New methodological approach to characterize dryland′s ecohydrological functionality on the basis of Balance between Connectivity and potential Water Retention Capacity (BalanCR)

Drylands are ecohydrologically-coupled ecosystems whose functioning depends on the interplay between hydrological connectivity between runoff source areas and the capacity of vegetation to retain water fluxes and associated resources. In this study we present a new easily applicable methodology for the ecohydrological characterization of dryland ecosystem functioning grounded in the balance between these two strongly interrelated processes using easily obtainable remote sensing data (e.g. UAV and SENTINEL-2 images), the BalanCR method (Balance between Connectivity and potential Water Retention Capacity). This methodology was first tested on synthetic hillslopes representing different configurations of the patchy and heterogenic distribution of vegetation in drylands. The analysis of these synthetic vegetation spatial patterns involving different vegetation patch densities, sizes, and fractional coverage values showed that BalanCR properly characterizes the expected ecohydrological interactions between potential conditions of runoff connectivity and water retention by plants operating in drylands. In a second step, we applied the BalanCR method on four semiarid hillslopes along an altitudinal aridity gradient covered by Mediterranean alpha steppes at very detailed spatial resolution (0.2 m) and at medium resolution (10 m). The obtained results were validated based on soil moisture data and vegetation greening and clearly recognized the four study sites as functional ecosystems, with very low water resource losses, and a pattern of increasing water redistribution processes as vegetation coverage declines. However, the sensitivity of methodology depends on the resolution of the input data (vegetation map and Digital Elevation Model; DEM), and the expected positive effect of small vegetation structures (vegetation patches smaller than the pixel size) on water redistribution is underestimated. Even in this case, the functionality and connectivity of the analyzed sites is correctly characterized as ecosystems showed similar values of both components for the methodology BalanC (hydrological connectivity component) and BalanR (potential water retention capacity component) than those obtained at very detailed scale, with a similar pattern of water allocation values in response to increased aridity. Thus, the proposed metric represents a promising tool for the proper evaluation of dryland conditions and to incorporate hillslope processes in climate change models, which is one of the main gaps to better understand the drylands response upon ongoing climate change.

Automated summary

In this study, a new methodology for characterizing the functioning of dryland ecosystems based on the balance between runoff connectivity and water retention capacity of vegetation was proposed. The method was tested on synthetic hillslopes and real hillslopes and showed good ability to assess ecohydrological interactions in drylands. This metric represents a promising tool for evaluating dryland conditions and incorporating hillslope processes in climate change models.