Water harvesting in the Jordanian Badia: Trade-offs between micro and macro structures

The Jordanian Badia is a desert region that covers roughly 90% of Jordan. Average annual rainfall in the Badia is below 200 mm and is only occurring during the winter months (Nov – Feb). Despite the dry conditions the Badia it is a vital region to the country, especially for ago-pastoralist communities. Over the past decades unsustainable land management and especially overgrazing have resulted in reduced vegetation cover, soil degradation, and loss of biodiversity. Rainwater harvesting structures are used to regenerate soils, improve vegetation cover and allow barley production in local depressions. Two types of water harvesting structures are currently tested in experimental catchments.

The first type is the Vallerani micro water harvesting structure. A Vallerani is constructed along a hillslope contour and consist of a ridge and furrow. The interspace area between two Vallerani’s is the surface runoff collection area. Inside the furrow native shrubs (Atriplex halimus) are planted and provide fodder for livestock. Vallerani structures are simple and cheap to construct, reduce soil erosion, conserve moisture and stimulate vegetation cover. The second type of rainwater harvesting is the Marab, which is a macro-scale water harvesting technique. A Marab consists of a series of earthen dams that are constructed parallel in a local depression. Surface water from a wadi enters at the upstream end in the Marab and is forced to flow in a zig-zag pattern around the constructed dams. The flow speed of the surface water is slowed down which results in enhanced infiltration. The stored water in the soil is used to grow a barley crop in the interspaces between the constructed dams. Apart from conserving moisture, Marabs retain sediments and help to reduce flash floods in the Badia.

When Vallarani’s and a Marab are both constructed in one catchment there is a trade-off between the two structures. The more Vallerani’s are implemented on hillslopes the less water will flow towards the downstream Marab. In this study modelling was used to optimize the location and number of Vallerani structures and quantify the available water running to the Marab in an experimental catchment. The Soil and Water Assessment Tool (SWAT) was used for this purpose. Rainfall, discharge and soil data were collected from the field during the 2018/2019 rainy season. The calibrated model showed good performance for large events but underestimated smaller events. A 30 year run was made, with and without Vallerani structures. Increasing the number and area of Vallerani structures from zero to the maximum decreased the number of significant runoff events by 45.3%, and the total discharge reaching the Marab by 36.2%. Using SWAT, the number and locations of the Vallerani’s were optimized to ensure that the Marab would receive enough water, while maximizing the number of hillslope water harvesting structures. It is concluded that implementation of Vallerani’s in sub-watersheds that produce low amounts of surface runoff results in most vegetation cover on hillslopes, while allowing sufficient barley production in the Marab.