SOIL AND WATER CONSERVATION.
Soil and water conservation involves all the practices carried out to prevent removal of soil from land and protect water sources and maintain supply.
Process by which soil is detached, removed and carried away from one place to another where it may not be useful.
Natural erosion is a normal geological process.
FACTORS INFLUENCING SOIL EROSION.
- Amount and intensity of rainfall.
Excess rainfall carries soil away. Rain drops hit the ground with force that splash up soil away.
- Slope of land (Topography)
Speed of runoff is determined by the slope of the land. The greater the speed of water the greater the erosive force.
- The type of soil.
Ability of water to infiltrate into the soil depends on the soil type. Sandy soils with coarse texture becomes saturated faster hence easily eroded.
- Soil depth.
Shallow soils becomes saturated with water quickly thus are easily eroded.
- Vegetation cover.
Forest protect soil against erosion by preventing direct exposure of soils to agents of soil erosion. The tree canopy reduces the impact of raindrops.
Leads to overgrazing leaving soil surfaces bare hence soils are exposed and become loose.
Trampling of land by animals has an erosive effect on the soil.
Indiscriminate removal of trees from forested areas.
Soils will be exposed to high temperatures and heavy rainfall.
- Planting of annual crops on steep slopes.
Leads to frequent cultivation hence exposure of soil to erosion.
- Indiscriminate burning of vegetation before cultivation.
This destroys the soil structure leaving soil loose and exposed to agents of soil erosion.
- Clean weeding.
Leaves soil unprotected against water erosion.
- Plough up and down the slope/across the contours.
SOIL AND WATER CONSERVATION
TYPES OF SOIL EROSION.
Soil erosion by water.
Soil splash resulting from the impact of water drops directly on soil particles.
Has greater impacts in bare soils. Kinetic energy of water dispersers the soil particles by detaching and transferring them in splashes.
Could results to carrying away of seeds planted shallowly.
Raindrop impact on bare soils decreases aggregate of soil particles leading to destruction
of soil structure.
Uniform removal of soil in thin layers from flat or gently sloping land.
Areas with loose, shallow top soil overlying a light sub soil are more susceptible.
The eroding power of water in the sheet flow depends on the amount and speed of surface run off.
Removal of soil by water from small but well defined channel (rills) or streamlets where there is a concentration of flowing water down the slope.
Occurs on slope with little vegetation or in ploughed fields.
Small enough to be filled easily by normal tillage operations.
Advanced stage of rill erosion.
Small channels get progressively deeper and wider. Characterised by deep long ditches made by running water,
Process in gully formation.
- Movement of water from water-shade.
- Channel erosion caused by flowing water.
- Wearing of the sides of the channel.
- Scouring of the floor of the channel by moving water.
Types of gullies.
Formed as a result of scouring of soil by concentrated run off in areas where soil is deep and there are unprotected depressions.
Occurs where there are resistant materials on gully floor.
Soil erosion by wind.
Removal and carrying away of soil by wind.
Severe in semi-arid areas where land is bare and cultivated land where soil is bare and exposed to sun.
Takes place by:
- Suspension of light soil in the air.
- Bouncing along the surface.
- Creeping over the land surface.
SOIL AND WATER CONSERVATION
Soil erosion through mans’ activities.
- Overstocking and overgrazing in ASALS.
- Indiscriminate clearing of forest and other vegetation.
- Earth moving operations e.g. mining, road and building construction, quarrying and sale of sand contribute to soil erosion.
EFFECTS OF SOIL EROSION.
- The eroded productive soil is lost forever. The top soil contains organic matter and plant nutrients hence lowering productivity of land.
- Soil micro-organisms are also carried away. Some break up soil organic matter to humus. Eroded soils have poor soil structure that does not support healthy crop growth.
- Deposition of eroded materials in dams and rivers makes them shallow by creating siltation problems.
- Sedimentation and silting in water bodies lead to decline in fish production.
- Excessive surface run-off causes damages by exposing underground water pipes and destroying roads.
River bank erosion.
Occurs along the river banks as a result of large volume of water and amount of materials carried by water.
Effects of riverbank erosion.
- Materials carried by water damage the banks depending on volume and speed of water.
- Widens the river bed reducing the potential size of cultivable land.
- Leads to sedimentation in dams and other water bodies.
- Construction of dams to regulate the flow of water.
- Construction of dykes e.g. River Nzoia to control flooding of Budalangi plains.
- Planting trees along the river banks to hold soil together.
- Observing government regulation on leaving a sizeable strip of uncultivated land along the river bank.
Solifluction. Gravitational flow of surface materials saturated with water.
The earth flows from steep slope due to heavy rainfall.
Solifluction is a form of mass wasting.
Mass wasting. Downward movement of weathered material on a slope under the influence of gravity.
