Soil erosion by gullies in Ethiopia is causing environmental and socioeconomic problems. A sound soil and water management plan requires accurately predicted gully erosion hotspot areas. Hence, this study develops a gully erosion susceptibility map (GESM) using frequency ratio (FR) and random forest (RF) algorithms. A total of 56 gullies were surveyed, and their extents were derived by digitizing Google Earth imagery. Literature review and a multicollinearity test resulted in 14 environmental variables for the final analysis. Model prediction potential was evaluated using the area under the curve (AUC) method. Results showed that the best prediction accuracy using the FR and RF models was obtained by using the top four most important gully predictor factors: drainage density, elevation, land use, and groundwater table. The notion that the groundwater table is one of the most important gully predictor factors in Ethiopia is a novel and significant quantifiable finding and is critical to the design of effective watershed management plans. Results from separate variable importance analyses showed land cover for Nitisols and drainage density for Vertisols as leading factors determining gully locations. Factors such as texture, stream power index, convergence index, slope length, and plan and profile curvatures were found to have little significance for gully formation in the studied catchment.

The saturated hydraulic conductivity (Ks ) is a key characteristic of porous media, describing the rate of water flow through saturated porous media. It is an indispensable parameter in a broad range of simulation models that quantify saturated and/or unsaturated water flow. The constant-head permeameter test is a common laboratory method to determine Ks on undisturbed soil samples collected from the field. In this paper we show that the application of this conventional method may result in a biased Ks in the case of highly permeable media, such as the top layer of Sphagnum peat and gravel. Tubes in the conventional permeameter, that collect water under the sample, introduce a hydraulic head-dependent resistance for highly permeable media and result in an underestimation of Ks . We present a simple and low-budget alternative of the constant-head permeameter test that overcomes the disadvantages of conventional permeameters. The new method was successfully tested on intact highly permeable peatmoss collected from a northern peatland. •Conventional constant-head permeameters underestimate Ks of highly permeable media due to flow resistance in tubing systems•We developed the low-resistance permeameter to overcome this disadvantage.•Testing of the low-resistance permeameter demonstrated no systematic bias and successful application for highly permeable media.

While previous studies conclude that riparian freshwater lenses are the result of buoyancy forces that drive river water circulation in the adjacent aquifer, only highly idealized conceptual models have been assessed. A numerical study is conducted to investigate the influence of evaporation from unsaturated bare soils, including accumulation and precipitation of salt, on riparian freshwater lenses, assuming otherwise similar conditions to those adopted in recent studies. Non‐evaporating simulations that account for the unsaturated zone are first compared to previous saturation‐only numerical and analytical results, and reasonable matches are obtained. The modeling analysis of evaporative simulations includes three different approaches to the treatment of solute in evaporated water: (a) no evaporative salt accumulation, (b) evaporative salt accumulation, and (c) as for (b) except resistance to evaporation caused by salt accumulation is considered. Significant evaporative fluxes caused gaining river conditions to change to losing conditions, leading to the cessation of buoyancy‐driven freshwater circulation and to freshwater bodies that are not lenticular. Rather, evaporation reduces regions where the water table and/or unsaturated zone are fresh relative to the freshwater lens volume. Evaporation rates depend on the hydraulic connection between the floodplain surface and water table, which is limited where the water table is deep, the soil hydraulic conductivity is low, and under higher potential evaporation rates. The results show that floodplain evaporation may control the freshwater‐saltwater distribution and lead to hypersaline groundwater under certain conditions, although earlier buoyancy‐driven conceptual models of riparian lenses appear to remain valid where the thickness of the unsaturated zone is sufficiently large.