Scales and processes of water and sediment redistribution in drylands: results from the Rambla Honda field site in Southeast Spain
Arid lands are characterised by a combination of high temporal variability of rainfall and spatial heterogeneity of soil surface properties. In response to these environmental conditions, sources and sinks of runoff water and sediments tend to be organised in mosaics with distinct spatial attributes. These patterns can be identified at several scales, each with a predominance of a different set of processes. The dynamic relationships between these patterns and processes are an essential aspect of spatial connectivity in arid landscapes. During the last six years, part of the research at Rambla Honda, a field site in Southeast Spain operating under the MEDALUS project, has been concerned with this subject. This paper reviews the results obtained up to date at the patch and the hillslope scales. The research at the patch scale focused on the role of vegetation as a source of spatial heterogeneity that affects short-range redistribution patterns of water and sediments. The approach has been to identify the dynamic relationships between plant clumps and bare ground in sparse vegetation mosaics, using field observations, experiments and simulation models. Field observations included runoff and sediment yield measurements on bounded plots and hillslope sectors, analysis of spatial correlation structures, as well as physiological and architectural properties of plant functional types. Experiments included rainfall simulation and runoff exclusion in the field, and soil fertility bioassays both in the field and the laboratory. A cellular automata model was built to explore the interactions between plant clumps and sediment movement. The research at the hillslope scale was concerned with the long-range transference of water and sediments from rocky upperslopes to their footslope sediment fill. The approach was based on an analysis of the available information about spatial patterns of soil moisture and discharge of runoff and sediments from plots and stream gauges in a first order catchment. Results show that, at the patch scale, in sparse vegetation, a range of positive feedback mechanisms lead to nucleation, or to the increase of spatial heterogeneity, by concentrating resources in the soil beneath plant clumps at the expense of the neighbouring bare ground. This spatial heterogeneity arises dynamically through the interaction between plant growth and hillslope fluxes of water and sediments. Within specific boundary conditions, this interaction is `tuned' towards the formation of mosaics of bare and vegetated patches with patterns that minimise redistribution lengths of water and sediments. The boundary conditions that affect the `tuning' process include factors that determine the potential distance and transport capacity of runoff, such as temporal variability of rainfall, slope angle, slope length, among others, and plant specific factors that affect the efficiency of plant clumps in trapping the resources that are redistributed on the hillslope. At the hillslope scale, the transference of sediment and water between hillslope elements requires very specific within-event temporal distributions of rainfall that allow for the widespread formation of a saturated layer at shallow depth and overland flow to reach first order channels. During most rainfall events these conditions are not met and, therefore, in most seasons, mean values of soil moisture do not increase downhill, and rather reflect variation in local soil properties than the effects of lateral redistribution processes. As a consequence, it may be expected that small changes of the frequency distribution of rainfall characteristics, in terms of within-storm temporal distribution of intensities, could lead to significant changes in soil moisture patterns and hydrologic connectivity between hillslope elements.