published_or_final_version / Geography and Geology / Doctoral / Doctor of Philosophy

published_or_final_version / Applied Geosciences / Master / Master of Science

This dissertation represents a unique contribution to the Spanish literature of soil erosion. The author of this document has written and published "Procesos y control de la erosion", a textbook about the engineering of soil erosion processes and the design of the most up-to-date methods and products used for erosion control. The text has been presented in Appendix A.Besides the above, the author of this dissertation is an active committee member at the International Erosion Control Association (IECA), specifically the International Development and the SOIL Fund programs. IECA members are drawn from individuals and erosion control business of different sizes. However, there was a lack of knowledge within IECA members about the formation of post-fire water repellent layers. For instance, Appendix B is represented by an article written by the author of this document, published in Environmental Connection (Volume 2, Issue 3), on July of 2008. The article gives a general description of what post-fire water repellency is, how is it formed, what factors affect it, its consequences on soil erosion, and how to control soil erosion on an efficient way when such a layer has been formed as a consequence of fire.Finally, Appendix C is a research paper in process of submission to the International Journal of Wildland Fire, about the formation of post-fire water repellent layers on Chilean Monterrey pine plantations. Since water repellent layers following wildfires have never been documented on Chilean soils, the need for such a publication is urgent, because Chilean land managers have been ignorant of such phenomenon.

O solo é um recurso natural e fundamental para toda a vida terrestre e a questão da degradação do solo assume importância não apenas ambiental, mas também de ordem social e econômica. A erosão pela água da chuva é o mais severo tipo de erosão, podendo ser encontradas diversas feições erosivas resultantes desse processo tais como erosão laminar, em sulcos ou em voçorocas. Ainda, são encontradas as feições erosivas subsuperficiais (pipes), que embora sejam menos estudadas e difíceis de quantificar, provocam perdas de solo que podem ultrapassar as quantias advindas dos processos erosivos mais tradicionalmente estudados. O objetivo deste trabalho foi o de entender a gênese das feições erosivas superficiais e subsuperficiais através da análise dos atributos físico-hídricos do solo e do relevo. Foi efetuado o mapeamento subsuperficial de pipes e a caracterização dos materiais de subsuperfície por meio da técnica geofísica da eletrorresistividade. Aliado a isso, utilizou-se a técnica da dendrogeomorfologia como ferramenta para datação, quantificação e reconstrução da dinâmica dos processos erosivos ocorrentes na área de estudo. Os resultados demonstraram que a gênese e desencadeamento dos pipes da área de estudo foi governada por fatores concorrentes, dada a complexidade da área de estudo, tais como gradiente de atributos físicos entre horizontes do solo (textura, estrutura, porosidade), estrutura e composição geológica, e gradientes topográficos. Fatores neotectônicos e morfoestruturais também poderiam estar afetando, entretanto devem ser melhor estudados. O método da eletrorresistividade foi eficiente na caracterização da geometria dos materiais em subsuperfície e na deteccção da presença de pipes, colapsados e ainda não colapsados, sendo importante para não subestimar os processos subsuperficiais. Os resultados obtidos também demonstraram o potencial da espécie Esenbeckia leiocarpa para estudos dendrogeomorfológicos, uma vez que as mudanças nos padrões de crescimento após a exposição das raízes possibilitaram datar o primeiro ano de exposição. A técnica da dendrogromorfologia se mostrou eficaz em entender a dinâmica do processo de sistemas complexos, tais como a abertura de voçorocas e voçorocas efêmeras e que a subsidência da superfície do solo causada pelos pipings é responsável pela formação e evolução das voçorocas efêmeras e pela ramificação da voçoroca principal. / Soil is a natural and fundamental resource for all terrestrial life and the question of soil degradation assumes not only environmental but also social and economic importance. Erosion by rainwater is the most severe type of erosion, and several erosive forms resulting from this process can be found such as sheet erosion, ephemeral gullies or gullies. Also, subsurface erosive forms (pipes) are found, although they are less studied and difficult to quantify, they cause soil losses that exceed the amounts coming from the more traditionally studied superficial erosive processes. The objective of this work was to understand the genesis of superficial and subsurface erosive forms from the perspective of soil and relief properties. Subsurface pipe mapping and the characterization of subsurface materials using the geophysical technique of electroresistivity, was performed. The dating, quantification and reconstruction of the dynamics of the erosive processes occurring in the study area were done using dendrogeomorphology as a tool. The results demonstrated that the genesis and the initiation of pipes in the study area was governed by competing factors, due to the complexity of the study area, such as the gradient of physical attributes between soil horizons (texture, structure, porosity), structure and geological composition, and topographic gradients. Neotectonic and morphostructural factors could also be affecting, however they should be better studied. The electroresistivity method was efficient in characterizing the geometry of the materials in subsurface and in detecting the presence of pipes, collapsed and not yet collapsed, aiding in avoiding the underestimation of the subsurface erosive processes. The results obtained demonstrated the potential of the species Esenbeckia leiocarpa for dendrogeomorphological studies, since the changes in the patterns of root growth after exposure allowed to date the first year of exposure. Dendrogromorphology also proved effective in understanding the process dynamics of complex systems, such as the opening of gullies and ephemeral gullies, and that the subsidence of the soil surface caused by pipings is responsible for the formation and evolution of ephemeral gullies and the branching of the main gully.

