A terrace is a levelled surface used in farming to cultivate sloping, hilly or mountainous terrain. They can be used on relatively flat land in cases where soil and climate conditions are conducive to erosion. Terraced fields are effective for growing a wide range of crops such as rice, potatoes, maize, olive trees, and vineyards. Terraces have four main functions (Gonzales de Olarte and Trivelli 1999):
1) Improve the natural conditions for agricultural production;
2) Decrease the rate of erosion;
3) Increase soil moisture; and
4) Generate positive environmental benefits.
Slow-forming terraces are constructed from a combination of infiltration ditches, hedgerows and earth or stone walls. This technology decreases superficial water run-off, increasing water infiltration and intercepting the soil sediment (UNESCO-ROSTLAC, 1997). Slow-forming terraces are called as such because they take between three and five years, and possibly even ten years, to fully develop.
Slow-forming terraces can be built where the land is marginally to steeply inclined and where the soil is sufficiently deep to create a drag effect. This leads to the formation of steps as sediment accumulates due to rainfall and natural gravity. Level ditches are traced and excavated along the contour line of a slope and then an embankment of earth, stones or plants is constructed at regular intervals. Eroded soil accumulates in these buffer strips every year and terraces slowly form. To avoid intensive rains breaking buffers strips, a one to two per cent inclination is recommended (Fantappiè, no date).
Depending on soil type, ditches should generally be dug 40 cm wide and 40 cm deep. The recommended length of the terrace is between 50 and 80 metres and the height of the slope should be the same as the height of the earth or stone ditches (Soluciones Prácticas-ITDG, 2007).
The best plants to cultivate along the buffer strips should be resistant to local conditions and grow well and fast. Where possible, plants should be used that can provide wood for fuel and feed for livestock. Where possible, leguminous species should be planted to improve nitrogen supply to the soil (Fantappiè, no date). The structure of slow- forming terraces is shown in Figure 1 below.
Lower investment options have been developed that are also effective in trapping sediment but do not require the building of physical structures. One option is the use of contour planted hedgerows (Young 1997), this system has been used on 10,000 hectares of land in the Philippines, Rwanda and Haiti. Double hedges of Leucaena, Gliricidia or similar shrubs are planted 4 to 8 metres apart along the contour. The shrubs are pruned two or three times per year and the leaf and branch material applied to the soil or against the stems of the shrubs, to trap the moving sediment. This leads to the formation of terraces up to 50 cm high in the first two to three years. Another alternative is to use deep rooting grass species such as Vetiver or Panicum bunch grass often used for cut and carry fodder. An even simpler method is to leave natural vegetative strips to when preparing the soil for planting, which gradually form the stabilised edges of terraces (ICRAF, 1996). These different live-barrier methods of terracing reduce erosion from half to just 2 per cent of the level without the live-barriers, and rainfall infiltration is also significantly improved.
This technology facilitates adaptation to climate change by optimising water use. This is particularly relevant in areas that depend on melting glaciers for their water supply and where there is uncertainty about future rainfall patterns, as occurs in Andean highland areas, for example. Climate variability also affects the soil, since heavy rainfall coupled with poor soil management give rise to landslides and mudslides. In this respect, slow-formation terraces reduce soil erosion and, consequently, the danger of large landslides occurring.
Terraces also provide a method for regulating the micro-climate for agricultural production. By capturing the sun’s heat in the rock walls, terraces absorb heat during the daytime and release this nightly helping to create a slightly warmer internal micro-climate which can protect crops from frosts, prolong the growing season and allow for crop diversification (Mars, 2005).
Slow-forming terraces allow for the development of larger areas of arable land in rugged terrain and can facilitate modern cropping techniques such as mechanisation, irrigation and transportation on sloping land. They increase the moisture content of the soil by retaining a larger quantity of water. They capture run-off which can be diverted through irrigation channels at a controlled speed to prevent soil erosion. They increase soil exposure to the sun and they replenish the soil and maintain its fertility as the sediments are deposited in each level, increasing the content of organic matter and preserving biodiversity.
Slow-forming terraces have also been shown to increase crop productivity. Research conducted in Peru found that the highest response to the effect of slow formation terraces on productivity was for peas (Table 1). Maize, fava beans and potatoes also improved their productivity. The most important reason for this increase is assigned to increased/enhanced water retention.
