ENVS 435: Watershed Management

Transcription

1 ENVS 435: Watershed Management Unit 4: Watershed degradation Soil & Water Conservation Approaches Erosion and Mass Movement Control INSTR.: R.M.Bajracharya

2 What is involved in watershed degradation? Watershed degradation involves several types of resource and biophysical condition degradation within watersheds. These include: Land degradation Forest and vegetative degradation Water quality and quantity decline Loss of biodiversity Climate deterioration The above forms of degradation are interrelated and eventually lead to overall deterioration of watershed conditions.

3 What are the causes of watershed degradation? There are two broad causes for watershed degradation, namely, natural and humaninduced causes. Natural causes include: Extreme weather/climatic events (torrential rain, high wind speeds, snow/ice/mud flows, flash-floods, etc.) Landslides and other mass-movements resulting from naturally unstable and active geology (mountain-building & down cutting processes) Natural wild fires (due to lighting) Earthquakes Volcanoes Extreme tidal action (coastal areas)

4 Human-induced causes of watershed degradation Improper land/soil management Cultivation of steep/marginal land (>30% slope) Excessive manipulation & tillage of soil Inadequate conservation practices (terracing, drainage/disposal of runoff water, soil exposure) Poor water management Inadequate source area protection Excessive damming and water abstraction Contamination of water sources & streams Improper forest/pasture management Excessive harvesting of timber/fuel wood Improper silvicultural practices Over-grazing of grass/shrub lands

5 Human-induced causes of watershed degradation cont d. Haphazard infrastructure development Road & trail construction + maintenance House/building location & construction Irrigation canal construction & maintenance Settlements & urbanization Improper disposal of waste water Inadequate sewage treatment/processing Lack of solid waste management Illegal hazardous waste dumping/disposal Modernized agricultural practices: Excessive use of pesticides & fertilizers Improper timing of applications

6 Human-induced causes of watershed degradation cont d. Socio-economic & demographic factors: Economic conditions and activities of rural or peri-urban house-holds Socio-cultural peculiarities, ethnicity, etc. Traditional land management & cultural practices Land tenure and fragmentation Population growth and movement In- or out-migration Poverty and economic disadvantage Social exclusion Gender roles & issues

7 Forms of watershed degradation: Land degradation Accelerated soil erosion Sheet/wash erosion Rill/gully erosion Splash erosion (rain drop) Slope destabilisation Slips & slumping of hill slope Terrace failure Land slides & mud flows Rock & debris slides Decline in productive capacity of soil Fertility decline; chemical degradation Organic matter decline; biological degradation Soil structural deterioration; physical degradation

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9 Forms of watershed degradation: water supply and quality Decline in availability of water drying up of source areas springs, wells, streams due to: Denudation of vegetation at source areas Excessive extraction/consumption of water Deterioration of water quality Contamination by agro-chemicals Contamination by untreated sewage Seepage of leachate from domestic or solid waste dumping sites Degradation of river/stream quality Loss of ecological balance of rivers & streams River training & flow control structures (straightening) Destruction of habitats & aquatic diversity Heavy sediment & chemical loading

10 Forms of watershed degradation: Forest and biodiversity decline Forest degradation includes: Decrease in forest crown cover (density) Decrease in understorey & ground vegetation Decline in the total biomass of vegetation Decline in the species richness or diversity Loss of biodiversity involves a decrease in the species richness and abundance of both plants, animals and microorganisms. This may occur in: forests, grassland (pasture) or in agricultural biodiversity. It results in a reduction of the ability of ecosystems to recover from disturbances and adapt to changes.

11 Forms of watershed degradation: Climate deterioration Changes in microclimate due to: Stripping of vegetation & exposed soil Bare ground & rock surfaces (greater fluctuations of temperature; increased droughtiness) Reduced infiltration of water into ground Greater runoff and flashiness of streams & rivers Longer-term changes in climate patters Larger, regional factors & patterns Increased ubanization (built-up areas, pavement & concrete cover)

12 Why the need for soil and water conservation? Rate of soil erosion and land degradation increases (beyond) natural rates with human intervention and use. Soil degradation takes numerous forms: Loss of fertile top soil Soil fertility and productivity decline Reduction of soil depth (restricted rooting & nutrient/water availability) Soil degradation (decline in quality) leads to decrease in productive capacity and crop yields. There is an increasing demand for food, fibre and other land-based resources.

