Valorando la conectividad ladera-cauce en una cuenca agrícola, utilizando óxidos de tierras raras como trazadores y modelos de selvas aleatorias

Soil erosion from agricultural areas is a large problem, because of off-site effects like the rapid filling of reservoirs. To mitigate the problem of sediments from agricultural areas reaching the channel, reservoirs and other surface waters, it is important to understand hillslope-channel connectivity and catchment connectivity. To determine the functioning of hillslope-channel connectivity and the continuation of transport of these sediments in the channel, it is necessary to obtain data on sediment transport from the hillslopes to the channels. Simultaneously, the factors that influence sediment export out of the catchment need to be studied. For measuring hillslope-channel sediment connectivity, Rare-Earth Oxide (REO) tracers were applied to a hillslope in an agricultural catchment in Navarre, Spain, preceding the winter of 2014-2015. The results showed that during the winter no sediment transport from the hillslope to the channel was detected. To test the implication of the REO results at the catchment scale, two contrasting conceptual models for sediment connectivity were assessed using a Random Forest (RF) machine learning method. The RF method was applied using a 15-year period of measured sediment output at the catchment scale. One model proposes that small events provide sediment for large events, while the other proposes that only large events cause sediment detachment and small events subsequently remove these sediments from near and in the channel. For sediment yield prediction of small events, variables related to large preceding events were the most important. The model for large events underperformed and, therefore, we could not draw any immediate conclusions whether small events influence the amount of sediment exported during large events. Both REO tracers and RF method showed that low intensity events do not contribute any sediments from the hillslopes to the channel in the Latxaga catchment. Sediment dynamics are dominated by sediment mobilisation during large (high intensity) events. Sediments are for a large part exported during those events, but the system shows a memory of the occurrence of these large events, suggesting that large amounts of sediments are deposited in and near the channel after these events. These sediments are gradually removed by small events. To better understand the delivery of sediments to the channel and how large and small events influence each other more field data on hillslope-channel connectivity and within-channel sediment dynamics is necessary.</p

Accuracy of high-resolution photogrammetric measurements of gullies with contrasting morphology

Field techniques allow accurate direct measurements on gully geometry, even in three-dimensional (3D) coordinates. Accuracy and detail reproduction is limited mainly by experimental setup and density of measurements, and less so by the precision of the measuring equipment. In contrast, remote-sensing techniques permit the coverage of large study areas with a minimum of time and effort. However, the indirect measurements from imagery are known to depend on factors like image resolution, quality of ground control, vegetation cover and image evaluation technique, which strongly influence the measurement accuracy. The objective of the present study was to investigate to what extent the accuracy of 3D gully measurement using photogrammetric techniques depends on gully morphology. At a study site in the Bardenas Reales (Navarre, Spain), field measurements of cross-sections were taken for five gullies with contrasting morphology and dimensions and used as reference data for analysing the errors associated with a corresponding dataset obtained using small-format aerial photogrammetry whose pixel size on the ground is 16 mm. Results show that volumetric gully measurements by means of photogrammetric techniques are strongly affected by the gully morphology; in particular by its width/depth (W/D) ratio, because of the increasing sun-shadowing and sight-shadowing effects associated with narrower gullies. Only wide, shallow gullies are little affected by this problem. For gullies of an intermediate typology (W/D between 0·5 and 2·5), the accuracy of photogrammetric measurements will much depend on the time of day and the period of the year when the photographs are taken, and narrow/deep gullies (W/D <0·5) will be likely to be highly inaccurate at any time. Although this study was conducted with a large measurement scale for small (mostly ephemeral) gullies, the W/D ratios judged challenging for photogrammetric analysis in this study are also common for larger-sized gullies of the (permanent) bank gully typ

Assessing hillslope-channel connectivity in an agricultural catchment using rare-earth oxide tracers and random forests models

