Leticia Gaspar

Soil properties and physiographic factors controlling the natural vegetation re-growth in a disturbed catchment of the Central Spanish Pyrenees

Large changes in land use have occurred in the Central Spanish Pyrenees during the twentieth century. This mountain area supported an intense human use since the Middle Ages, that greatly transformed the natural landscape. The land uses changed dramatically during the first half of the twentieth century, due to major socioeconomic forces which lead to depopulation and land abandonment. Since then, a process of natural vegetation recovery has taken place. The anthropogenic impact during centuries deteriorated the soil quality, threatening the sustainability of agroecosystems and the natural vegetation re-growth. In this study, a soil survey was performed to identify the soil types and the physico-chemical properties of the soil that are relevant for maintaining the soil function for the recovery of vegetation after land abandonment. The study was conducted in the Arnás River catchment, which is representative of the region. The highest cation exchange capacity (CEC), nutrients, water retention, water availability and infiltration rates, as well as the lowest pH and carbonates, occurred in Lithic Hapludolls and Calcic Haploxerolls at the shady aspect, under a forest cover. Lithic Ustochrepts and Lithic Xerorthents on the sunny aspect, which were covered by shrubs, were of lesser quality than were the deeper and better-developed soils on the shady aspect. This is seen as an evidence of different patterns of soil degradation, which conditioned the re-growth of natural vegetation after land abandonment. The results provide insights into the main soil factors that have affected the re-establishment of natural vegetation in recent years. Recommendations are given for designing effective strategies for soil conservation after land abandonment in similar mountain environments.

Reconstructing the history of sediment accumulation in the Yesa reservoir: an approach for management of mountain reservoirs

Abstract Navas, A., B. Valero-Garcés, L. Gaspar and J. Machín. 2008. Reconstructing the history of sediment accumulation in the Yesa reservoir: an approach for management of mountain reservoirs. Lake Reserv. Manage. 25:15–27. The Aragón River was impounded at the foothills of the Pyrenean Internal Depression in 1959; since then sediments accumulations have decreased the reservoir storage capacity. In this work, we interpreted the history of the sediment accumulation in the Yesa reservoir based on the detailed study of two sediment cores collected at the more stable area in the reservoir. The identification of main sedimentological facies together with the analysis of the grain size distribution of the materials accumulated at the bottom of the reservoir was used for interpretations of the sedimentary dynamics. These data were compared with records of known flood events to derive a tentative chronology of the infilling process by assigning main changes observed in the facies types and sediment components to specific years. In addition to grain size data of sediments accumulated in the river channels, suspended sediments from representative sites of the Aragón Basin to the Yesa reservoir were collected during high and low waters and analysed. Grain size data and sediment composition (organic matter, carbonates) were used to assess the characteristics and the pattern of the sediment transport through the Aragón River network and the role played by lithology and land use. The results provide information on the sediment transport. This approach can be used to assess the siltation processes in Mediterranean mountain reservoirs to improve the management of water bodies by preventing their infilling.

Comparing catchment sediment fingerprinting procedures using an auto-evaluation approach with virtual sample mixtures

Information on sediment sources in river catchments is required for effective sediment control strategies, to understand sediment, nutrient and pollutant transport, and for developing soil erosion management plans. Sediment fingerprinting procedures are employed to quantify sediment source contributions and have become a widely used tool. As fingerprinting procedures are naturally variable and locally dependant, there are different applications of the procedure. Here, the auto-evaluation of different fingerprinting procedures using virtual sample mixtures is proposed to support the selection of the fingerprinting procedure with the best capacity for source discrimination and apportionment. Surface samples from four land uses from a Central Spanish Pyrenean catchment were used i) as sources to generate the virtual sample mixtures and ii) to characterise the sources for the fingerprinting procedures. The auto-evaluation approach involved comparing fingerprinting procedures based on four optimum composite fingerprints selected by three statistical tests, three source characterisations (mean, median and corrected mean) and two types of objective functions for the mixing model. A total of 24 fingerprinting procedures were assessed by this new approach which were solved by Monte Carlo simulations and compared using the root mean squared error (RMSE) between known and assessed source ascriptions for the virtual sample mixtures. It was found that the source ascriptions with the highest accuracy were achieved using the corrected mean source characterisations for the composite fingerprints selected by the Kruskal Wallis H-test and principal components analysis. Based on the RMSE results, high goodness of fit (GOF) values were not always indicative of accurate source apportionment results, and care should be taken when using GOF to assess mixing model performance. The proposed approach to test different fingerprinting procedures using virtual sample mixtures provides an enhanced basis for selecting procedures that can deliver optimum source discrimination and apportionment.

