Soil hydrology for ecohydrology in drylands: examples from Tunisia, China and Cape Verde Islands: a review

A fundamental factor for developing and maintaining the susceptibility of dryland ecosystems is the efficient use of available water. Because of pressure on fresh water resources, practical tools are needed for using more efficiently the rain water, this by regulating soil hydrological processes. For this purpose, 'water harvesting' techniques, referring to small hydraulic structures to collect runoff water, are widely used in arid and semi-arid regions. This paper evaluates three such structures. In Tunisia the indigenous 'jessour' terraces and ground water recharge systems are described. It was found that the CCR (catchment to cropping area ratio) should be larger than 7.4 in order to provide sufficient amounts of runoff water to the terrace for the cultivation of olive trees for an 'average' annual rainfall amount of 235 mm. In China a field experiment was set up in the semi-arid region of the loess plateau to evaluate different tillage practices on their effect on increasing the water storage in the soil profile for better crop growth. It was concluded that subsoiling was the best practice in terms of water conservation. It resulted in the highest increase in water storage during the fallow period, and hence more water was made available to winter wheat during the growing season. On the Cape Verde Islands, with low annual rainfall (100-500 mm), mainly occurring during a few major rainfall events, three land use systems were compared in three adjacent small catchments. A catchment with traditional agriculture and one with natural vegetation, both without soil and water conservation systems, were compared with a catchment with trees in water collecting bench terraces and microcatchments. Lower runoff amounts measured during the third year of monitoring reflected the newly installed soil and water conservation practices in the afforested watershed.