Soil water management is a critical aspect in land cultivation. Farmers practice various techniques in soil water management. Whether contour bunding/ creating furrows, building terraces, molding ridges, or compartmental bunding, the most important guiding factor in choosing the right soil conservation measure, is your purpose. There are no practices of soil water management that would be suited for every location and therefore, it is important to choose structures that would give maximum results depending on the climatic conditions of your area, soil type and the cost of the technique you. The ultimate goal of every soil water management technique is to slow down runoff, collect water that would have otherwise been lost, avoid soil erosion and consequently increase your crop yield as a result of improved soil moisture storage.
Managing water activity in the soil is done for various purposes including; to modify the soil slope, influence the run off or to grow crops in a steep slope.
The structures that will modify the slope are; Fanya juu, level terraces and bench terraces. If the goal is to influence the surface runoff by either controlling or reducing loss of water then; contour bunds/furrows ,compartmental bunds and ridges /tied ridges is the best option. Finally, the hill side ditches and intermittent terraces will enable you grow crops in a steep slope.
| Figure 1: contour bunds in a field crop |
Contour Bunds/ contour furrows
This is also referred to as “strip farming”. This type of manipulation of land requires less soil movement compared to bench terracing and therefore, is suitable for areas that receive lower amounts of rainfall. The crops are grown along the strips while the catchment area is left to fallow. With this type of structure, you will collect the runoff water and improve the available water in your cropped area. The water movement into the soil is slow and therefore, this is a limiting factor in areas that have “heavy” soils ( a heavy soil is a fine-grained soil, made of clay or silt, excluding sand). For example, some parts of western Kenya have heavy soils and Eastern side have sandy soils. With heavy soils, you risk waterlogging. It is important to lay your contour bunds accuratelyto avoid having uneven depths of ponding water behind the banks. This can be done by having smaller bunds at right angles and should be lower in height than the main ridges so that in case of overtopping ,the water will move laterally and not over the bund. Contour bunds can be done by hand labour or ox drawn.
Terraces
Terracing is a common practice in slopy regions. Terraces are channels constructed across a field slope. They channel runoff water into a stable outlet, where it is carried safely away. The crops are grown along the slope and the channels are covered using grass or legumes. In the dryer regions, It is able to catch all the rain that falls but, in areas that experience heavy rainfall, there is need to create a way for the excess water to flow down the slope. In case of over saturation.
| Figure 2: fanya juu terraces |
One of the common ways of building terraces, that has been largely used in Eastern Kenya is a technique locally referred to as ‘Fanya Juu’ Directly translated in English ‘Throw upwards’.Fanya-juu terraces are constructed by throwing soil up slope from a ditch to form a bund along a contour. The trench is 60 cm wide by 60 cm deep, and the bund 50 cm high by 150 cm across at the base. Several of these terraces are made up the slope following the contour lines. The distance between bunds depends upon the slope and may be from 5 m apart on steeply sloping lands to 20 m apart on more gently sloping lands. In rocky areas, rocks can be used to enhance the terraces. Fanya juu requires less labour compared to other terracing techniques, and has been successfully used in Machakos and its environs to manage soil erosion and conserve water.
| Figure 3: tied ridges |
Ridges and tied Ridges
Ridges are furrows dug along contour lines to collect the rain that falls from unplanted slopping basins into its ridges and furrows and therefore, increase accumulation of water which consequently allows more water to be absorbed in the soil.
Planting is done on either side of the furrows, where the water is absorbed. The spacing between the ridges depends on the spacing between rows of the crop but the height of the ridges is 30cm high.
Basins are made by digging out shallow furrows along the contour lines of the slope and making ridges on the downside of the furrows. These are then “tied” together by slightly lower ridges which are made at regular intervals along the furrows. The ties should be spaced at 1 and 3 meter interval along the furrows and half to two thirds of height of the ridges. These ties can be done once a year or during the growing season. In addition, you can minimize the water loss by evaporation through mulching. The setting up can be done by hand or ox drawn.
| Figure 4 compartmental bunding |
Compartmental bunding
Compartmental bunding involves building earthen embankments across the slope of the land, following the contour as closely as possible. A series of such bunds divide the area into strips and act as barriers to the flow of water. The size of the bunds depends upon slope of the land. A height of 0.5 meters is recommended. This technique is suitable for areas with low rainfall, that is less 700mm of rainfall per year.
Compartmental bunding conserves the rainwater, recharges soil profile uniformly, reduces runoff, arrests soil and nutrient losses and increases crop yields on a sustainable basis. This is a technology that can easily be adopted by framers because the cost of construction is low and simple. In regard to the soil type, it is best suited for medium to deep black soils with high clay content.
Steps in forming compartmental bunds.
- Till the land
- Form the bunds along and across the slope at required intervals.
- With the help of the spade, shaping the bunds at the corners of the compartments after field layout.