Sediment source fingerprinting and the temporal variability of source contributions

Sediment source fingerprinting has been progressively developed and refined over the past 40 years or more and now represents a widely used and valuable technique, with important practical applications. However, relatively little attention has been given to the target samples and the extent to which they are able to provide meaningful information on short- or longer-term relative source contributions for a given study catchment. A key issue here is the inherent short- and longer-term temporal variability of source contributions and the extent to which such variability is taken into account by the target samples. The objective of this study was to investigate the temporal variation of source contributions from the Qiaozi West catchment, a small (1.09 km²) gully catchment located within the Loess Plateau of China. The target samples represented a suite of 214 spot suspended sediment samples collected during eight representative wet season rainfall events occurring over two years. A suite of geochemical properties was used as fingerprints and standard source apportionment calculations indicated that the gully walls contributed the most sediment (load-weighted mean 54.5%) and, together with cropland (load-weighted mean 37.3%), and gully slopes (load-weighed mean 6.6%) were the main sediment sources. The 214 individual target samples indicated that the contribution of cropland sources varied between 8.3% and 60.4%, gully walls between 22.9% and 85.8% and gully slopes between 1.1% and 30.7%, representing ranges of 52.1%, 62.9% and 29.6% respectively. In order to explore whether the temporal variability of source contributions demonstrated by the study catchment should be seen as typical, equivalent information was abstracted from 14 published studies for other catchments of varying size and located in different environments worldwide. This information demonstrated similar temporal variability of the relative contributions of the major sources, which were typically characterized by ranges of the order of 30-70%. The temporal variability associated with the estimates of relative source contributions provided by target samples has important implications for the uncertainty associated with such estimates derived using source fingerprinting techniques based on a limited number of target samples. Further attention needs to be directed to the design of sampling programmes used to collect such samples and to taking account of such uncertainty in source apportionment calculations.

Priority setting for restoration in surrounding savannic areas of the Brazilian Pantanal based on soil loss risk and agrarian structure

Soil health is at the core of the sustainability agenda. As in many agroecosystems in the tropics, soil erosion is a major issue in poorly managed pasturelands. A noteworthy case is located in the Upper Taquari River Basin (UTRB), as part of the Upper Paraguay Basin on the plateau with drainage waters for the Taquari megafan in the Brazilian Pantanal. Here we combine slope (S-factor), erodibility (E-factor), rainfall-rainy day ratio (R-factor), and vegetation and soil indices (C-factor) to locate erosion risk and prioritize eco-engineering interventions via palisades and small dams in UTRB. The method consisted of assessing distinct weights between Universal Soil Loss Equation (USLE) factors in a GIS platform, providing 35 combinations of classes as low, moderate, high, and very high erosive risk. The validation of the method was based on the ravine and plain ground truths obtained from high-resolution raster data. The best weight of USLE factors aids to locate critical erosive sites and vegetation patterns. Then, erosion risk and interventions were analyzed according to land use and rural property sizes in the government's Rural Environmental Registry (CAR) database. Overall, the natural factors of slope and erodibility in a proportion of 25% and 75% in GIS algebra provided the best mapping accuracy result. About 65% of the UTRB has high or very high erosion risks, and 70% of the available area can be acknowledged as degraded pasturelands. A total of 4744 erosion interventions were recorded, with an accuracy of 65.28% and 61.15% for check dams and palisades interventions, respectively. The number of necessary interventions in areas of native vegetation was almost 50% higher than in pasturelands. Even though micro landowners occupy most of the watershed, large properties have about ten times as many areas at high risk of erosion. The mutual cooperation between properties, independently of size, is supported by governmental public policies like incentives for ecosystem services restoration of critical gullies, with CAR compliance and fiscalization.

Unique and Potentially Mars-Relevant Flow Regime and Water Sources at a High Andes-Atacama Site

A field expedition in the High Andes/Atacama Desert region revealed two types of flow-produced structures and a unique flow regime. Gullies somewhat smaller than those on Mars (width: 0.2-1 m, depth: 0.2-0.6 m, length: 4-60 m) were observed as mainly erosional structures. The other flow-related feature called infilled valleys showed activity only in specific, spatially discrete areas during the daytime. The active sections were composed of a source depression where liquid H₂O was produced from subsurface buried sources, which flowed down and percolated into fine-grained infilling material of the valley. Several such active sections could be present along one valley, separated by inactive ones. Three types of H₂O sources fed them: buried snow, surface snow, and ice left behind from the liquid water that had emerged the preceding day. This latter source has not yet been suggested for Mars. Some aspects related to the formation of the gullies and infilled valleys (H₂O budget, albedo, erosional processes) may be similar with the formation processes hypothesized for the recurring slope lineae on Mars. The observed diurnal spatially discrete activity of the infilled valleys is related to the interaction of insolation and mass movement of exhumed subsurface snow, which is also believed to exist on Mars. The Ojos del Salado site is unique in that, despite it being located in the hyperarid High Andes/Atacama Desert region, material from rare snowfall events has been protected due to burial by grains transported by strong winds, supporting ephemeral melting in the long term.

Analysis of methods to determine the sediment retained by check dams and to estimate erosion rates in badlands

Check dams are useful structures at a catchment scale to determine erosion rates using the sediment retained in their wedges. Several studies employ different complex, accurate methods to estimate them. We likewise evaluate which catchment variables affect these erosion rate values. Our study aims to compare five frequently used methods (two prismatic and three topographic) to evaluate how they estimate total sediment yield (TSY) at two locations in Central Spain. For this purpose, we determine the sediment trapped in each area by 25 check dams built during the twentieth century. We also evaluate the catchment variables influencing TSY rates. Results show differences between methods in TSY determinations at the 90 % confidence level, although there are no differences between locations or the location-method iteration. We found that the natural logarithm of the drainage area was the factor that best explained TSY, presenting a negative trend (partial correlation coefficient, 0.83). Vegetation cover factor had no influence in estimating TSY for the two locations. We conclude that the determination of TSY rates depends on the chosen method and four related variables: check dam length, check dam height, wedge length and check dam drainage area. Furthermore, it is important to analyse and select the best method to estimate erosion rates based on the sediment retained by check dams depending on the characteristics of the study area (channel and sediment wedge shape), method accuracy and field effort. A more accurate estimation of erosion rates will allow researchers to determine the role of check dams in controlling sediment in each specific restoration project.