The role of check dams in retaining organic carbon and nutrients. A study case in the Sierra de Ávila mountain range (Central Spain)

Study on Soil Water Infiltration Process and Model Applicability of Check Dams

As the primary ecological construction measure on the Loess Plateau, check dams play an essential role in developing agricultural production, improving people’s production and life, and replenishing groundwater. Soil water infiltration is the most important way to replenish groundwater in the dam land. In order to investigate the water infiltration process of check dams, an empirical model suitable for the simulation of the dam land infiltration process was selected. The soil water infiltration process of the check dam was studied by a field test and a model simulation. The results showed that there were few macropores in the dam, and the water mainly moved downwards in the form of matrix flow. Moreover, the stable infiltration rate of the dam site was low, and its infiltration process could be divided into three stages: rapid infiltration, fluctuating infiltration, and stable infiltration. In addition, the infiltration rate of a non-silted dense layer was 2.4~5 times that of a silted dense layer. The Horton model had a good fitting effect on the water infiltration process of the check dam and thus was suitable for the simulation and prediction of the water infiltration process of the dam. The results can provide a theoretical basis for efficient soil water utilization and infiltration simulation of check dam land.

Assessing, mapping, and optimizing the locations of sediment control check dams construction

Check dams are considered to be one of the most effective measures for conservation of the soil and water resources. However, identifying the most suitable sites for the installation of check dams remain quite demanding. This research investigates and compares five machine learning algorithms (MLAs) - boosted regression trees (BRT), multivariate adaptive regression spline (MARS), mixture discriminant analysis (MDA), random forest (RF), and support vector machine (SVM) - for generating check-dam site-suitability maps (CDSSMs) and assessing them in Firuzkuh County, Iran. First, the locations of 475 existing check dams were monitored, registered, and divided into calibration (70%) and testing datasets (30%) for training and validation of the models. Fourteen check-dam conditioning factors (CDCFs) were selected and checked for multicollinearity. The relative importance of the CDCFs assessed using the elastic net (ENET) algorithm. Results demonstrated that distance from river (DFR) and drainage density (DD) to be the most significant factors for mapping the suitable sites for the erection of check dams. This research revealed that all of five MLAs had excellent accuracy for predicting the check-dam site-suitability with high AUC values: RF (0.966), SVM (0.878), MARS (0.878), MDA (0.844), and BRT (0.843). The most accurate model (RF) showed that 16.95%, 35.55%, 31.08%, and 16.42% of study area comes under low, moderate, high, and very high suitability classes. The outcome achieved by this research will be helpful to sustainability planners and managers in constructing check dams at suitable sites for better conservation of soil and water resources.

Using Landsat satellites to assess the impact of check dams built across erosive gullies on vegetation rehabilitation

Gully erosion, a process of soil removal due to water accumulation and runoff, is a worldwide problem affecting agricultural lands. Building check dams perpendicular to the flow direction is one of the suggested control practices to stabilize this process. Though there are many studies on the effect of erosive controls on land stabilization, few examine its effect on the rehabilitation of vegetation. Here we use information from the satellites Landsat-7 (1999-2018) and Landsat-8 (2013-2018) to assess the effect of soil check dams built during 2012 across three gullies with distinct structures in a dryland area on vegetative cover and water status. We use a time series analysis technique to decompose Landsat-derived soil adjusted vegetation index (SAVI) into woody (SAVI_W) and herbaceous (iSAVI_H) contributions. The integral over the seasonal signal of the normalized difference water index (iNDWI) was used to assess changes in water status in the gully. We used herbaceous biomass collected in the field in 2014-2017 to validate iSAVI_H as a proxy of herbaceous biomass. Our results show that following the construction of the check dams, the change in woody vegetation cover is best described by a sigmoid model with an increase of ~57% (95% CI: 39%-76%; p < 0.0001), while the herbaceous vegetation increases linearly at a rate of ~71% per year (95% CI: 48%-93% y^-1; p < 0.0001). The correlation between iSAVI_H and herbaceous biomass (R² = 0.56; n = 16; p < 0.001) corroborates this increase. We found higher herbaceous productivity in the deeper gully compared to the shallower gullies but not statistically different increase rates. An increase in iNDWI of ~68% (95% CI: 43%-95%; p < 0.0001) likely implies an improved water infiltration rate that favored the vegetation expansion. Our satellite-based approach can be used to assess the impact of erosive control practices on vegetation rehabilitation in heterogeneous gullies.

'Green', rammed earth check dams: A proposal to restore gullies under low rainfall erosivity and runoff conditions

Gully restoration check dams are usually made with construction materials and have a permanent impact on the landscape. Although earthen dams have not been used to restore gullies, they have been used around the world for water reservoirs and to control flooding. Here, we propose 'green', rammed earth check dams as a better environmental solution to restore gullies and integrate check dams in the natural landscape. This new design of earthen dam aimed at achieving a reduction in construction volume, as we propose a straight profile for the earthen wall upstream and an impervious core made of rammed earth wall. Moreover, the sloping, revegetated profile downstream enables integrating the check dam into the landscape, while the rammed earth wall allows the use of traditional materials and construction methods better adapted to the natural environment. We also present the design process, calculating its stability versus overturning, sliding and ground resistance for its weight. In addition, we compare the construction costs and carbon footprint of the rammed earth check dam with those of other frequently used check dams, such as concrete or gabion check dams. It shows that although rammed earth check dams may have a greater cost (>21%), they also have a sink effect of 23,639.36 kg CO₂ due to the vegetation cover on the downstream embankment. This work shows their feasibility in gullied areas and ravines of fine textured soils under low runoff and rainfall erosivity conditions.