Towards Standardising Runoff Connectivity Assessment at the Hillslope Scale in Drylands Using Structural Trait (De)composite.

Hydrological connectivity at the hillslope scale is a complex, spatially explicit phenomenon where surface and subsurface processes converge and interact, including infiltration, runoff, and lateral flow occurring during a singular rainfall event under specific antecedent soil moisture conditions.

In drylands, where Hortonian runoff generation prevails, such complexity has been conceptually simplified for operational purposes by using connectivity as a proxy for assessing ecosystem "health" or land degradation. Grounded in the current source-sink paradigm, a binary scheme of vegetation (pure sinks) and bare (pure sources) areas is used to distribute potential overland flow according to topography. The connectivity character is then distilled through the concept of Flow Length, with different metrics proposed under this rationale.

Despite this operational simplicity, the quantification of connectivity has yet to reach a standardized status, hindering intercomparison studies and the establishment of assessment baselines for land degradation.

Within the same framework umbrella, we recognize its shortcomings and propose decomposing the connectivity issue into three spatially explicit traits, each representing distinct structural features that emerge at the hillslope scale. This analytical approach aims to separately evaluate the contributions of vegetation patterns and flow routing, without the constraint of the hillslope shape. Facing the challenge of integrating these traits into a unified, synthetic metric for assessing runoff connectivity, we discuss several alternatives. The study is conducted at the experimental site in Benidorm (Alicante, Spain), using UAS-derived orthophotos and DEMs, where lateral variations within a small catchment serve to test the suitability of the proposal. This methodological proposal aims to advance the conceptual discussion toward developing a standardized approach for runoff connectivity evaluation and to inform land degradation assessments in drylands.