To complement the Inabanga Watershed Project (BSWM, 2005), the study reported here was conducted to assess erosion and water resources degradation focussed on the Upper Inabanga Watershed using the Water Erosion Prediction Project (WEPP) erosion model and geographic information system and GIS tools. The study was divided into two sections. The first section was the assessment of the impact of land uses and farm management practices using five runoff experimental plots and two watersheds. The second section of the study was an application of the WEPP and GeoWEPP erosion models. Except for most of the crop management parameters, local climate, soil and topographic parameters were determined and used as inputs to run the model. The WEPP-GeoWEPP watershed simulations predicted that any increase in agricultural areas increased on-site soil loss and sediment yield from the watershed. / Master of Science (Hons)

Graduation date: 1995

La producción de sedimentos en zonas de montaña es un motivo de preocupación no sólo por la pérdida de la fertilidad de la capa superficial de suelo, sino también debido a sus efectos aguas abajo, tales como la deposición de sedimentos en embalses. En este trabajo, la producción de sedimentos se estudio a dos escalas espaciales: una cuenca pequeña y una grande en el noreste de los Pirineos orientales. La cuenca pequeña es Cal Rodó (4,2 km2), y la grande es la cuenca Alta Llobregat del Río Llobregat (504 km2), donde a la salida se encuentra el embalse de La Baells, la primera cuenca está anidado dentro de la última. En Cal Rodó, el objetivo fue determinar y comparar los intervalos de confianza de la carga de sedimentos en suspensión a escala de evento y a escala anual. Los datos utilizados abarcan 10 años (1996-2005). Estos incluyen muestras discretas de agua recogidas por un muestreador automático de agua, lecturas de turbidez proporcionada por un sensor de infrarojo de turbidez, un sensor de ultrasonidos, y mediciones de la lámina de agua. Todo conectado a un registrador de datos. La incertidumbre de producción de sedimentos fueron evaluados por simulaciones de Monte Carlo usando la función de distribución de la carga de sedimentos y modelación de residuales. La producción total para el periodo estudiado y estimada a través de la calibración de sensores de turbidez fue 17,217.0 Mg (4,1 Mg/ha/año) y su intervalo de confianza al 90% está entre 16,311.4 Mg y 18,395.8 Mg. Por otra parte la producción de sedimentos a través de las curvas de transporte de sedimento a escala anual fue de 103,441.0 Mg (24,6 Mg/ha/año), y su intervalo de confianza oscila entre 49,107.0 Mg y 254,803.0 Mg. Las conclusiones generales son que la incertidumbre de la carga de sedimentos por las curvas de transporte de sedimentos son amplias (> 300%) y que la producción de sedimentos se sobreestimo en un orden de magnitud. Una buena distribución de las muestras en todo el hidrograma es necesaria, a fin de desarrollar las curvas de transporte más fiable. La segunda fase de este estudio fue realizada en la cuenca Alta del Río Llobregat donde la cubierta del suelo es principalmente bosque de coníferas y pastos, con presencia de algunas zonas intensamente erosionadas (badlands). El promedio anual de erosión de suelo se estimó cuantitativamente a través de RUSLE integrado en SIG. Los datos tratados provienen de las precipitaciones registradas en ocho estaciones meteorológicas que abarcan 14 años (1991-2004), las propiedades físicas del suelo, el inventario de cubierta vegetal, las características de gestión del suelo y un modelo digital de elevaciones. Todo ello se utilizó como entradas (inputs) para generar cada uno de los factores de RUSLE, las imágenes tienen una resolución de 20 m. La producción de sedimentos se calculó mediante la aplicación de una tasa de entrega de sedimentos (sediment delivery ratio SDR) a los resultados obtenidos por RUSLE. También se llevaron a cabo análisis de sensibilidad para cada uno de los factores de RUSLE y una evaluación de la incertidumbre global de erosión de suelo. Adicionalmente diversos escenarios de erosión de suelos debido al Cambio Global fueron desarrollados. La producción de sedimentos media anual fue de 3,35 Mg/ha/año y el intervalo de confianza al 90% fue entre 0,95 y 13,7 Mg/ha/año. Estos resultados son similares a los resultados de un estudio previo de batimetría del embalse el cual fue de 4.54 Mg/ha/año, y su intervalo de confianza al 90% fue entre 4,29 Mg/ha/año y 4,79 Mg/ha/año. El mapa de riesgos de erosión muestra que pequeñas áreas altamente erosionables (5%) producen el 50% de sedimento. / A better understanding of soil erosion and sediment yield in mountainous areas is needed, for management purposes and to envisage uncertainties when developing soil erosion models. In this thesis, sediment yield estimates and associated uncertainties were studied at two spatial scales: a small catchment (Cal Rodó 4.2 km2) and a relatively large basin (Upper Llobregat Basin 504 km2), the former is nested within the latter, in the SE Pyrenees, Spain. In Cal Rodó stream which is characterized by an event based regime, is assessed the uncertainty of suspended sediment load (SSL) at event and annual scales. SSL in streams are