Implications of land-use alteration on discharge and sediment delivery using hydrological modelling

Sedimentation is a major issue at the Wala Dam, Jordan, and its impact on the serviceable lifetime of the reservoir is the primary driver for proposals to raise the height of the dam. The approach followed in this study involves the application of an optimized SWAT model of the Wala catchment to examine hypothetical and object-based catchment management scenarios, including land-use alteration, on a one-at-a-time basis to simulate discharge and sediment yield delivered to the dam over the period 1979 – 2013. Plantation scenarios study the response of the catchment to cultivation of barley and olive in selected areas. The simulated effect of altering plantation vary spatially with both location and scale. Changes in annual sediment and water delivery to the Wala reservoir are linked to a simple model of dam functional lifetime to establish a rational model framework for integrating hydrological and ecological decision-making in this highly-stressed setting. Considering the Wala Dam raising plan, it is concluded that the current capacity is hypothetically expected to fill up with sediment in 65.63 years based on the existing land-use conditions of the catchment. The ongoing 17 Mm³ expansion of the dam is predicted to lengthen life-span up to about 283 % of the current estimations based on the existing land-use. Longest life-span of the Wala Dam is expected to be achieved by cultivating the northern areas with olive in all cases considereing exisiting and expanded capacity within extreme and average climate conditons. Shortest life-span is relevant to cultivating barley over the whole catchment. Life-span estimated based on an extreme flood condition that occurred in year 1992 varies between 2.92 and 10.02 year. Although such conditions can be rare and highly unpredictable, they must be taken into consideration while designing dams. Land-use alteration plans do not necessarily improve life-span of the dam and therefore, careful studies must investigate end goals and feasibility of management plans. Catchment-scale water and sediment management within the Wala basin is part of a complex system of inter-relationships within the overall framework of the water-energy-food nexus. Retention of water in the landscape for ecological benefit, to the detriment of available resource to support water supplies, carries a significant cost in this context; on the other hand, the model results here suggest that land restoration (at least under the cropping scenarios tested) can be achieved with marginal impacts relative to the benefits of raising the Wala dam. This, however, has its own consequence; by further eroding the downstream export of water to the Dead Sea, schemes such as the Wala contribute to the progressive deterioration of that unique water body, with impacts of global ecological, cultural, economic and geopolitical scale. The variation observed between simulated land-use scenarios emphasises the potential use of catchment modelling to target land restoration measures to those areas where net ecological benefits, including water conservation and reduction of erosion, are maximised.