Field experimentation for a better understanding of the occurrence and evolution of ephemeral gullies in field conditions

Ephemeral gully erosion is responsible for a great part of the erosion that occurs in agricultural areas. Like any complex system, the formation of an ephemeral gully has an inherent complexity that has led many researchers to study in isolation, through experimentation and modeling, the processes involved in this phenomenon. However, by definition, in a complex system there are many interactions between processes that cannot be ignored. Therefore, it is also necessary to carry out studies that consider all the processes simultaneously. We believe that, to date, there is a lack of studies of this type on the formation of an ephemeral gully that would allow us to advance in the characterization and understanding, to test the hypotheses put forward about it, formulating other alternatives if necessary, and to evaluate the existing models, such as QAnnAGNPS. In order to fill this gap, an experiment has been initiated in November 2023 in which, first of all, an agricultural plot has been selected in an area of highly erodible silty loam soils located in Pitillas (Navarra). The plot has been tilled with conventional tillage to replicate the initial conditions of an average agricultural plot, which has been kept free of vegetation by using herbicide, and in which a rain gauge has been installed as well as moisture probes at different depths. After each precipitation event, drone flights have been carried out to obtain digital elevation models (DEM) with a resolution of less than one centimeter and orthomosaics. The DEMs and orthomosaics generated in each flight make it possible to locate the origin of the gullies formed and to determine their dimensions and their temporal evolution, in this case until November 2024, when the plot was tilled again to restart the observations. Our observations confirm the enormous complexity of the erosive phenomena, highlighting the formation and evolution of hundreds of headcuts of very different typology and size, many of them linked to ephemeral gullies. The first gullies appeared four months after tilling, after a rainfall event of 17.7 mm and high soil moisture conditions. Prior to this rainfall event, 55.4 mm accumulated in different storms, but with an intensity that did not cause the formation of gullies. Subsequent events lengthened and widened the first gullies and created new ones, resulting in a dense drainage network and very high soil losses. Our first results suggest the complexity of the phenomenon, with the formation and migration of headcuts playing a major role, confirming the suitability of the modeling of the phenomenon centered on these headcuts. The proposed experimentation represents a great opportunity to advance in the understanding of the formation and evolution of ephemeral gullies in real agricultural conditions, considering all the typical variables that affect this phenomenon.