Costs and benefits of optimizing small rainwater harvesting systems for preventing land degradation

The availability of water resources is one of the main factors governing the development of arid regions. Rainwater agriculture can be implemented as a sustainable solution for the preventing desertification. Efficiency of rainwater harvesting depends on two main components: 1) water collecting; and, 2) water storing. The first aim of the study is increasing efficiency of scarce rainwater resources management and providing  the secure tool for preventing land degradation by erosion by means of small rainwater harvesting systems – microcatchments. The second aim of the study is cost vs benefits analysis of the suggested management approach. The results show that increasing runoff coefficient beyond 12% may be achieved only by mulching the runoff generating area, at least at loessial soils. Deepening the pit depth has a significant effect on soil water losses through evaporation with almost linear efficiency increasing with pit’s depth increasing. The practice that gave the best results both water and cost wise was collecting  rainwater into a shallow trench of 20 cm deep and 1m wide with sealed trench walls and additional crop planted at the trench bottom and an tree or shrub next to the trench. In order to increase the efficiency of water collection and conservation, we built an experimental system that included the following characterisitics: 1) the runoff generation area was mulched with polyethelene sheets and 2) the water collection area was built in two variants, 2.1) as a shallow trench of 20 cm deep and 1m wide with sealed trench walls and  Narcissus flowers planted at the trench bottom and an olive tree next to the trench, and 2.2) a deep trench of 1m deep and 1m wide with  olive trees planted inside. In both cases, we got at least a 10-fold runoff amount increase when even very low intensity rain created runoff over the polyethelene mulching. In the 2.1 case, water infiltrated only through the pit bottom to the deeper layer, and evaporation was better prevented. The main idea of planting flowers was to show the possibility of growing additional plants along with trees in microcatchments.  Since flower roots take water from a very shallow depth and don't compete with the tree root , such an approach could be considered beneficial. Flowers naturally destroy the crust at the pit bottom during the flower bursting and enrich the soil with organic material after the flowers die. Monitoring five years after the system’s implementation showed the succes of the system, both in runoff accumulation and in plant growth. In the 2.2 case, water infiltrated through the bottom and the walls, but evaporation was minimal because of the system depth. The cost of microcathment building depends on the type has supplied enough runoff for excellent tree growth. The third part of the study was cost vs benefits analysis of the suggested optimization. The results show that the highest benefit may be achieved by mulching the runoff generation area with the locally available materials (gypsum, stones etc) and applying the variant 2.1 for water collection.