Evaluating functional connectivity in a small agricultural catchment under contrasting flood events by using UAV

Concentrated erosion, a major feature of land degradation, represents a serious problem for soil and water resources management and a threat to ecosystems. Understanding the internal mechanisms (de-)coupling sediment pathways can improve the management and resilience of catchments. In this study, concentrated erosion and deposition forms were mapped accurately through field and aerial unmanned aerial vehicle (UAV) campaigns, in order to assess the evolution of connectivity pathways over a series of three contrasted and consecutive flood events occurring between October 2016 and January 2017 (return period ranging from 0.5 to 25 years) in a small Mediterranean agricultural catchment (Can Revull, Mallorca, Spain; 1.4 km²). In addition, a morphometric index of connectivity (IC) was used to identify the potential trajectories of different concentrated erosion forms and deposition areas. IC predictions were calibrated by identifying the optimal critical thresholds, i.e. those most consistent with field observations after each of the events studied. The results found that the index performed well in predicting the occurrence and the length/area of the different type of landforms, giving kappa (κ) coefficients of variation ranging between 0.21 and 0.92 and linear correlations R² between 0.33 and 0.72. The type of landform affected the correspondence of IC predictions and field observations, with lower thresholds the greater the magnitude of their associated geomorphic processes. Rainfall magnitude proved to be a very important factor controlling the development of erosion and deposition landforms, with large differences in length/area between the contrasted events. The evolution of the observed trajectories revealed feedback dynamics between the structural and functional connectivity of the catchment, in which morphological changes determined the spatial distribution of the processes’ activity in the successive events and vice versa. © 2020 John Wiley & Sons, Ltd.     

Post‐fire hydrological response and suspended sediment transport of a terraced Mediterranean catchment

In July 2013, a wildfire severely affected the western part of the island of Mallorca (Spain). During the first three post‐fire hydrological years, when the window of disturbance tends to be more open, the hydrological and sediment delivery processes and dynamics were assessed in a representative catchment intensively shaped by terracing that covered 37% of its surface area. A nested approach was applied with two gauging stations (covering 1.2 km² and 4.8 km²) built in September 2013 that took continuous measurements of rainfall, water and sediment yield. Average suspended sediment concentration (1503 mg L^?1) and the maximum peak (33 618 mg L^?1) were two orders of magnitude higher than those obtained in non‐burned terraced catchments of Mallorca. This factor may be related to changes in soils and the massive incorporation of ash into the suspended sediment flux during the most extreme post‐fire event; 50 mm of rainfall in 15 min, reaching an erosivity of 2886 MJ mm ha^?1 h^?1. Moreover, hysteretic counter‐clockwise loops were predominant (60%), probably related to the increased sensitivity of the landscape after wildfire perturbation. Though the study period was average in terms of total annual precipitation (even higher in intensities), minimal runoff (2%) and low sediment yield (6.3 t km^?2 y^?1) illustrated how the intrinsic characteristics of the catchment, i.e. calcareous soils, terraces and the application of post‐fire measures, limited the hydrosedimentary response despite the wildfire impact. Copyright © 2017 John Wiley & Sons, Ltd.     

How pixel size affects a sediment connectivity index in central Belgium

Connectivity has become an increasingly used concept in hydrological and sediment research. In order to quantify it, various indices have been proposed since the start of the 21st century including the index of connectivity. This index is based on a limited number of factors, the most important one being topography. Sediment connectivity indices values probably depend on the digital elevation model (DEM) resolution. The aim of this study was, first, to compare the effect of DEM pixel size (between 0.25 and 10 m, using an UAV) in the Belgian loess belt, a lowland area. We show that the index values were lower when the pixel size decreased (a difference of about 20% in value between 0.25 and 10 m). In addition, the impact of linear features in the watershed (e.g. grass strip, bank and road) was lower with the largest pixel sizes, and the connectivity pattern was affected with a pixel size of 5 m or more. At lower pixel sizes (1 m or below), some more disconnected regions appeared. These corresponded with zones where there had been water stagnation during and after rainfalls, and was corroborated by field observations. This confirmed the need for a proper resolution according to the objectives of the study. The second aim of this study was to deduce a minimum pixel size for connectivity study, helping local erosion or sedimentation location and consequent land management decisions. In our context, 1 m stands as the optimum DEM resolution. This pixel size permitted location of all ‘key areas’ in terms of erosion. Very high resolutions (<0.5 m) did not generate much more information, and their calculation time was far greater. Copyright © 2017 John Wiley & Sons, Ltd.     

Assessment of CHADFDM satellite-based input dataset for the groundwater recharge estimation in arid and data scarce regions

Aquifer natural recharge estimations are a prerequisite for understanding hydrologic systems and sustainable water resources management. As meteorological data series collection is difficult in arid and semiarid areas, satellite products have recently become an alternative for water resources studies. A daily groundwater recharge estimation in the NW part of the Lake Chad Basin, using a soil–plant-atmosphere model (VisualBALAN), from ground- and satellite-based meteorological input dataset for non-irrigated and irrigated land and for the 2005–2014 period is presented. Average annual values were 284 mm and 30°C for precipitation and temperature in ground-based gauge stations. For the satellite-model-based Lake Chad Basin Flood and Drought Monitor System platform (CHADFDM), average annual precipitation and temperature were 417 mm and 29°C, respectively. Uncertainties derived from satellite data measurement could account for the rainfall difference. The estimated mean annual aquifer recharge was always higher from satellite- than ground-based data, with differences up to 46% for dryland and 23% in irrigated areas. Recharge response to rainfall events was very variable and results were very sensitive to: wilting point, field capacity and curve number for runoff estimation. Obtained results provide plausible recharge values beyond the uncertainty related to data input and modelling approach. This work prevents on the important deviations in recharge estimation from weighted-ensemble satellite-based data, informing in decision making to both stakeholders and policy makers.