13 Some key principles Loss of soil productivity (critical to sustainability & enhancing rural livelihoods) Erosion is a consequence of how the land is used (traditional vs. improved practices) The farm household has been insufficiently studied/researched to identify and address critical issues in production & sustainability. Plant yields are adversely affected by both water and nutrient deficiency (or excess) Often adoption of conservation technology is a bigger problem than development of the tech. Farmers need to be convinced of the shortand long-term benefits. The technologies must address specific farmer concerns and needs.

14 1. Bench terraces Bench terraces are used on sloping land with relatively deep soils to retain water and control erosion. They are constructed by cutting and filling to produce a series of level steps or benches. Bench terraces are reinforced by retaining banks of soil or stone on the forward edges.

15 1.1 Bench terraces Advantages Effectively controls soil and water runoff and erosion. Traps sediment in the drainage ditches built along the terrace Reduces slope length. Every 2-3 meters of slope length is levelled to terraces. Improves soil fertility over the long run. Limitations Initially disturbs the soil, reducing productivity in the first 2-3 years. Needs intensive labour and investment for construction and maintenance. Needs skills for proper construction. Terraced fields with an interval slope consume much land.

16 2. Composting Compost is a type of organic fertilizer derived from the decomposition of plants and animals. It is an excellent source of plant nutrients. It is common in home gardens. The use of compost is a traditional soil fertility management practice throughout Southeast Asia.

17 2.1 Composting Advantages: Decaying compost generates nutrients for crops. Decaying composting generates heat. Surface applied compost may also act to protect the soil from erosion. Limitations: Compost mounds requires a large quantity of plant material. Cannot be used in the lowlands. Cannot be practiced on steep slopes. High labor requirement.

18 3. Contour Tillage Contour planting is practiced on sloping lands to reduce soil erosion and surface runoff. This may involve construction of soil traps, bench terraces or bunds, or the establishment of hedgerows. Different combinations of crops can be planted following different patterns. The SALT system is a good example of contour farming.

19 3.1 Contour Tillage Advantages: Reduces runoff and soil erosion. Reduces nutrient loss. Cultivation is faster and or easier. Limitations: Improperly laid out contour lines can increase the risk of soil erosion. Labor intensive maintenance. Needs special skills to determine contour lines.

20 4. Cover crops Cover crops are grown to protect the soil from erosion and to improve it through green manuring. These are usually short term crops, planted in fields or under trees during fallow periods. Cover crops are also inter planted or relay-planted with grain crops such as maize, or planted once in a cropping cycle.

21 4.1 Cover crops Advantages: Improves soil fertility and physical and chemical properties. Reduces soil erosion and water loss. Suppresses weeds. Reduces need for fertilizer and herbicides. Provides human food and animal forage. Increases soil organic matter. Helps retain moisture in the soil and prevent soil from drying. Limitations: May compete for soil moisture and nutrients with the perennial crops. Involves additional farm and inputs. May result in weed problems. May be alternate host for pests. Some cover crops species may contain chemicals which inhibit subsequent crop growth. Rats and snakes may hide in dense cover crop foliage.

22 5. Crop rotation In crop rotation various crops are grown in sequence, one after another, in the same part of the farm or field. Each crop places a different demand on the soil in which it is grown.

23 5.1 Crop rotation Advantages: Very effective in improving soil fertility. Reduces nutrient drain. Helps sustain crop production. Helps controls pests and diseases. Limitations: May be difficult where input supplies are poor. Less applicable for long-term crops. May require a farmer to plant a crop which is not the highest priority. Demands more skills of the farmers.

24 6. Diversion ditches Diversion ditches are constructed along the contour lines and across slopes for the purpose to intercept surface runoff and divert it to suitable outlets. Diversion ditches are dug at varying intervals, depending on the steepness of the slope.

25 6.1 Diversion ditches Advantages: Protects cultivated land hillside runoff. Controls gully erosion. Slows down the erosive power of runoff. Limitations: If not properly designed, the ditches can overflow on to the farms during heavy rain. Needs support structures such as check dams and drops to effectively control erosion. Needs continuous repair and de-silting.

