Comparing overland flow processes between semiarid and humid regions: Does saturation overland flow take place in semiarid regions?

•Hortonian overland flow (HOF) commonly occurs in arid regions.•Saturation overland flow (SOF) commonly occurs in humid regions.•Reports that SOF takes place in semiarid regions are critically analyzed.•The possible occurrence of SOF in semiarid regions should be re-examined. Two main processes of runoff are commonly thought to take place in a drainage basin: infiltration-excess overland flow, known also as Hortonian overland flow (HOF), and saturation overland flow (SOF). While HOF mainly takes place in arid and semiarid regions once the infiltration capacity of the upper skin of the soil (usually ~1 mm thick) is surpassed, SOF may takes place in wet humid and subhumid climates. SOF takes place once the groundwater table reaches the surface (mainly close to the stream channel), or due to an impermeable subsurface soil layer. Despite the low precipitation in semiarid regions, a number of publications advocated the occurrence of SOF also in semiarid regions, subsequently underplaying the centrality of biocrusts to effectively generate runoff. These publications are mainly based on moisture measurements that were conducted with dielectric-based sensors. In the present work, it will be argued that: (a) the amount and duration of the rain cannot possibly result in SOF, (b) the location of the alleged SOF, as reported by some publications (at the up- or mid-slope), is unlikely to facilitate SOF, (c) the likelihood that the electrical pulses produced by the dielectric-based sensors will distinguish between completely saturated and nearly saturated conditions is low, (d) in certain occasions even when the moisture content, as detected by these sensors, was slightly below saturation, the authors tended to assume that the soil was saturated, (e) while claiming that the subsurface soil layer reached saturation, the upper layer of the soil (where overland flow takes place) was occasionally reported to remain below saturation, (f) no supportive evidence for the occurrence of SOF or even subsurface flow was provided, such as visible wet belts with dense vegetation. It is therefore concluded that the occurrence of SOF in semiarid regions should be cautiously considered and re-examined.