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.
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.
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.
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.
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.
for more information contact:
dr. erwin bastian
geographisches institut der universitat hannover
schneiderberg 50, 3000 hannover 1
federal republic of germany