The importance of the spatial pattern of vegetation for hydrological behavior in semiarid environments is widely acknowledged. However, there is little empirical work testing the hypothetical covariation between vegetation spatial structure and hillslope water and sediment fluxes. We evaluated the relationships between vegetation structural attributes (spatial pattern, functional diversity), soil surface properties (crust, stone, plant, and ground cover, and particle size distribution) and hillslope hydrologic functioning in a semiarid Mediterranean landscape; in particular, we tested whether decreasing patch density or coarsening plant spatial pattern would increase runoff and sediment yield at the hillslope scale. Runoff and sediment yield were measured over a 45-month period on nine 8 • 2-m plots that varied in vegetation type and spatial pattern. We grouped vegetation into functional types and derived plant spatial pattern attributes from field plot maps processed through a GIS system. We found that there was an inverse relationship between patch density and runoff, and that both runoff and sediment yields increased as the spatial pattern of vegetation coarsened. Vegetation pattern attributes and plant functional diversity were better related to runoff and sediment yield than soil surface properties. However, a significant relationship was found between physical crust cover and plant spatial pattern. Our results present empirical evidence for the direct relationship between the hydrologic functioning of semiarid lands and both the spatial pattern and the functional diversity of perennial vegetation, and suggest that plant spatial pattern, physical crust cover, and functional diversity may be linked through feedback mechanisms.

[1] The connectivity of runoff sources is considered one of the main factors controlling the hydrology of sparsely vegetated landscapes. However, the empirical demonstration of this role is very limited, partly because of the scarcity of suitable connectivity metrics. In this work, we derived and tested a spatial metric, Flowlength, for quantifying the connectivity of runoff source areas considering both vegetation pattern and topography. Flowlength is calculated as the average of the runoff pathway lengths from all the cells in a raster-based map of the target site. We evaluated the relationships between the connectivity of runoff sources, measured with Flowlength, and the runoff and sediment yields from six plots and three catchments in semiarid southeast Spain. Flowlength distinguished varying degrees of connectivity between differing vegetation patterns with similar vegetation cover. The connectivity increased with the grain size of the bare areas and was positively related to plot runoff and sediment yields. Flowlength also correctly ranked the three catchments according to total runoff yielded during the study period. The inclusion of microtopographic information in the quantification of Flowlength improved the relationships between the pattern of runoff sources and the measured fluxes, highlighting the importance of topographic features in the connectivity of surface flows. In general, the microtopography had a net decreasing effect on the connectivity, which was mainly attributed to an increase in the amount of runoff sink areas caused by the sediment terracettes developed upslope of plants. Our results confirm that the connectivity of runoff sources is a key factor controlling runoff and erosion in semiarid lands and support the potential of Flowlength as a surrogate for the hydrological functioning of ecosystems with patchy vegetation.Citation: Mayor, Á . G., S. Bautista, E. E. Small, M. Dixon, and J. Bellot (2008), Measurement of the connectivity of runoff source areas as determined by vegetation pattern and topography: A tool for assessing potential water and soil losses in drylands, Water Resour. Res., 44, W10423,

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In semiarid ecosystems, the transfer of water, sediments, and nutrients from bare to vegetated areas is known to be crucial to ecosystem functioning. Rainfall simulation experiments were performed on bare-soil and vegetated surfaces, on both wet and dry soils, in semiarid shrub-steppe landscapes of SE Spain to investigate the spatial and temporal factors and interactions that control the fi ne-scale variation in water infi ltration, runoff and soil loss, and hence the water and sediment fl ows in these areas. Three types of shrub-steppe landscapes varying in plant community and physiography, and four types of plant patches (oak shrub, subshrub, tussock grass, and short grass mixed with chamaephytes) were studied. Higher infi ltration and lower runoff and soil loss were measured on vegetation patches than on bare soils, for both dry and wet conditions. The oak-shrub patches produced no runoff, while the subshrub patches showed the highest runoff and soil loss. Despite these differences among patch types, the infl uence of vegetation patch type on the variables analysed was not signifi cant. The response of bare soil surfaces clearly varied between landscape types, yet the differences were only relevant under dry soil conditions. Stone cover, particularly the cover of embedded stones, and crust cover, were the key explanatory variables for the hydrological behaviour of bare soils. The study documents quantitatively how bare soils and vegetation patches function as runoff sources and runoff sinks, respectively, for a wide range of soil moisture conditions, and illustrates that landscape-type effects on bare-soil runoff sources may also exert an important control on the site hydrology, while the role of the vegetation patch type is less important. The effects of the control factors are modulated by antecedent soil moisture, with dry soils showing the most contrasting soil water infi ltration between landscapes and surface types.