26 7. Drop structures Drop structures are constructed to slow the flow of water in channels. Drop structures are more effective when combined with check dams.

27 7.1 Drop structures Advantages: Controls the upstream water velocities to reduce erosion. Drops the water flow to a lower level. Dissipates the excess energy of water flow. Controls downstream erosion. Limitation: - Requires some skill to construct. - Higher cost involved.

28 8. Grass Strips Planting grasses along the contour lines creates barriers to minimize soil erosion and runoff. It induces a process of natural terracing on slopes as soil collects behind the grass barrier, even in the first year. Types of grasses: Napier grass Amliso (broom grass) NB-20 (Napier-Amliso cross) Vetiver

29 8.1 Grass Strips Advantages: Controls soil erosion and runoff Provides fodder Grass can be used as mulch Limitations: Labor is required for management of grass strips. Ruzi grass can root itself from cuttings. Mulching of grass cuttings may contribute to the weed problem. Uses land which may other wise be used for food production.

30 9. Hedgerows Hedgerows are one of the simplest erosion control practices on sloping land. Various tree and crops species are established in the hedgerow to enhance farm income and diversity. Hedgerows help slow down the passage of rainwater and trap soil to gradually form natural terraces.

31 9.1 Hedgerows Advantages: Reduces soil erosion. Improves soil fertility and soil moisture. Provides biomass for green leaf manure. Provides shading for young plants. Serves as a source of fodder, fuelfood and light construction materials. Improves soil structure and water infiltration. Provides a source of mulch. Limitations: Loss of land for cultivation due to establishment of contour hedgerow. Hedgerows compete with food crops planted between the rows of light. Hedgerow plants may be hosts to pests. Effective retention of excess water may result in soil slippage on steep slopes.

32 10. Minimum tillage/zero tillage Simple farm implements such as hoes and digging sticks are used to prepare land and plant food crops. Minimum tillage is common and effective in controlling soil erosion, particularly on highly erodible and sandy soils.

33 10.1 Minimum tillage Advantages: Lessens the direct impacts of raindrops on bare soil, thus minimizing soil erosion. Minimizes degradation of soil structures. Slows down the rate of mineralization, leading to more sustained use of nutrients in the organic matter. Limitations: Inadequate seedbed preparation may lead to poor establishment and low yield of crops such as maize and sweet potato. Rooting volume may be restricted in soils with massive structures.

34 11. Mulching Mulching includes covering of cut grass, crop residues or other organic materials over the ground, between rows of crops or around the trunks of trees. It is commonly used in areas subject to drought and weed infestation. The choice of the mulch depends on locally available materials.

35 11.1 Mulching Advantages: Intercepts the direct impact of raindrops on bare soil and reduces runoff and soil loss. Suppresses weeds and reduces labor costs of weeding. Increases soil organic matter. Improves soil chemical and physical properties. Increases the moistureholding capacity of the soil. Helps to regulate soil temperature. Limitations: Possible habitat for pests and diseases. Not applicable in wet conditions. Difficult to spread evenly on steep land. Lack of available materials suitable for mulching. Some grass species used as mulch can root and become a weed problem.

36 12. Ridge Terraces A ridge terrace consists of a ridge and furrow, constructed along the contour on sloping land. Grasses and legume trees are usually used to stabilize the ridge. Fruit trees, banana and cassava are also commonly used.

37 12.1 Ridge Terraces Advantages: Effectively controls runoff and erosion on moderate slopes. The furrow behind the ridge traps sediment and nutrients. Relatively low labor inputs are required compared to bench terracing. There is minimum disturbance of soil particularly important on shallow upland soils. Limitations: Less effective in controlling erosion than bench terraces. Takes time and labor to establish a stable ridge. Needs proper maintenance, since the ridge can break, channel runoff and result in rills.

38 13. Shifting cultivation It is also commonly referred to as swidden or slash-n-burn cultivation. In this system, the underbrush is cut, then most of the trees are felled. Certain tree species are left to stand and branches are pruned. In most places, underbrush is burned.

39 13.1 Shifting cultivation Advantages: Slowly releases nutrients from the forest biomass to the soil. Helps control weeds in the first three months, enabling crops to grow quickly. Retains soil moisture. Easy method of clearing rainforest for permanent agriculture. Minimizes direct impact of raindrops on soil surface. Suitable for root crops and banana-based cropping systems. Limitations: Increases soil and nutrient loss. Soil nitrogen is lost by burning. Only simple land preparation is possible.

