A review of the magnitude and response times for sediment yield reductions following the rehabilitation of gullied landscapes

Gully erosion is a major driver of elevated sediment yields across many areas of the globe, and considerable rehabilitation has occurred to reduce the amount of sediment eroded from gullies. However, compared to other forms of erosion, there has been little systematic review of the effectiveness of gully rehabilitation on reducing sediment yields. This study reviewed the global literature to provide an understanding of the potential sediment yield reductions that can occur following the rehabilitation of gullied landscapes. We focused on studies reporting a measured response on how gully and catchment sediment yield has changed since treatment. A total of 37 studies were found that met this criterion. The studies were partitioned into three broad categories, including those focused on: (i) treating the catchment above the gully; (ii) installing treatments in the actual gully channel; and (iii) a combination of approaches which include treating both the catchment and the gully channel. All the studies demonstrated a reduction in sediment yield following gully rehabilitation, with reported values ranging between 12 and 94%. The timeframes associated with the reductions in sediment yield varied considerably (2–80 years). Applying a variety of rehabilitation measures, which generally includes treating both the hillslope above the gully, and trapping sediment within the gully, appears to result in shorter (median) timescales for sediment yield reduction. Overall, this review indicates that gully rehabilitation strategies combining both engineering and vegetation measures are often the most successful. Engineering measures such as check dams are important for stabilizing sites in the early phases to support the revegetation of gullies and adjacent hillslopes. However, vegetation is the key to the long-term success of gully rehabilitation. This is because many engineering structures eventually fail, or they have a limited life span as an active sediment trap. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Predicting gully densities at sub-continental scales: a case study for the Horn of Africa

Despite its environmental and scientific significance, predicting gully erosion remains problematic. This is especially so in strongly contrasting and degraded regions such as the Horn of Africa. Machine learning algorithms such as random forests (RF) offer great potential to deal with the complex, often non-linear, nature of factors controlling gully erosion. Nonetheless, their applicability at regional to continental scales remains largely untested. Moreover, such algorithms require large amounts of observations for model training and testing. Collecting such data remains an important bottleneck. Here we help to address these gaps by developing and testing a methodology to simulate gully densities across Ethiopia, Eritrea and Djibouti (total area: 1.2 million km²). We propose a methodology to quickly assess the gully head density (GHD) for representative 1 km² study sites by visually scoring the presence of gullies in Google Earth and then converting these scores to realistic estimates of GHD. Based on this approach, we compiled GHD observations for 1,700 sites. We used these data to train sets of RF regression models that simulate GHD at a 1 km² resolution, based on topographic/geomorphic, land cover, soil and rainfall conditions. Our approach also accounts for uncertainties in GHD observations. Independent validations showed generally acceptable simulations of regional GHD patterns. We further show that: (i) model performance strongly depends on the amount of training data used, (ii) large prediction errors mainly occur in areas where also the predicted uncertainty is large and (iii) collecting additional training data for these areas results in more drastic model performance improvements. Analyses of the feature importance of predictor variables further showed that patterns of GHD across the Horn of Africa strongly depend on NDVI and annual rainfall, but also on normalized steepness index (k_sn) and distance to rivers. Overall, our work opens promising perspectives to assess gully densities at continental scales. © 2020 John Wiley & Sons, Ltd.     

An Electron-Balance Based Approach to Predict the Decreasing Denitrification Potential of an Aquifer

Numerical models for reactive transport can be used to estimate the breakthrough of a contaminant in a pumping well or at other receptors. However, as natural aquifers are highly heterogeneous with unknown spatial details, reactive transport predictions on the aquifer scale require a stochastic framework for uncertainty analysis. The high computational demand of spatially explicit reactive-transport models hampers such analysis, thus motivating the search for simplified estimation tools. We suggest performing an electron balance between the reactants in the infiltrating solution and in the aquifer matrix to obtain the hypothetical time of dissolved-reactant breakthrough at a receptor if the reaction with the matrix was instantaneous. This time we denote as the advective breakthrough time for instantaneous reaction (τ_inst ). It depends on the amount of the reaction partner present in the matrix, the mass flux of the dissolved reactant, and the stoichiometry. While the shape of the reactive-species breakthrough curve depends on various kinetic parameters, the overall timing scales with τ_inst . We calculate the latter by particle tracking. The effort of computing τ_inst is so low that stochastic calculations become feasible. We apply the concept to a two-dimensional test case of aerobic respiration and denitrification. A detailed spatially explicit reactive-transport model includes microbial dynamics. Scaling the time of local breakthrough curves observed at individual points by τ_inst decreased the variability of electron-donor breakthrough curves significantly. We conclude that the advective breakthrough time for instantaneous reaction is efficient in estimating the time over which an aquifer retains its degradation potential.     

长江口及邻近海域表层沉积物中重金属元素含量分布及其影响因素

根据2003和2006年长江口及邻近海域表层沉积物样品的粒度组成和元素铬、铜、镍、铅、锌、铝、钙和锶的含量,分析了重金属元素含量的分布特征,探讨了含量的变化趋势及其对人类活动的响应。自2003年三峡工程一期蓄水完成到2006年6月,长江输沙量逐年减少,长江口海域沉积物中黏土的百分含量明显增加,但是沉积物分布的总体格局并没有发生明显的变化。重金属元素大多在泥质区沉积物中富集,高值区沿岸线呈带状分布,在最大浑浊带和口外羽状锋处达到峰值。在长江大量物质输入的背景下,研究区沉积物中重金属元素含量相对其他类似河口较低。河流的陆源颗粒输入、水动力条件、细颗粒物质的吸附以及絮凝作用是控制沉积物中重金属元素含量分布的主要因素,氧化还原条件也对重金属元素含量变化有一定的影响。人类活动(重大工程的建设和人为污染)对长江口外泥质区中重金属元素含量有重要影响,尤其对铅和锌的含量及其分布的影响显著。自2003年三峡工程一期蓄水以来,长江口海域表层沉积物中重金属元素沉积机制未有明显的变化,但是重金属元素含量有逐渐降低的趋势。