Introducing a new methodology for applying field experiments to evaluate gully evolution processes

Gullies found in various climatic regions are a dominant erosional factor. Gully activation, sediment yields and evolution are critical when assessing land management practices, geomorphological landscape conservation and modelling. Gully headcut migration and widening are the main proxies of gully activity, affected by run-off discharge and in situ characteristics such as lithology, slope, biocrusts and vegetation. Because of these variations and the difficulties that lie in examining gully headcut evolution under field conditions, experiments are often undertaken in flumes and from historic aerial photos and post-event surveys. Few studies report actual gully growth processes in nature. We have developed a simple experimental field methodology for generating in situ discharges to undertake field experiments on gully evolution in various environments and for different research applications. Our objective for the development of this apparatus was to monitor the hydrologic and erosional response of first-order headcuts in biocrusted sands located in the north-eastern Negev, Israel. For this goal, a device was required that could generate specific discharges and could be easily carried in the field. We developed a Marriott tube-based flow apparatus, constructed of a sealed water tank with a faucet and a vertical open tube with a sealed slip mechanism set on its top. This apparatus generates discharges in the range of 0.5–20 L/s, and continuous filming allows documentation of processes on the headcut face and of surface water velocity, while permitting the definition of run-off discharge steps. Thirteen gully headcuts, situated in a sandy valley dissected by upslope to midslope gullies, with an average drainage basin area of 81 m², were chosen to test the new run-off-generating methodology. In each gully headcut, nine increasing water discharges were applied. We include an in-depth description of the developed apparatus and present preliminary results from our experiments, exemplifying the potential of this tool in various research conditions.