Table 1: Crop yields (kg/ha) for the main crops in La Encanada, Peru
|Crops||Terraced fields||Non-terraced fields||Increase (%)|
Source: Ganoza, 1988; CONDESAN, 1995; 60.
In Bolivia, slow-forming terraces have been also found to improve crop yield by between 25 and 75 per cent (Table 2).
Table 2: Crop Yields (t/ha) in the Community of Chullpa K'asa, Bolivia
|Crops||Terraced fields||Non-terraced fields||Increase (%)|
Source: based on Delgadillo and Delgado, 2003
Terraces made using hedgerows or grass strips have the advantage that the material pruned from them can be used as fodder for livestock. Also, the system takes up less space occupying only 10-15 per cent of the land as opposed to 20-30 per cent for ditch and bank terraces, they also require considerably less work to establish.
In terms of limitations, an economic analysis of terrace investments in the Peruvian Andes has shown that if implemented on a regional-scale, slow-forming terraces can produce varied and sometimes limited returns. Where farmers must pay the full costs of investments, returns can be as low as 10 per cent (Antle et al, 2004). Profitability will depend on additional factors such as interest rates, investment costs and maintenance costs. Cost-benefit analysis should, however, take account of other factors including increased soil productivity and conservation benefits. In addition, slow-formation terraces are formed over a long period of between three and five years, which means that their positive effects are not immediate.
Terraces formed with hedgerows or grasses can compete with associated crops is they are not sufficiently pruned. Generally, this technology is less effective on slopes of more than 30 per cent if hedges are more than 4 m apart.
The most costly component of terrace construction is labour which will depend on average local daily wages. The time required to construct a slow-forming terrace will depend on available manpower, the type of soil and the time of year. The basic tools required (such as picks and shovels) usually belong to the farmer and can be used at no extra cost. Once built, annual maintenance costs are minimal (Treacey, 1989). Research indicates that two people can build 7m² of wall in one day. Assuming a common size terrace wall of dimensions 1.8m high and 50m long, two people could restore an entire terrace in two weeks, or build an entirely new one in a slightly longer period of time (Valdivia, 2002). In a project in northern Peru, an initial investment of $350/h was required with $86/h per annum for maintenance (Yanggen et al, 2003).
Knowledge of terrace design, construction and maintenance, including contouring or levelling techniques as well as knowledge of crops suited to slow-terrace irrigation is required. The most reliable method for defining the contour is the A-frame method (http://www.appropedia.org/The_'A'_frame_(Practical_Action_Brief)). To make a judgement on the cost-benefit ratio of a slow-forming terrace scheme, information on capital investments and likely economic returns will be necessary.
Slow-forming terraces can be implemented at farm-level without specific institutional and organisational arrangements. Notwithstanding, local government agencies can provide assistance in the form of technology transfer and training and subsidies. In terms of social organisation, advantage should be taken of communal work ethics and other mutual cooperation systems for faster construction and more efficient maintenance.
Barriers to implementation include lack of access to credit by farmers and the slow-rate of return in terms of the time it takes for crop yields to increase, which can take as long as ten years (Yanggen et al, 2003). This can lead to farmers abandoning the technology if long-term benefits are not fully understood. Yanggen et al (2003) estimate that in the Peruvian case, subsidies of around 40 per cent of the total cost of implementing slow-forming terraces would be required to make this technology an attractive alternative to farmers. Given the length of time required for results, lack of access to land or land rights could prevent a farmer from adopting this technology over traditional practices. This is because farmers with precarious forms of land tenancy tend to have shorter planning horizons and view permanent structures requiring long-term investments as riskier (Dvorak, 1996).
The reduction in available land area for cultivation due to the space taken by the ditch and banks, or vegetation strips can be a significant disincentive for farmers with very limited access to land. Also the land cultivated is rented from another land-owner there is little incentive to invest in soil conservation.
Terrace construction can provide an opportunity for improvements in soil, crop and water management practices. These in turn can provide opportunities for farmers to increase crop yields and diversify agricultural production to generate additional income.
Antle, J. M., J. J. Stoorvogel and R. O. Valdivia (2004) Assessing the economic impacts of agricultural carbon sequestration: Terraces and agro-forestry in the Peruvian Andes, Agriculture, Ecosystems & Environment 122(4), 435-445
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