Factors influencing mass wasting/ Solifluction.
- Slope of land.
In steep slope movement of the materials is fast compared to gentle lying plains.
- The nature of material.
Materials containing a lot of water move easily downslope.
If large rocks overlie weak sedimentary rocks which have clay or shale materials underneath, mass wasting occurs easily.
Areas with high rainfall have wet materials which are easily moved under the effect of gravity.
Bare soils are easily carried away downslope.
- Human activities.
Deforestation, building, quarrying and cultivation interferes with stability of surface layers thus initiating mass wasting.
- Forces within the earth’s crust.
E.g. earth tremors and volcanic eruptions cause large and widespread movement of materials.
Landslide. Sudden rotational movement of a mass of soil or rock along a more or less semicircular slip surface.
Water content of materials is minimal compared to Solifluction.
Types of landslides.
Intermittent movement of earth or rock masses for a short distance involving backward rotation.
May lead to reverse slope.
Independent units of slip leads to numerous step-like terracets.
Slumping can be initiated by undercutting of a slope by a steam, waves or human activities.
Debris slide/earth slide/soil slip.
Materials move at great speed and may lead to loss of life and property.
Movement of materials along vertical or overlying cliff. It is a sudden movement.
SOIL AND WATER CONSERVATION
Occurs on very steep and nearly vertical slopes where blocks of rocks move down a stiff cliff.
Common in steep slopes and in wet seasons.
Masses of rock materials that slide down along a bedding plane, a joint or a fault line.
EFFECTS OF MASS WASTING AND SOLIFLUCTION.
- Soil fertility.
Materials derived from fertile origins ends up in different destinations leading to fertile soils in regions where they are deposited.
- Creation of lakes.
Debris/blocks of rocks block river courses creating temporal lakes.
- Damaging property and causing loss of life.
Destroys buildings, homes, infrastructural networks and leads to loss of lives.
- Soil erosion.
Facilitates soil erosion in steep slopes.
- Permanent scars on the landscape.
These areas remain unattractive to settlement since they are bare and cannot support vegetation.
- Tourist attraction.
Rock fall are tourist attraction e.g. the weeping stone of Kakamega and kit mikai in seme.
METHODS OF SOIL AND WATER CONSERVATION.
S Biological/cultural control. S Physical/structural control.
BIOLOGICAL/CULTURAL CONTROL MEASURE.
- Grass strips/filter strips.
Uncultivated strips 1-2 metres wide along the contour between cultivated strips (30M).
The strips are composed of grass and they gradually form terraces.
- Reduce speed of run off and filter out soil.
- Limit machine use.
- Harbour pest increasing pest infestation to the crops.
- Cover cropping.
Involves establishment of crops that spread over the surface of the soil to provider a cover.
Such crops include Desmodium, sweet potatoes etc.
- Decreases raindrop impact.
- Prevent soil from being baked by the sun thus preserving soil moisture and volatile soil nutrients.
- Reduces the speed of runoff and increases water infiltration into the soil.
- Contour farming.
Tillage and planting are done across the hill to create ridges of earth which hold up water and prevent rill erosion by reducing water run- off. It checks erosion on gentle slopes.
Includes: contour ridging and contour planting.
SOIL AND WATER CONSERVATION
Role of mulching in soil and water conservation.
- Prevents splash erosion.
- Reduces speed of run-off and thus increases water infiltration.
- Reduces evaporation.
- Increases organic matter and water retention capacity.
- Cropping systems.
- Rotational grazing. In ASALS to allow grass time to recover after each period of grazing.
- Crop rotation. A grass-legume should be included to maintain soil structure.
- Timely planting, correct spacing of crops and application of manure.
- Soil conservation measure. Crops with inadequate ground cover should be intercropped with crops with a good ground cover such as legumes.
- Strip cropping.
Crops with little soil cover are grown in alternate strips with those having good ground cover.
The different strips control movement of soil particles hence controlling soil erosion.
- Grassed/vegetated waterways.
Continuous depression man-made or natural through which water flows.
Grass/vegetation is planted in the depressions to slow the speed of water and traps eroded soil preventing further erosion.
Should not be used as a track or grazing place.
- Afforestation and reaforestation.
- Planting of trees where they never existed.
- Planting trees where forests have been cleared.
Roles of tree in soil and water conservation.
- Protect the soil below from raindrop erosion by reducing the force with which it falls onto the ground.
- Provide shade and reduce loss of moisture through evaporation.
- Acts as windbreaks.
- The roots of trees bind soil particles together.
- Reduces speed of running water thus reducing its erosive power.
- The leaves decay to supply humus which improves soil structure and increases water infiltration into the soil.
Trees in this system helps in reducing soil erosion among other benefits..
PHYSICAL/STRUCTURAL CONTROL MEASURES.
Involves mechanical constructions.