Selecting Color-based Tracers and Classifying Sediment Sources in the Assessment of Sediment Dynamics Using Sediment Source Fingerprinting

The use of sediment color as a fingerprint property to determine sediment sources is an emerging technique that can provide a rapid and inexpensive means of investigating sediment sources. The present study aims to test the feasibility of color fingerprint properties to apportion sediment sources within the South Tobacco Creek Watershed (74 km) in Manitoba, Canada. Suspended sediment from 2009 to 2011 at six monitoring stations and potential source samples along the main stem of the creek were collected. Reflectance spectra of sediments and source materials were quantified using a diffuse reflectance spectrometry, and 16 color coefficients were derived from several color space models. Canonical discriminant analysis was used to reclassify and downsize sediment source groups. After the linear additive test and stepwise discriminant function analysis, four color coefficients were chosen to fit the Stable Isotope Analysis in R model. Consistent with the conventional fingerprinting approach, the color fingerprint results demonstrated a switch in the dominant sediment source between the headwaters and the outlet of the watershed, with the main sources being topsoil in the upper reaches, whereas outcrop shale and stream bank materials dominated in the lower reaches. The color fingerprinting approach can be integrated with conventional fingerprints (e.g., geochemical and fallout radionuclide properties) to improve source discrimination, which is a key component for source ascription modeling. We concluded that the use of color fingerprints is a promising, cost-effective technique for sediment source fingerprinting.

Radionuclides and soil properties as indicators of glacier retreat in a recently deglaciated permafrost environment of the Maritime Antarctica

Many ice-free environments in Maritime Antarctica are undergoing rapid and substantial environmental changes in response to recent climate trends. This is the case of Elephant Point (Livingston Island, South Shetland Islands, SSI), where the glacier retreat recorded during the last six decades exposed 17% of this small peninsula, namely a moraine extending from the western to the eastern coastlines and a relatively flat proglacial surface. In the southern margin of the peninsula, a sequence of Holocene raised beaches and several bedrock plateaus are also distributed. A main issue in this environment is the role of glacier retreat and permafrost controlling the recently formed soils. To this purpose, a total of 10 sites were sampled along a transect crossing raised beaches and moraine materials following the direction of glacier retreat. At the selected sites surface samples were collected until 12cm depth and sectioned at 3cm depth intervals to analyse main properties, grain size, pH, electrical conductivity and carbonates. Besides, elemental composition and fallout (FRNs) and environmental radionuclides (ERNs) were analysed. To assess if profile characteristics within the active layer are affected by glacier retreat variations of organic carbon and carbon fractions and 137Cs contents were examined. The presence of organic carbon (range: 0.13-3.19%), and 137Cs (range: bdl-10.1Bqkg-1) was only found at the raised beaches. The surface samples had abundant coarse fractions in rich sandy matrix with increasing acidic pH towards the coast. Significant differences were found in the elemental composition and the radionuclides between the moraine and raised beaches. Soil forming processes are related to the time of exposure of the landforms after glacier retreat. The results obtained confirm the potential for using geomorphological, edaphic and geochemical data to assess the influence of different stages of glacier retreat in recent soils and sediments.

