rain drops splash effect, runoff and cattle trampling. At the same time, stabilization has the purpose of making productive the surface area occupied by the structure. Stabilized structures would need less maintenance and damages are less likely to occur, even during heavy rainstorms. Trees or shrubs help to demarcate farm and homestead boundaries, thus provide additional sense of ownership. Particular plants are also hosts for insects able to control pest incidence. Stabilized areas are an additional source of timber, fuelwood, fibre, food and forage, palatable grasses and legumes, fruits and other products (dyes, gum, medicinal, etc.). Vegetative Fencing Objectives: Vegetative fencing (VF) is a conservation practice which consist of a combination of vegetative planting materials resistant to cattle grazing planted in rows and with grass/legume plant species sowed behind these rows. Used to protect and enrich reclaimed areas like closures and gullies, farm boundaries and specific community assets like ponds. By doing so it also helps controlling runoff and erosion. It is the “first level of defence” against animal interference that allow other valuable trees to be planted after the fence is established. VF can be exercised as a “grid system” in closures, around reclaimed gullies, and farms, providing increased sense of ownership to users. Ley Cropping Objectives: Ley cropping is a cropping system in which legume based pastures are rotated with purely grown crops. Legume based pastures are grown on fallow lands for a few years to improve fertility of the soil and thus the yields of subsequent crops (mainly cereals). The establishment of dense, productive forage crops during the fallow period (1-3 years) provides a thick ground cover, supply forage of good quality after the rainy season, prevent soil erosion, restore soil fertility quicker than bare (and overgrazed) fallow, increase the water holding capacity of the soil and have a beneficial effect on future crop yields. Integration of Food/Feed Legumes into Cereal Cropping Systems Objectives: Better and full use of production resources such as water, radiation, nutrients, space and time. Integration of cropping optimises productivity by producing higher combined yields from the crops and greater economic returns from a given area of land than the same crop grown in monoculture. Lower risk in intercropping than with mono cropping. Better distribution pattern of labour demand. For example, land preparation is done only once for all crops and therefore the labour situation is less constrained. Control pests, diseases and weeds in the absence of biocides. Better control of soil erosion realised. Intercropping Objectives: Intercropping is a practice of growing two or more crops along the contour simultaneously in the same plot in a fixed pattern in one season. The aim of intercropping is to increase crop production and provide protection to the soil against erosive forces. Different planting times and different length of growing periods spreads the labour requirement of planting and harvesting, but also allows midseason change of plan according to the rain in the early part of the season. The various leaf arrangements of different plants allow light to be better intercepted over time. The contrasting patterns of root growth, which utilize different soil layers, optimize the use of available soil moisture and nutrients. Mixed stands protect the soil surface more effectively than pure stands. Overall output per unit area can be much greater from intercrops than single crops and chances of total crop losses are lower than in pure stands. Crop Rotation Objectives: Crop rotation is one of the oldest practices known to man for fertility restoration and pest/ disease control and it consists of growing different crops one after the other on the same piece of land. Plants of the same crop develop their roots at the same depth of soil profile and thus the proliferation of the root systems in the same depth results in a strong competition for moisture and nutrients. Therefore, if the same crop is grown on the same land year after year, the soil nutrient in that layer decreases sharply and the crop yield consequently declines. On the other hand, if different crops are rotated, the depletion of soil nutrients and decline of crop yields are not as serious as when the same crop is grown year after year. Different crops have different characteristics that enable them to exploit the soil at different depths. Crops also differ in terms of their effect to the soil. Some crops restore or build fertility while others deplete fertility. Strip Cropping Objectives: Strip cropping is a cropping practice where strips of two or more crops are alternately established on the contour or, it is a system of establishing more than one crop in alternate strips following a contour pattern for the purpose of erosion control, crop diversification, and decrease the risks associated to the use of single crops only. This cropping system is designed as a defence mechanism against soil erosion in areas where the cropping system is dominated by row/sparsely growing crops that exposes the ground to erosive forces. For instance, crops like sorghum and maize are susceptible to erosion and need to be grown alternately with soil conserving crops.
Flood Control and Improved Drainage	Waterways (Vegetative and Stone Paved) Cut-off Drains Graded Soil Bund Graded Fanya Juu Improved Surface Drainage for Increasing Productivity of Vertisols and Soils with Vertic Properties
Physical Soil and Water Conservation	Level Soil Bunds Stone Bunds Stone Faced Soil Bunds Level Fanya Juu Bench Terracing Conservation Tillage using Maresha and Broad Bed and Farrows Maker (BBM) Hillside Terraces Hillside Terrace with Trenches
Water harvesting	Hand-dug Wells Low cost Water Lifting Low Cost Micro-ponds Underground Cisterns Percolation pit Percolation Pon Farm Pond Construction Spring Development Family Drip Irrigation System Roof Water Harvesting System Farm Dam Construction River-bed or Permeable Rack Dams Small Stone Bunds with Run-on and Run-off Areas Narrow Stone Lines Along the Contour Stone Faced/Soil or Stone Bunds with Run-off/ Run-on Areas Conservation Bench Terraces (s) (CBT(s)) Tie Ridge (s) The Zai and Planting Pit System Large Half Moons Diversion Weir Design and Construction
POLITICAL MEASURES
Education and training
Income generating measures
Participatory approach

  

Terracing in the Gobecho catchment (Ethiopia)	Gully control near Ambo (Ethiopia)

Box 1 shows three examples of how (rather simple) interventions can improve the environmental condition of the catchment. A more detailed list of mitigation and intervention techniques and the respective application can be found in DESTA, L. ET AL. (2005): Part 1: Community Based Participatory Watershed Development.  Addis Ababa: Ministry of Agriculture and Rural Development, pages 69 – 165 and DESTA, L. ET AL. (2005): Part 2: Community Based Participatory Watershed Development: Annex. Addis Ababa: Ministry of Agriculture and Rural Development, pages 43 - 48.

Box 1: Fenced vs. unfenced springs/wetlands Springs and wetlands not only guarantee the discharge in dry season, also they have a great retention capacity during and after rains. Springs are sensitive areas that react to disturbances easily. Protection of springs and wetlands is thus crucial for the sustainable use of the MHP projects. Protection measurements might include fencing (to protect the areas from cattle step and thus soil compaction), protection of natural vegetation around the springs, protection of a natural humus layer (in order to sustain sufficient infiltration and thus groundwater recharge). Furthermore fencing can help to improve the water quality, since pollution due to agricultural and pastoral use can be prevented. Buffered vs. unbuffered riverbanks/wetlands A buffer typically consists of a band of vegetation along a wetland or water body, preferably natural habitat, but including previously altered, stable native or introduced species. A buffer can perform a variety of functions, which can improve the environmental condition of a watershed, such as sediment removal and erosion control, runoff reduction through infiltration, reduction of human impacts by limiting easy access as well as barrier to invasion of exotic species. Thus buffers can benefit the sustainable use of the MHP and should hence be established and/or protected. Eucalyptus vs. indigenous plants Eucalyptus easily adapts to any soil and water condition, it is characterized by fast growth, high survival, long roots and hard leafs. Those features make it economically very beneficial. When it comes to watershed management, its negatives sides should be considered though: it does not only take a lot of nutrients and water and is hence a high competition to companion plants and decreases biodiversity, also soil erosion can occur which can negatively affect the MHP’s performance. FAO thus recommends that large scale monocultures of eucalypt plantations be excluded from watersheds, aforestation projects should avoid monocultures and the use of eucalypts. The only way to include eucalyptus is to adopt it in agroforestry systems, but the proportion of each species should be planned out carefully.

References

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