Helps to drain or infiltrate the excess water and to retain the required moisture content in the soil.
- Trash line/ stone line.
Trash made of crop residues or stones are heaped along the contour.
Helps to trap soil being carried away.
Heaps of soil along the contour.
Grass may be planted on top of the bund to hold the soil together.
The banks of earth are 1-2M wide at the base and 60CM high.
Built on contour with short-ties every 5-10M to channel above the bund.
Suitable for fairly small cultivated areas on moderate slopes.
Should not be more than 30CM apart.
- Cut-off drains/diversion ditches.
Open trench with an embankment on the lower side.
Large quantities of water e.g. fro, road ditches and overgrazed hills emptying water into cultivated land cannot be stopped by terraces and grass strips and have to be diverted to areas where soil erosion cannot occur.
Should have an embankment on lowers side of the channel which holds any excess water that may overflow.
Embankment should be stabilised by grass or trees.
Cut-off drain discharges its water to.
- Natural waterway such as a river.
- Unto a non-erodable stony or rock ground.
- Onto grassland with well-established grass cover. S Into an artificial waterway.
Constructed to reduce the surface flow of water and to carry away excess water that cannot be absorbed by soil.
Done where slope is between 13-55%.
SOIL AND WATER CONSERVATION
Types of terraces.
- Broad-based terraces.
Wide at the base (3M or more) and 2/3M high at the crest.
Have a shallow drain (channel) on top.
250M long and heavy machinery are needed to level the terrace thus expensive.
- Narrow –based terraces.
Level and shorter and closer than broad-based terraces.
Constructed manually. Are planted with grass or long term crops. Built along the contour or on slight gradient with channels on upper and lower sides.
Suitable on 12-20% slope.
- Bench terraces.
Constructed on steeper slope (35-55%).
- Growing of high value crops.
- Where there is an acute shortage of suitable land.
The top surface layer is kept aside and spread on top after construction.
At the top, there is an embankment planted with grass.
Top bank is protected by stone wall or grass and is built as vertically as possible.
The banks and upper parts may harbour weeds so the cultivated area usually become unproductive because of exposed soil
May develop from vegetative strips of “fanya juu” terraces or by excavating deep soil for arable farming. The land is reshaped into a series of steps.
- d) “Fanya juu” terraces.
A ridge made by digging a channel (60cm wide and 60-90cm deep) and throwing the soil uphill.
Grass should be planted on the ridge to protect it.
Suitable in arid areas where grass strips without ridges are unreliable especially in steep slopes and where grass may be eaten by termites during the dry spell.
After some time, they develop into bench terraces.
- Gabions/porous dams.
Boxes of galvanised wire mesh filled with stones which are built across slopes and gullies.
May be prefabricated or made on site.
Trap soil as it flows through the stones and reduce the erosive force of run-off by reducing its speed. The soil then fill up the gull.
- Dams and reservoirs.
- Wall or barrier built across a river or waterway to hold and store water. It also reduces the speed of water.
- (Tanks.) Holds excess water from roof tops.
- Check dam. Constructed across a channel or a gulley. Reduces the speed of water and allows soil to settle. Made of plant debris, concrete or stone.
Any watershed manipulation carried out to decrease surface run-off.
METHODS OF WATER HARVESTING.
- Using weirs and dams. a)
Barrier constructed across the river to raise water level and still allow water to flow over it.
Used to facilitate pumping of water or flow by gravity.
Barrier constructed across a river or a dry valley to collect and hold large volume of water. Raise the water and forms a reservoir or lake for storing water.
Should have a spillway to allow excess water to flow away.
Bottom part should have an impervious layer to prevent water seepage. Plant grass on the embarkment to prevent erosion. Built across a river, a valley or low lying area to harvest and collect water.
Natural means of harvesting and storing water. May be used as source of drinking water for livestock or rearing fish.
Not suitable for human consumption.
- Roof catchment.
Volume of water collected depends on:
- Rainfall intensity and distribution.
The higher the amount of rainfall, the higher the volume collected.
- Surface area provided for the water catchment.
The larger the surface area, the higher the volume of water collected
- Gradient of the catmint area.
Where the catmint is well sloped and hard 75% of annual rainfall may be harvested.
- Retention ditches/level terraces.
Artificially constructed water reservoirs serving the purpose of collecting and storing water.
- Use of wells.
Holes artificially sunk into the ground below the water table to enable water to seep in for use.
- Rock catchment.
Harvesting of rain water from big rocks.
Concrete channels are constructed at the base to direct water into a reservoir.
Concrete wall may be constructed around the rock to act as a tank and water taps are fitted to the wall to drain the water. E.g. Mutomo in Kitui, Mwingi and Makueni.
- Micro climate.
Micro-environments designed to enhance conserving soil and water around growing crops like bananas or citrus tree