Terracing

Description

The ancient Andean technique of terracing consists of making cuts in steep slopes to form contour ridges and establish cultivation surfaces that are supported by stone walls. Because the terraces are positioned perpendicular to the flow of water they reduce erosion, retain soil and moisture and thus generate a microclimate conducive to crop growth. While their main purpose is to increase the amount of cropland, they also reduce the slope of the hillsides and thus prevent landslides that might affect structures, dwellings and crop areas that are downstream. Traditional terracing construction and restoration techniques require a large investment of labour, which must be provided by the community.

Location

Historically, terraces were built in the Andean Altiplano to adapt agricultural production to extreme conditions, but they may be built at any altitude range with slopes between 10% and 35%. They are particularly useful on hill or mountain sides with eroded soils for the purpose of increasing the agricultural area and preventing landslides. In the southern hemisphere, if terraces are constructed from east to west, and facing north, the walls absorb more heat, which promotes crop growth.

Threats and Impacts Attended

Transforming the landscape of hillsides prone to erosion into cultivation terraces increases agricultural productivity and food security. Terraces prevent landslides and erosion by reducing the intensity of runoff. They also reduce the risk of drought by raising the soil moisture content and allowing water to slowly infiltrate. Heat is absorbed through the walls, which provides greater thermal regulation and decreases the effects of sudden temperature changes and the likelihood of frost.

Implementation Steps

1. Study the characteristics of the site, such as soil type, relief, precipitation, runoff, sun exposure and the availability of materials with which to construct the terraces.
2. Plot the contour ridges and calculate the slope of the drains.
3. Dig trenches 50 cm deep following the contour ridges.
4. Construct the retaining wall above the trenches, using the largest stones first. The wall will not normally exceed 2 m in height and, if needed, may be reinforced with cementing agents.
5. Fill the terrace with the excavated material and add a surface layer of fertile soil. (6) Perform annual maintenance on the walls to ensure stability. For the reconstruction of ancestral structures, the assistance of technical specialists is recommended.

Inputs and Costs

The construction cost of a 700 m2 terrace on a hillside with a 10% slope, with a retaining wall 100 m long by 1.5 m high, is given below. The main inputs are the labour for excavation, wall construction and filling; stone and seeds; and production and application of organic fertilizers. Five days of annual maintenance and two days of training are assumed.

Economic and Ecosystemic Benefits: Agricultural terraces raise productivity considerably. For example, Altieri (1999) reports that after terraces were restored in a project in Peru the first yields increased by between 43% and 65% for potatoes, maize and barley compared with conventional hillside cultivation. Erosion and soil loss were also prevented to a significant degree. Chow and others (1999) note that annual soil loss for potato crops declined from 20 t/ha on hillside crops to 1 t/ha on terraces with contour channels. Terraces have a highly beneficial economic effect. With the increased yields, the investment is recouped in the first year after construction, and net revenue generated over the following ten years is twice as high as that from conventional hillside cultivation (Rist and San Martín, 1993).

Limiting Factors

The main difficulty in building these structures is the amount of labour required. The reconstruction of one hectare of terraces is estimated to require up to 2000 worker-days (Altieri, 1999). In addition, on slopes greater than 35% construction and maintenance are more difficult. As the measure has to be implemented by the community, good organization is essential, as is motivation so that participants will assume ownership of the production method and be responsible for its maintenance.

Lessons Learned

Community support schemes in terrace-recovery and restoration projects have been successful when low-interest loans are granted for agricultural inputs in exchange for labour to reconstruct specific areas of the terraces. It is advisable that civil society organizations provide support by coordinating efforts and giving technical advice. Implementation is more effective if farmers are organized and have specific production aims based on sustainable practices.

Additional Considerations

Terraces can be used as an element of risk management, benefiting entire communities. The Andean highlands are estimated to have more than 500,000 ha of terraces, of which 75% need restoration. Given the reported benefits, this measure has a high value for ecosystem-based adaptation because it increases communities’ overall resilience.

Units to Monitor Project Progress

Area of terraces constructed or restored (m2).

Unites to Monitor Measure's Impact

Additional income (US$/year); inhabitants with terrace protection (number).

References

• Altieri, M. (1999). “Applying agroecology to enhance the productivity of peasant farming in farming systems in Latin America”, Environment, Development and Sustainability vol. 1 No. 3-4, pp. 197–217.
• Rist, S. and J. San Martin (1993). Agroecología y saber campesino en la conservación de suelos. 2nd ed. AGRUCO. Universidad de Cochabamba, Bolivia.
• Chow T., W. Rees and J. Daigle (1999). “Effectiveness of terraces/grassed waterway systems for soil and water conservation: A field evaluation”, Journal of Soil and Water Conservation vol. 54, No. 3 577-583
• Proyecto de Manejo de Recursos Naturales (2008). Guía metodológica para la rehabilitación y construcción de terrazas prehispánicas. La Paz, Bolivia.