Abstract
AbstractThe development of surface hydrological connectivity is a key determinant of flood magnitude in drylands. Thresholds in runoff response may be reached when isolated runoff‐generating areas connect with each other to form continuous links to river channels, enabling these areas to contribute to flood hydrographs. Such threshold behaviour explains observed nonlinearities and scale dependencies of dryland rainfall–runoff relationships and complicates attempts at flood prediction. However, field methods for measuring the propensity of a surface to transmit water downslope are lacking, and conventional techniques of infiltration measurement are often inappropriate for use on non‐agricultural drylands. Here, we argue for a reconceptualization of the dryland surface runoff process, suggesting that the downslope transfer of water should be considered alongside surface infiltration; that is, there is a need for the “aggregated” measurement of infiltration and overland flow hydraulics. Surface application of a set volume of water at a standardized rate generates runoff that travels downslope; the distance it travels downslope is determined by infiltration along the flow, integration of flow paths, and flow resistance. We demonstrate the potential of such a combined measurement system coupled with structure‐from‐motion photogrammetry to identify surface controls on runoff generation and transfer on dryland hillslopes, with vegetation, slope, surface stone cover, and surface roughness all having a significant effect. The measurement system has been used on slopes up to 37° compared with the flat surface typically required for infiltration methods. On average, the field workflow takes ~10–15 min, considerably quicker than rainfall simulation. A wider variety of surfaces can be sampled with relative ease, as the method is not restricted to stone and vegetation‐free land. We argue that this aggregated measurement represents surface connectivity and dryland runoff response better than standard hydrological approaches and can be applied on a much greater variety of dryland surfaces.
Highlights
Drylands cover approximately 41% of the Earth’s land surface (Middleton & Thomas, 1997) and are home to over 38% of the planet's population (Huang et al, 2017)
Representing the runoff response of dryland surfaces via the use of standardised surface flows is fundamentally different from conventional methods that typically focus on point measurements of infiltration
We have shown that the aggregation of infiltration and surface runoff dynamics into measurements of runoff dimensions is readily applicable to a range of dryland surfaces
Summary
Drylands cover approximately 41% of the Earth’s land surface (Middleton & Thomas, 1997) and are home to over 38% of the planet's population (Huang et al, 2017). The seasonal (or permanent) moisture deficit of drylands means that many river flows are ephemeral; when floods occur, they can be torrential and flashy. Modelling the development of such floods is challenging because large surface runoff generating areas may be initially isolated from the channel network by downslope infiltration (Ambroise, 2004). When surface hydrological connections between surface runoff source areas and river channels are eventually established, nonlinearities are introduced in rainfall-runoff relationships as source areas begin to contribute towards river discharge (Bracken & Croke, 2007; Smith, Bracken, & Cox, 2010; Wainwright & Bracken, 2011). Expected to generate some catchment runoff response” Understanding how ground surface characteristics, infiltration and overland flow hydraulics influence surface hydrological connectivity in drylands is paramount to identifying areas vulnerable to erosion and flash flooding and informing targeted catchment management strategies
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