Using a natural field experiment to characterize the temporal and spatial evolution of ephemeral gully erosion on a conventionally tilled plot, Southern Navarra, Spain

Within the historically cultivated region of Navarra, Spain, ephemeral gully erosion dominates soil losses, threatening agricultural productivity and landscape degradation.  Effective management requires soil erosion prediction technology as directly informed by regionally collected field data.  In particular, soil erosion models must replicate the emergence and evolution of ephemeral gullies in time and space to accurately assess and implement conservation practices.  Here we report on the results of a field campaign located in Pitillas, Southern Navarra.  An agricultural field composed of a highly erodible silty loam was tilled in November 2023, but it was kept out of production for the 2023-24 growing season.  The evolving landscape was captured by drone flights after every major precipitation event, producing DEMs at centimeter-scale resolution.  Ephemeral gullies appeared four months after initial tilling, which over time created incised drainage network systems.  Centimeter-to-decimeter scale rills occupy the furrows that are concordant to the tilled topography, whereas decimeter-scale ephemeral gullies occupy the major swales that are discordant to the tilled topography.  One such rill and ephemeral gully system at the conclusion of the season is interrogated to define the morphometric characteristics of the eroded channels and to calculate total soil losses.  This third-order ephemeral gully system exhibits a trellis drainage pattern occupying an area of 0.266 ha.  Channel dimensions and longitudinal profiles are characterized along nine continuous channel reaches.  The results show that scaling relationships for channel width and depth conform well to upstream drainage area using a hydraulic geometry framework.  Exponent values derived for the longitudinal variation in width, average depth, and maximum depth as a function drainage area are 0.21±0.06, 0.31±0.11, and 0.34±0.11, respectively.  That is, these channels incise more deeply than they widen in response to erosive runoff events consistent with previous observations within the region.  While variations in slope concavity indicate localized zones of flow acceleration and deceleration, a headward migrating wave of degradation, located mid-slope by season’s end, demarcates the transition from areas of net erosion to net deposition.  By comparing original and final surface topographies, total soil loss from this ephemeral gully system is estimated to be 3.23 kg/m²-yr, also consistent with previous work in this region.  Current efforts are now focused on (1) correlating specific rainfall events to channel development and evolution, and (2) validating models to ensure that these discrete erosional processes can be predicted accurately in time and space.  The field campaign has proven to be invaluable in the further development and refinement of soil erosion prediction technology required for effective resource management regionally.