NDVI, 137Cs and nutrients for tracking soil and vegetation development on glacial landforms in the Lake Parón Catchment (Cordillera Blanca, Perú)

The present dominant trend of retreating and shrinking glaciers is leading to the formation of new soil in proglacial zones. The Cordillera Blanca located in the Peruvian Andes includes the Lake Parón catchment known for the Artesonraju Glacier and its rapid retreat, forming the largest proglacial lake in the region. This work aims to gain knowledge of soil and vegetation development on the most representative proglacial landforms existing in the Parón catchment. Previous research in proglacial environments suggests that soil properties might indicate different ages of ice retreat besides the normalised difference vegetation index (NDVI), which is known to be a powerful tool for assessing vegetation development. In the area surrounding Lake Parón up to the glacier tongue, an altitudinal transect (4200-4700 m a.s.l.) was established for sampling topsoils. A total of 40 surface soil samples (0-3 cm) were collected from the main glacial landforms, moraines, colluvium, glacio-fluvial terraces and alluvial fans, developed after different stages of glacier retreat. Soil organic carbon (SOC) and SOC fractions (active and stable), total nitrogen (TN) and 137Cs were analysed. A multitemporal analysis of NDVI was performed to assess the vegetation dynamics in the Parón catchment and over the different glacial landforms over time (1987-2018). The NDVI increase in recent decades indicates an expansion of vegetation cover and density. We compared NDVI values with the SOC and TN content to assess the relationships with vegetation growth in mountain soils. NDVI and the distribution of SOC and TN content show a positive correlation between vegetation evolution and the enrichment in soil nutrients that are more abundant in older moraines in coincidence with highest NDVI. These results outline the effect of shrinking mountain glaciers on generating new soils in parallel with the growth of vegetation.

A deconvolutional Bayesian mixing model approach for river basin sediment source apportionment

Increasing complexity in human-environment interactions at multiple watershed scales presents major challenges to sediment source apportionment data acquisition and analysis. Herein, we present a step-change in the application of Bayesian mixing models: Deconvolutional-MixSIAR (D-MIXSIAR) to underpin sustainable management of soil and sediment. This new mixing model approach allows users to directly account for the ‘structural hierarchy’ of a river basin in terms of sub-watershed distribution. It works by deconvoluting apportionment data derived for multiple nodes along the stream-river network where sources are stratified by sub-watershed. Source and mixture samples were collected from two watersheds that represented (i) a longitudinal mixed agricultural watershed in the south west of England which had a distinct upper and lower zone related to topography and (ii) a distributed mixed agricultural and forested watershed in the mid-hills of Nepal with two distinct sub-watersheds. In the former, geochemical fingerprints were based upon weathering profiles and anthropogenic soil amendments. In the latter compound-specific stable isotope markers based on soil vegetation cover were applied. Mixing model posterior distributions of proportional sediment source contributions differed when sources were pooled across the watersheds (pooled-MixSIAR) compared to those where source terms were stratified by sub-watershed and the outputs deconvoluted (D-MixSIAR). In the first example, the stratified source data and the deconvolutional approach provided greater distinction between pasture and cultivated topsoil source signatures resulting in a different posterior distribution to non-deconvolutional model (conventional approaches over-estimated the contribution of cultivated land to downstream sediment by 2 to 5 times). In the second example, the deconvolutional model elucidated a large input of sediment delivered from a small tributary resulting in differences in the reported contribution of a discrete mixed forest source. Overall D-MixSIAR model posterior distributions had lower (by ca 25–50%) uncertainty and quicker model run times. In both cases, the structured, deconvoluted output cohered more closely with field observations and local knowledge underpinning the need for closer attention to hierarchy in source and mixture terms in river basin source apportionment. Soil erosion and siltation challenge the energy-food-water-environment nexus. This new tool for source apportionment offers wider application across complex environmental systems affected by natural and human-induced change and the lessons learned are relevant to source apportionment applications in other disciplines.

Spatial variability of the sediment transport in the headwater catchment of the Aragón river

The Aragon river was dammed at the foothills of the Pyrenean Inner Depression in 1959. Since then sediments are accumulated and its initial water storage capacity has been reduced. In this work, a study of the transport of sediments in the hydrological network of the Aragon river catchment until the