Terraces and filtration trenches
Terraces and filtration trenches are principal soil conservation measures for the catchment restoration and protection. In agriculture, a terrace is a leveled section of a cultivated slope, designed as a method of soil conservation to slow or prevent the rapid surface runoff of rainwater. Less steep slopes means the water runs slower and has more time to infiltrate. The aim is to store water in the soil profile (or replenish the groundwater), and keep nutrients and soil particles from washing away from the field.
Failure of terraces is often related to the lack of participation in the design and decision making. A terrace is a profound change in the landscape and in the agricultural practices of the landowner. Men and women have to understand, the need for investment to get the benefits for the ecosystem and their farm. Then they need to express the demand and take the decision or terraces before the work can start. Seeing is believing. Having a pilot show case in the area helps to convince the farmers taking the step to invest in terraces. For learning, adaptation and collective work the farmers are organized in self help groups of around 20 neighbouring farmers as in the Farmer Field School.
During the preparation and organization the households build a vision for the future of their farm. Simple mapping of the resources and the visualizing the plans on paper strengthen the bond of the farmer with its farm and environment. These groups build on self reliance and resourcefullnes and create confidence to invest in the farm. Rwanda has policies in place to support cooperative forms of organization.
There are different types of terraces suitable to different area and slope including radical terraces and progressive terraces. Progressive terraces are formed by establishing contour bunds with soil or stones in combination with ditches and vegetation as in the Fanyaa Juu. The progressive terraces are formed in time by the natural process of erosion and sedimentation. Contour bunds, a more passive and slower option are mostly preferred.
Bench- or radical terraces are constructed on terrain with steeper slopes. Construction of the bench terraces is labor intensive and expensive.Terracing design takes in consideration the soil and subsoil. With a thin arable layer on rocky surface it is not possible to build terraces. Radical terraces might have lower initial production due to removed top-layer with the organic material and soil micro-organisms. The soil layers below are usually hard and not fertile. By removing and storing the top-layer and putting the top-layer back at the end, the production dip can be reduced. Still then it takes 3 seasons to get back to earlier levels. It is therefore recommended to add additional manure and organic material.
Due to higher infiltration rate on the flattened terrace, nutrients easily dissolve and infiltrate with the water towards the deeper layers and become out of reach of the plant roots. The high water content due to infiltration of rainwater can increase the risk of unstable terraces and even provoke local landslides.
In Rwanda it is recommended to add soil improvers as lime, manure and compost. 30 tons of manure and compost per hectare are necessary to increase the productivity and justify the high construction costs. In the acidic soils it is also recommended to add a minimum of 2,5 tons of lime per hectare.
Instability of newly build terraces can be mitigated by regulating the water flow by diversion ditches and by planting shrubs or low trees and other perennial vegetation on the lower end of the terrace. The roots hold the soil together. Higher trees, on the contrary, might move in the wind causing the soil to loosen making the terrace unstable.
Terraces can be put under hillside irrigation. The parcels right under the irrigation canal are irrigated through furrows or inundation. The terraces that lay 10 meter below the feeder canal can be irrigated by sprinkler or drip irrigation. Managing the irrigation to water the crops requires a high level of skills of the farmers.With the adequate training and assistance the farmers can acquire a sufficient high level over 3 harvest cycles of practicing.
The steep end of the terrace (or riser) needs be protected by ground covering vegetation. The choice should be informed by the function in the production system. It could be for example a crop with natural pest management characteristics (push and pull system) and or other uses like animal feed, mulch trying to avoid competition with the main crop. Vetiver (www.vetiver.org) and Napier are grasses commonly used for erosion control. Napier grass is cut and used as fodder for the cows. The manure of the cows is brought back on the land to replace the nutrients that are taken away with the harvest.
Often land is formed into multiple terraces, giving a stepped appearance. Terraces are common in Rwandas rainfed agriculture. Terraces have the benefit that they are easier for both mechanical and manual sowing and harvesting and irrigation than a steep slope would be. the hill that capture the superficial runoff to increase infiltration on the slope.
Filtration ditches or ditches are like small channels along the contour of the hill that capture the superficial runoff to increase infiltration on the slope. Filtration ditches can be dug in forest or agricultural fields. Some are like sections of a few meters that fill with rainwater (cut-off drains) others are like channels under a small slope or to evacuate the excess water to safe place. This has to be carefully done protecting the soil to avoid the formation or erosion gullies..
Filtration ditches, contour ditches or cutt-off drains are like small chan
And organization last
Without maintenance progressive might dissappear and radical terraces might become unproductive or cause landslides. Progressive terraces take time to have impact so the farmers need to be patient. The loss of terrain by the ditches and vegetation strips and trees (agro forestry) will be initially seen as a loss. to the farmers. Without understanding of the functioning of the terrace and without monitoring by experts many farmers ignore the ditches and vegetation strips.
Therefore, terracing should always be accompanied by an additional production package for the farmers containing seeds, fertilizer and training on crop cultivation to boost the production.
One Acre Fund has ample experiences with financial products for the small holder farmers in Rwanda. With professional support of the One Acre Fund trainers and in a learning environment the farmers use credit to lift the production levels.
The government stimulates the use of chemical fertilizer by giving a subsidy of 50%. The production of fertilizer based on the methane gas from lake Kivu will lower the cost of chemical fertilizer substantially. People need to find out what levels of fertilization best fits the different agroecological zones in Rwanda.
WOCAT RAB 2014 Brochure on radical terraces
Rwanda Agricultural Board (RAB) Land Husbandry, Irrigation and Mechanization Department: Interventions
Research evaluation 2014: Benefits of terraces
Ministry of Agriculture 2015 Profiling of terraces
The social. economic and environmental benefits of terraces
Fanya juu Terraces (Kenya)