currently estimated using diverse types of sediment rating curves (SRC) that allow the interpolation and extrapolation of the discrete observations of sediment concentrations to the whole range of recorded discharges. However, the accuracy of outcomes using SRCs in mountain event based streams is still unknown. The parameters of the non-linear relationship are obtained by a log transformation and then a bias correction factor is applied, based on the residual analysis. The uncertainty of SSL can be assessed by using Monte-Carlo approaches based on the simulation of residuals. Nevertheless, when these methods are going to be applied in streams with a regime characterized by events, it is necessary to verify if different events have different discharge-concentration relationships; if this occurs, the role of events must be taken into account in the load analyses, as they modify the structure of the residuals. Results showed that when SRCs are used for interpolation between samples, paying attention to the role of events allows to avoid nonlinear bias overcorrection and to reduce the uncertainty associated with the sediment load estimates. The sediment yield from annual and seasonal rating curves were 6.0 and 5.1 times larger than the estimated by sensor calibrations. Therefore a good distribution of samples throughout the hydrograph needs to be ascertained in order to develop reliable SRCs. The second part of this research deals with soil erosion and sediment yield assessment and its associated uncertainties in the Upper Llobregat River Basin. Land cover is mainly coniferous forest and pastures with some intensely eroded areas (badlands). Annual average soil erosion was quantitatively estimated with an integration of GIS and the Revised Universal Soil Loss Equation (RUSLE). Maps for each of the factors involved where obtained with a 20 m resolution. Rainfall dataset from eight weather stations and spanning 14 years, soil properties, land cover inventory, land management features and digital elevation model were used as resource datasets to generate each of the RUSLE factor maps. Annual average sediment yield was computed by applying a sediment delivery ratio to the results obtained by RUSLE, and this result was compared with an existing bathymetric survey. Uncertainty and sensitivity analyses were undertaken for each of the RUSLE factors, in order to assess its magnitude and determine which of the factors influences the most the soil loss estimate. Results show that the annual average sediment yield was 3.35 Mg km-2 yr-1 and its confidence interval lied between 0.95 and 13.7 Mg km-2 yr-1 with 90% confidence. These results are in agreement with results from reservoir bathymetric survey. Additional comparisons of estimated sediment yield were done with empirical methods such as PSIAC and FSM. Erosion risk maps showed that about 5% of the area generated 50% of soil loss. These areas are located in steep slopes and coincide in many cases with badlands which are near the streams. Finally, sediment yield changes under climate conditions for the late 21st Century where not relevant, whilst spreading agricultural activity as in the 1950's had a relevant increase in sediment yield.

Meander migration is a complex and dynamic process of the lateral movement of a river due to erosion on one bank and deposition on the opposite bank. As a result, the channel migrates in a lateral direction, which might be a major concern for the safety of bridges during their life span of 75 years. Although there are several existing models for predicting meander migration of a river, none of them are based on the physical model tests on a specific type of soil. A total of eight flume tests are conducted to develop a prediction equation of meander migration in clay. The test results of migration rate follow a hyperbolic function, and spatial distribution of the maximum migration distance is fitted with the Pearson IV function. The proposed equations of the initial migration rate and the maximum migration distance, obtained by a multiple regression technique, are validated with the laboratory data. A new methodology for risk analysis is developed to process a number of predicted channel locations based on each future hydrograph generated in such a way that all the hydrographs have the same probability of occurrence. As the output from risk analysis, a CDF map is created for a whole river representing a general trend of migration movement along with the probability associated with new location of the river. In addition, a separate screen is generated with a CDF plot for a given bridge direction so that bridge engineers can read a specific migration distance along the bridge corresponding to the target risk level (e.g. 1 %). The newly developed components through this research are incorporated with the other components in the MEANDER program which is a stand-alone program and the final outcome of the research team. Verification study of the MEANDER program is conducted with full-scale field data at the Brazos River at SH 105, Texas. The prediction results matched quite well with the measured field data. However, a more extensive verification study for other sites is highly recommended.

