THE PRINCIPLES OF WATER HARVESTING IN CONSERVATION AGRICULTURE FOR ARID AND SEMI ARID AREAS

 RETENTION DITCHES

                  

Retention ditches are large ditches, designed to catch and retain all incoming runoff and hold it until it infiltrated into the ground. They are sometimes also called infiltration ditches. In semi-arid areas retention ditches are commonly used for trapping rainwater and for growing crops that have high water requirements, such as bananas. These crops can be planted in the ditch and thereby get increased supply of moisture. The design of retention ditches is usually determined by trial and error. Often the ditch is about 0.3-0.6 m deep and 0.5-1 m wide. In very stable soils it is possible to make the sides nearly vertical, but in most cases the top width of the ditch needs to be wider than the bottom width. The spacing between the ditches varies according to slope. On flat land the ditches are usually spaced at 20 m and have close ends so that all rainwater is trapped. On sloping land the spacing is between 10-15m and the ditches might have open ends so that excess water can exit. Retention ditches can also be made for the purpose of harvesting water from roads or tracks. The location of such ditches will be specific to the site. When constructing the ditches, the soil is thrown to the lower side to form an embankment that prevents soil from falling back in. In order to stabilize the structure grass can be planted on top of the embankment.

  CONDITIONS FOR RETENTION DITCH

Retention ditches are particularly beneficial in semi-arid areas where lack of soil moisture is a problem. They should be constructed on flat or gentle sloping land and soils should be permeable, deep and stable. Retention ditches are not suitable on shallow soils or in areas prone to landslides.

CONTOUR FARMING

Contour farming means that field activities such as ploughing, furrowing and planting are carried out along contours, and not up and down the slope. The purpose is to prevent surface runoff down slope and encourage infiltration of water into the soil. Structures and plants are established along the contour lines following the configuration on the ground. Contour farming may involve construction of soil traps, bench terraces or bunds, or the establishment of hedgerows. The first step in contour farming is to determine a contour guide line. All subsequent water conservation measures are related to the contour guidelines. Contour ploughing ensures that rainfall and runoff are spread evenly over a field by making furrows parallel to the contours

If you don’t plough along the contour, water will run down the furrows and erode soils when it rains Small dams made of earth can be made at regular intervals in the furrows, to trap rainwater and prevent it from flowing along the contour; these are known as tied ridges

CONDITIONS

 Contour ploughing is successful on slopes with a gradient of less than 10%. On steeper slopes contour ploughing should be combines with other measures, such as terracing or strip cropping. The fields should have an even slope, since on very irregular slopes it is too time-

consuming to follow the contours when ploughing

CONTOUR FURROWS

Contour furrows are, small earthen banks, with a furrow on the higher side which collects runoff from the catchment area between the ridges. The catchment area is left uncultivated and clear of vegetation to maximize runoff. Crops can be planted on the sides of the furrow and on the ridges. Plants with high water requirements, such as beans and peas are usually planted on the higher side of the furrow, and cereal crops such as maize and millet are usually planted on the ridges The distance between the ridges varies between 1m and 2 m depending on the slope gradient, the size of the catchment area desired and available rainfall amounts. The drier the area, the larger the distance between the furrows. Small cross–ties in the furrows can be constructed at regular intervals and at right angle to the ridges to prevent flow of runoff and to ensure an evenly distribution of captured water.

CONDITIONS FAVOURABLE FOR FULLOWS

Contour furrows are suitable for areas with annual rainfall amounts of 350-700mm. The topography should be even to facilitate M an even distribution of the water. Contour furrows are most suitable on gentle slopes of about 0.5-3%. Soils should be fairly light. On heavier, more clayed soils they are less effective because of the lower infiltration rate

GRASS STRIPS

Grass strips are a cheap alternative to terracing. Grass is planted in dense strips,  up to a meter wide, alone the contour. These lines create barriers that minimise soil erosion and runoff. Silt builds up in front of the strip, and within time benches are formed. The spacing of the strips depends on the slope of the land. On a gentle sloping land the strips should have a wide spacing (20-30 M). On a steep land the spacing needs to be less (10-15M). Grass strips can be planted along ditches to stabilise them, or on the rises of bench terraces tom prevent erosion.

The grass needs to be trimmed regularly to prevent them from shading and spreading to cropped area between strips. The cut grass can be used as livestock fodder or mulch. Any grass varieties can be used depending on what is locally available. Vetiver grass is a good grass to reduce erosion and resists drought well. Other examples of grasses that can be used are Napier guinea and Guatemala grass. Alternatively a local veld grass can be used.

 PLANTING PITS

Planting pits are the simplest form of water harvesting. They have proved especially successful for growing sorghum and millet in areas with minimal rainfall amounts. Small holes are dug at a spacing of about 1 m. During rainstorms the planting pits catch runoff and concentrate it around the growing plant. Crops are planted in the pits and thereby benefit from the increased moisture availability in the pits.  Compost or manure is placed in the pits before planting to improve soil fertility. It is not necessary to follow the contour when constructing planning pits. Dimensions of the pits vary according to the type of soil in which they are dug. Usually they are between 10-30 cm in diameter and 5-15 cm deep. In the second year, farmers may sow into the existing holes or, if spacing of the pits is large, they may dig new ones in-between the existing ones

CONDITIONS

Planting pits have proven successful in areas with annual rainfall of 200-750 mm. They are particularly useful for rehabilitate barren, crusted soils and clay slopes, where infiltration is limited and tillage is difficult. The slope should be gentle (below 2%) and soils should be fairly deep. Where soils are already shallow, they become even shallower when planting pits are dug. In those cases farmers should not plant in the pit, but in top of the ridge of excavated soils in order to maximize rooting depth.

SEMI-CIRCULAR BUDS

Semi-circular bunds are earth bunds in the shape of a semi-circle with the tip of the bunds on the contour. The size of the bunds varies, from small structures with a radius of 2 m to very large structures with a radius of 30 m. They are often used to harvest water for fruit trees and are especially useful for seedlings. Large structures are used for rangeland rehabilitation and fodder production. The entire enclosed area is planted. When used for tree growing, the runoff water is collected in an infiltration pit, at the lowest point of the bund, where the tree seedlings also are planted. The bunds are laid out in a staggered arrangement so that the water which spills round the ends of the upper hill will be caught by those lower down

CONDITIONS

Semi-circular bunds are suitable on gentle slopes (normally below 2%) in areas with annual rainfall of 200-750 mm. The soils should not be too shallow or saline

 COVER CROPS

Cover crops are usually creeping legumes which cover the ground surface between widely spaced perennial crops such as fruit trees and coffee, or between rows of grain crops such as maize. Often cover crops are combined with mulching. They are grown to protect the soil from erosion and to improve soil fertility. Cover crops protect the soil from splashing raindrops and too much heat from the sun. Most of the plants used as ground cover are legumes, such as different varieties of beans and peas. Pigeon peas and other crops with strong tap roots and longer growing season than maize and beans make good mix and can be used to break hard-pans in semi-arid areas. Over 100 species of cover crops are in use around the world.

For the cover crop to compete with the main crop as little as possible the cover crop should be of a low yielding variety. Cover crops should be planted as soon as possible after tillage to be fully beneficial. This can be done at the same time  as sowing the main crop, or after the main crop has established, to avoid competition at crop nutrition level