Water harvesting techniques based on terrain modification enhance vegetation survival in dryland restoration

Abstract

To successfully restore drylands, where the scarcity of water is one of the main limiting factors for plant survival, water inputs should be enhanced as much as possible. A specific type of water harvesting that concentrates runoff generated in bare areas upslope (runoff source areas) in a planting area downslope (runoff sink area) is an effective technique that, unlike traditional restoration, does not entail much over-cost. The objective of this study was to analyze the effectiveness of terrain modification techniques, aimed at water harvesting, on a degraded hillslope at a quarry restoration site in an arid area (Almeria, southeast Spain). Two different terrain modifications, to harvest surface runoff and increase soil water retention, were applied at two different hillslope sectors: a) micro-catchments were constructed on the upper section of the hillslope, where the slope gradient was high and b) narrow terracing was done in the middle part of the hillslope, which was characterized by moderate slope gradients. An additional sector, not subjected to terrain modification treatment, was used as a control, and was set in the lower part of the hillslope where the slope gradient was the lowest. A similar planting system was adopted for all experimental treatments; planting was done in 12 fertility island patches. Within each patch 12 different autochthonous plant species were planted in a staggered pattern with 80 cm separation. All plant species, which were from a nursery, were planted within a 1 L container made from organic fibers and containing compost from plant residues enriched with zeolites. To determine the microtopographical modifications required in the analysis of the optimal water harvesting strategy, a digital elevation model, with 0.2 m spatial resolution, was created from the data generated using a terrestrial laser scanner. Runoff flow direction and contribution areas, as well as a set of topographical attributes strongly related to the terrain water retention capacity (wetness index, slope gradient, aspect, USLE LSF factor, etc.), were calculated. Results indicated that micro-catchments had the highest plant survival rates and vegetation coverage values because they were the terrain treatment areas with the largest capacity to retain run-on from contributing areas. The areas without terrain modification (control areas) showed the highest plant mortality.