"medias lunas' rejuvenate bolivian farmland

the tarija basin of bolivia lies between the central southern mountain ranges of the sub-andean sierra zones and the eastern cordillera. fine water deposited sediments composed primarily of silty or clay soil types make up the predominate soil type in the central area of the basin. the basin has a semiarid climate with distinct rainy and dry seasons. the precipitation occurs as brief showers often separated by dry spells which endanger new growth. during the dry season the soil root zone dries out completely.

for the past three centuries the basin area has been severely degraded. the dry forest was destroyed, and followed by improper working of the soil and overgrazing by cattle, sheep and goats. today the degradation continues. the topsoil in many areas has been completely washed away, leaving a surface nearly devoid of vegetation. the major erosive processes in the basin include sheet wash, gully and gorge formation.

a joint german-bolivian development project funded by gtz is using a unique water catchment technique-'medias lunas' or crescent-shaped depressions-to improve the soil conditions and permit revegetation. medias lunas were developed by the ancient nabateans of the middle east over 2,000 years and recently 'reinvented' in the negev desert of israel.

the purpose of the medias lunas is to: prevent surface runoff, improve soil-water conditions within the medias lunas, and to provide an optimum environment for plant growth. in this project the farmers planted native and exotic multipurpose trees for the production of fodder to be fed to goats kept in stalls. the researchers are also exploring the use of medias lunas for producing food crops in a preliminary trial.

constructing medias lunas

the area to be afforested is covered by a network of medias lunas with a wall of earth piled up on the downhill side. each media luna comprises a catchment area of about 14m2. the open side of the crescent is on the uphill side. maximum acceptable surface slope for medias lunas is about 15.

medias lunas should be established during the last 11/2 months of the rainy season when the topsoil is moist and the ridge of earth on the downhill side can be compacted more easily than during the dry season. ridges made in the dry season are too loose and are damaged by rain.

effort must be spent during the first year to ensure that the earth ridges do not break or that newly established medias lunas are not filled with sediment too quickly. the medias lunas can be allowed to fill in over several years since by this time the improved soil-water conditions in the medias lunas have led to the development of a vegetative cover.

planting medias lunas

medias lunas can be planted as soon as about 30 mm of rain have fallen, while traditional hole planting must wait for uniformly distributed rainfall. thus planting in medias lunas can extend the first growing season for the young trees by about 2 to 3 months. species planted in the medias lunas included: acacia cyanophylla, a. aromo, a. visco, a. caven, prosopis alba, and leucaena pulverulenta. all of these trees are multipurpose legumes and were chosen based on their livestock feed value as well as their soil improvement capacity.

farmer participation

the project began in 1984 with an intensive campaign by project staff to inform the local people of the necessity of carrying out a soil conservation and afforestation practices. regular meetings with them led to the formation of a group of interested farmers which then drew up a contract with the researchers to define their responsibilities. a clause was included in the contract which guaranteed that existing property rights would not be changed in any way in order to allay the farmers' fears that the land once improved might be taken from them.

persons living in the project region were then commissioned on a contract basis and paid by the project to make the medias lunas, but the farmers carried out the planting work themselves-two trees per media luna, thus significantly reducing the afforestation cost per hectare. the owners were contractually obliged to provide subsequent protection of the trees against livestock and ants. the farmers pledged to protect the area from browsing animals by surrounding it with a 'dead' hedge made of thorny branches, around which they planted a live fence to guarantee that would keep livestock away permanently. the materials for these purposes were provided and delivered to the farmers free of charge. if the medias lunas are allowed to develop undisturbed, then within 3 to 4 years the ground will be covered to 90% by vegetation during the rainy season.

about 60 ha were afforested during the first project year. the total cost of making, planting and maintaining the medias lunas was u.s. $280 per ha. planting materials were the most expensive, at u.s. $150 per ha.

impact on soil conditions

measurements were carried out throughout the rainy season to study the effect of the medias lunas on water catchment and infiltration. they could then compare the treated to the untreated sites. while as much as 70% of the rainfall ran off without being absorbed in the unaltered areas, infiltration rates of 50-60 mm/h, and in extreme cases up to 150 mm/h, were observed in medias lunas. in medias lunas both surface runoff and soil loss (less than 1 mt/ha/y) remained low even with high-intensity rainfall. other measurements taken during the rainy season showed significantly higher moisture content under the medias lunas than under unimproved surfaces. the infiltration study of the medias lunas also showed that newly established medias lunas are just as effective at improving soil moisture as those that are several years old.

the improvement of soil moisture in the medias lunas permitted spontaneous revegetation, and in some cases the conditions were wet enough that even river bank species were found in medias lunas. it is only in the medias lunas that plants are able to complete their developmental cycles and bear ripe seeds. as a result of the large amounts of leaf litter which accumulate on the ground, an intensive network of roots develops in the soil, causing a high rain interception rate and good shading of the ground. the measurements confirm that soil moisture even improves satisfactorily in silty and clayey substrates, which have so far posed problems for afforestation.

crop production in medias lunas

since 1987, trials have been under way with the aim of integrating crop production into the afforestation sites. in the trials they experimented with planting lablab (lablab purpureus/highworth) on the earth ridges of the catchments, and have had promising results so far.

in view of the known high nutritive value of lablab as animal forage, this represents an important step in the direction of the cultivation of livestock feed for stall-fed goats. a fodder mixture consisting of acacia cyanophylla leaves, prosopis pods and lablab, for example, was readily accepted by the animals. an agricultural component of the medias lunas, if successful, would motivate other farmers to construct and plant medias lunas, and the highly beneficial effects for the soil would improve overall productivity of the area and halt the devastating erosion.

bastian, erwin, and werner grfe (1989) afforestation with 'multipurpose trees' in 'media lunas', a case study from the tarija basin, bolivia. agroforestry systems 9:93-126, 1989.

for more information contact:

dr. erwin bastian
geographisches institut der universitat hannover
schneiderberg 50, 3000 hannover 1
federal republic of germany