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

Abstract

[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,

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

“…Thus, results support previous research finding that woody encroachment in drylands can result in a major change in ecosystem structure [Bai et al, 2009;Bird et al, 2002;Havstad et al, 2006;Schlesinger et al, 1990], characterized by an increased heterogeneity of vegetation and SOC distribution [Puttock et al, 2013]. Reduction in vegetation cover following woody encroachment decreases vegetative protection of the soil [Ludwig et al, 2005] and increases connectivity [Mayor et al, 2008;Wainwright et al, 2011], leading to accentuated water erosion fluxes [Parsons et al, 1996;Wilcox et al, 2003a]. Greater variation in SOC contents at the woody sites (Table 2) supports the theory that the alteration of hydrological functions associated with woody encroachment results in the occurrence of positive feedback mechanisms [Peters and Herrick, 2001] reinforcing the heterogeneous woody landscape structure into islands of fertility [Schlesinger et al, 1990].…”

Soil indicators to assess the effectiveness of restoration strategies in dryland ecosystems

“…For instance, the Flowlength is a spatially based index that effectively relates the connectivity of source areas and vegetation distribution and topography (Mayor et al, 2008). Borselli et al (2008) developed a GIS-based connectivity index and a field validation procedure that assesses the links between source and sink areas at the hillslope and landscape scales.…”

Integrating knowledge exchange and the assessment of dryland management alternatives – A learning-centered participatory approach

“…However, as general recommendations, data gathering should focus on simple metrics and standard methods which could be easily adopted for future monitoring programs, and should meet the basic requirements of replicability and comparability. For example, a variety of indices that are based on vegetation cover and pattern metrics have proven to be useful as indicators for water (and soil) conservation potential in drylands (e.g., Tongway and Hindley, 2004;Mayor et al, 2008;Mayor and Bautista, 2012). Regarding carbon sequestration, the most frequently used indicators are soil organic carbon (SOC) and above-ground biomass.…”