40 14. Soil barriers Soil barriers may be made of wood or rocks: over time they may develop into live fences of trees and shrubs. Fences are usually with logs and branches across the slope. These are placed against wooden stakes driven into the ground. The upper side of the barrier is filled with grass and other materials to act as a sediment trap.

41 14.1 Soil barriers Advantages: Slows down surface runoff. Retains sediment behind the fences. If properly maintained, natural terraces develop over time. Allows cultivation even on steep slopes that may not otherwise be feasible to crop. Limitations: Wooden barriers do not usually last for more than 2-5 years. Barrier construction requires significant labor.

42 15. Soil traps Soil traps are constructed to harvest soil eroded from the upper slopes of the catchment. The most common types of soil traps are check dams and trenches, built in diversion ditches or waterways.

43 15.1 Soil traps Advantages: Prevents widening and deepening of gullies. Promotes the deposition of nutrientrich, highly fertile sediments. Reduces the velocity of runoff in gullies. The area where soil accumulates can be used for growing crops. Limitations: Requires continuous desilting to prevent overtoping during heavy rains. Check dams require regular repair and maintenance.

44 16 Water harvesting Small scale impoundments of water for different purpose is called water harvesting. It is most successful when operated as a system with three components: the watershed or catchment area; the reservoir; and the service area.

45 16.1 Water harvesting Advantages: Improves food production. Promotes conservation and ecological balance. Involves low investment cost per hectare. Easy to construct. Provides alternative uses to offset sacrificed land area. Protects against drought. Allows irrigation by gravity. Mostly individually owned. Limitations: Requires large amount of labor. High seepage and evaporation losses possible. Floating vegetation may infest reservoir. Uncontrolled runoff in high intensity rainfall areas can overtop and damage the embankment. Poor design and management can lead to erosion and flooding.

46 Soil erosion and mass wasting Prevention/mitigation measures Soil erosion is the detachment and transportation of soil particles by erosion agents Water (rain drops, flowing water) Wind (abrasion) Gravity Ice/glaciers (freeze & thaw action; scouring) The expression of erosion processes depends on the scale at which it is viewed: Plot scale (interrill erosion: sheet/wash & splash) Hill-slope scale (rill & interrill erosion processes) Catchment scale (rill, gully & interrill erosion) Basin/regional scale (mass-wasting processes)

47 Mass movement processes Land slides movement of soil, rocks, vegetation, en-mass down-slope due to: Action of gravity on unstable slopes Super-saturation of profile Earthquakes or other natural forces Slips and slumping small slope failures, generally debris not moving far down slope. Rock falls/slides Debris falls Mud flows River/stream bank collapse Headward erosion (gully expansion)

48 Control of erosion & mass movement Agricultural land Terracing (outward, inward, level, step, etc.) Planting on terrace sideslopes (grass, shrubs, trees) Contour tillage Drainage structures (channels, grassed water ways, bunds, etc.) Hedge-rows, grass strips, stone barriers Ridges, furrows, pits Reduced or zero-tillage Non-agricultural Roadside slopes Steep lands (sloping) Landslide reclamation Rehabilitation of denuded/degraded lands Construction sites Stream/river banks

49 Terrace types Step terraces Level bench terraces Platform terraces Outward-sloping terraces Inward-sloping terraces Irrigated khet bench terraces

50 Terrace formation Progressive terracing using stone walls Stone & straw lines to reduce erosion Terracing without burying topsoil

51 Contour ditches across hill slope Grass strips Contour tillage done by tractor Tied ridges for soil and water trapping Broad-bed & furrow made with oxen-drawn implement Tractor-drawn ridging implement

52 Non-agricultural erosion control Jute netting Excelsior Gabion wire Erosion net Mulch net Various types of geotextiles

53 Roadside, hill slope and gully stabilization Concrete network structure with vegetation Gully stabilization check-dam Jute netting for hill slope stabilization (top); Propwall for support (below) Herring-bone drainage with jute netting (top); Gabion retaining walls

54 Other bioengineering techniques Live fascine drain construction Brush wattle construction