Soil erosion from agricultural areas is a large problem, because of off-site effects like the rapid filling of reservoirs. To mitigate the problem of sediments from agricultural areas reaching the channel, reservoirs and other surface waters, it is important to understand hillslope-channel connectivity and catchment connectivity. To determine the functioning of hillslope-channel connectivity and the continuation of transport of these sediments in the channel, it is necessary to obtain data on sediment transport from the hillslopes to the channels. Simultaneously, the factors that influence sediment export out of the catchment need to be studied. For measuring hillslope-channel sediment connectivity, Rare-Earth Oxide (REO) tracers were applied to a hillslope in an agricultural catchment in Navarre, Spain, preceding the winter of 2014-2015. The results showed that during the winter no sediment transport from the hillslope to the channel was detected.To test the implication of the REO results at the catchment scale, two contrasting conceptual models for sediment connectivity were assessed using a Random Forest (RF) machine learning method. The RF method was applied using a 15-year period of measured sediment output at the catchment scale. One model proposes that small events provide sediment for large events, while the other proposes that only large events cause sediment detachment and small events subsequently remove these sediments from near and in the channel. For sediment yield prediction of small events, variables related to large preceding events were the most important. The model for large events underperformed and, therefore, we could not draw any immediate conclusions whether small events influence the amount of sediment exported during large events. Both REO tracers and RF method showed that low intensity events do not contribute any sediments from the hillslopes to the channel in the Latxaga catchment. Sediment dynamics are dominated by sediment mobilisation during large (high intensity) events. Sediments are for a large part exported during those events, but the system shows a memory of the occurrence of these large events, suggesting that large amounts of sediments are deposited in and near the channel after these events. These sediments are gradually removed by small events. To better understand the delivery of sediments to the channel and how large and small events influence each other more field data on hillslope-channel connectivity and within-channel sediment dynamics is necessary.La erosión del suelo en zonas agrícolas es un problema grave debido entre otras razones a los efectos que tiene aguas abajo, tales como la colmatación de embalses. Para evitar que estos sedimentos lleguen a los cauces, embalses y cualquier tipo de lámina de agua, es importante entender la conectividad entre laderas y cauces así como la conectividad dentro de la cuenca. Para determinar el funcionamiento de la conectividad ladera-cauce y el posterior transporte de sedimentos en el propio cauce es necesario obtener información sobre el transporte que se produce desde las laderas a los cauces. De la misma manera, es necesario estudiar los factores que influencian la exportación del sedimento fuera de la cuenca. Con el fin de estudiar la conectividad ladera-cauce, se aplicaron óxidos de tierras raras (REO, del inglés Rare-Earth Oxide) como trazadores en una ladera de una cuenca agrícola en Navarra, en el periodo previo al invierno de 2014-2015. Los resultados mostraron que durante el invierno no hubo transporte de sedimentos desde las laderas al cauce. Para valorar el alcance de los resultados obtenidos a partir de los trazadores REO a escala de cuenca, dos modelos conceptuales de conectividad fueron evaluados a partir del método “Selvas aleatorias” (RF, del inglés Random Forest). Este método se aplicó utilizando datos de transporte de sedimentos medidos en la desembocadura de la cuenca durante un periodo de 15 años. El primer modelo conceptual propone que los pequeños eventos son los que proporcionan sedimentos a los eventos de mayor magnitud, mientras que el segundo modelo propone que sólo son éstos últimos los responsables de la erosión y los eventos pequeños son los que movilizan el sedimento en las zonas próximas al cauce y dentro del cauce. A la hora de predecir el sedimento producido por los eventos pequeños, las variables relacionadas con los eventos previos de gran magnitud fueron las más importantes. Las simulaciones para eventos de gran magnitud no fueron lo suficientemente buenas por lo que no fue posible llegar a ninguna conclusión respecto a si los eventos pequeños influyen en la cantidad de sedimento producido durante eventos de gran magnitud. En la cuenca de Latxaga, tanto los trazadores REO como la metodología basada en RF mostraron que durante los eventos de baja intensidad las laderas no aportan sedimento a los cauces. La dinámica de los sedimentos está controlada por la movilización de los mismos durante eventos de gran magnitud y alta intensidad. La mayor parte de los sedimentos se exportan durante estos eventos, pero en la “memoria” del sistema queda registrada la ocurrencia de estos eventos de gran magnitud, de manera que grandes cantidades de sedimento quedan depositadas en el cauce y zonas próximas a él. Estos sedimentos depositados son gradualmente retirados por los eventos pequeños. Para entender mejor el aporte de sedimentos desde las laderas a los cauces y cómo los eventos de pequeña y gran magnitud interaccionan entre sí se necesitan más datos de campo sobre conectividad ladera-cauce y sobre la dinámica de los sedimentos en los cauces