Soil erosion is a long standing problem in the Scotland District of Barbados. Many workers, including pedologists, geologists, hydrologists and soil conservationists have recently undertaken investigation into the causes of soil erosion and the possible means of its control. These studies have focussed on a problem in a restricted field. As accelerated soil erosion is an important ecological process, it was felt that an investigation into the problems of erosion from an ecological viewpoint was necessary. [...]

Soil water erosion is recognized as the principal mechanisms behind soil organic carbon (SOC) losses from soils, a soil constituent essential for ecosystem functions. SOC erosion can thus be far-reaching, affecting the future human welfare and the sustainability of ecosystems. Little research has yet been done to investigate the main mechanisms involved in the lateral translocation of SOC on the landscape. Understanding the effects of the different water erosion mechanisms, which control SOC losses (SOCL) at the hillslope level, creates scope for further scientific studies. Empirical data from 357 plots, with a range in slope length from 1 (n=117) to 22.1m (n=240) were analysed to estimate the global variations of particulate organic carbon content (POCC), POC losses (POCL) and sediment POC enrichment ratio (ER). The global average POCL rate was calculated to be 12.1 g C m-2 y-1. Tropical clayey soil environments revealed the highest POCL (POCL=18.0 g C m-2 y-1), followed by semi-arid sandy (POCL=16.2 g C m-2 y-1) and temperate clayey soil environments (POCL=2.9 g C m-2 y-1). The global net amount of SOC displaced from its original bulk soil on an annual basis was calculated to be 0.59±0.09 Gt C, making up an approximated 6.5% of the net annual fossil fuel induced C emissions (9 Gt C). POCL data for different spatial scales revealed that up to 83% of the eroded POC re-deposits near its origin in hillslopes, and is not exported out of the catchment. The low organic carbon sediment ER obtained from the data of clayey soils (ER of 1.1) suggests that most of the eroded POC remains protected within soil aggregates. Consequently, erosion-induced carbon dioxide (CO2) emissions in tropical areas with clayey soils are likely to be limited (less than 10%), as the process of POC re-burial in hillslopes is likely to decrease the rate of organic matter (OM) decomposition and thus serve as a potential carbon sink. Water erosion in sandy and silty soils revealed organic carbon sediment ER as high as 3.0 and 5.0, suggesting that in these soils the eroded POC is not re-buried, but is made vulnerable to micro-decomposers, thus adding to the atmospheric CO2 influx. The results obtained in the review study only reaffirm that large variations of POCL are evident across the different pedo-climatic regions of the world, making it a scientific imperative to conduct further studies investigating the link between SOC erosion by water and the global carbon cycle. A field study was designed to quantify the POC exported in the eroded sediments from 1x1m2 and 2x5m2 erosion plots, installed at different hillslope aspects, and to further identify the main erosion mechanisms involved in SOC erosion and the pertaining factors of control. The erosion plots were installed on five topographic positions under different soil types, varying vegetation cover, and geology in the foothills of the Drakensberg mountain range of South Africa. Soil loss (SL), sediment concentration (SC), runoff water (R) and POCL data were obtained for every rainfall event from November 2010 up to February 2013. Scale ratios were calculated to determine which erosion mechanism, rain-impacted flow versus raindrop erosion, dominates R, SL and POCL. Averaged out across the 32 rainfall events, there were no significant differences in R and POCL between the two plot sizes but SL were markedly higher on the 5m compared to the 1m erosion plots (174.5 vs 27g m-1). This demonstrates that the sheet erosion mechanism has a greater efficiency on longer as opposed to shorter slopes. Rain-impacted flow was least effective where soils displayed high vegetation coverage (P < 0.05) and most efficient on steep slopes with a high prevalence of soil surface crusting. By investigating the role of scale in erosion, it was possible to single out the controlling in situ (soil surface related conditions) and ex situ (rainfall characteristics) involved in the export of SOC from soils. This information will in future contribute toward generating SOC specific models and thus further inform erosion mitigation. / M. Sc. University of KwaZulu-Natal, Durban 2013.