Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / The understanding of processes such as the generation of surface runoff, sediment yield and their relations with the rainfall regime is the basis for the planning and effective management of soil and water resources in a watershed. In this context, the present study aims to identify the main processes that influence the generation of surface runoff and sediment yield in small watersheds, and determine parameters for empirical sedimentological models, as well as the Sediment Delivery Ratio (SDR) for the tropical, semi-arid region of Brazil. The study area is the Iguatu Experimental Basin (IEB), which comprises a watershed of 16.74 km2, three small nested watersheds (from 1 to 3 ha) and three erosion plots of 20 m2. One of the watersheds had been under regenerating Caatinga for 35 years, another subjected to a management, which involved thinning the Caatinga, and the last faced deforestation followed by burning and the cultivation of grass. The period of study was six years (2009 to 2014). Collections to quantify surface runoff and sediment yield were taken for each erosive rainfall event in an accumulated period of 24 hours. Surface runoff in the watersheds was quantified using Parshall flumes, and sediment yield was measured with towers and trenches to collect suspended sediment and bedload. On the watershed scale, runoff was measured by means of a spillway, and sediment yield by a turbidimeter. Cluster analysis was used to determine rainfall regimes and groups of similar rainfall-runoff events. With data from the erosion plots and watersheds, the C factors and coefficients of the Modified Universal Soil Loss Equation (MUSLE) were calibrated and validated. From data measured in the basin, sediment delivery ratios were determined. The occurrence of dry spells and the formation of cracks in the soil were important factors in controlling the generation of runoff and consequently sediment yield. The dry spells made it possible for the soil to dry out, with the formation of cracks, which acted as pathways for preferential flow, generating higher initial abstraction during the start of the rainy season. Changes in ground cover had little influence on accumulated flow, demonstrating that the soil characteristics and conditions as moisture and the presence of cracks, best explain the generation of runoff on expansive soils. The greatest runoff losses in all the watersheds under study were for Rainfall Regime II, characterized by higher depth, intensity and occurrence of the rainfall. For the ground covers under study, values for the C factors and the fit of the coefficients "a" and "b" of MUSLE, proved to be appropriate and recommended according to the statistical indices employed. Values for sediment delivery ratio for individual rainfall events ranged from 0.08 to 1.67%, with an average of 0.68%. In the basin, extreme events may cause high disaggregation of soil particles, but without sufficient transporting energy for dragging the sediment, generating low SDR. / O entendimento de processos como geraÃÃo do escoamento superficial, produÃÃo de sedimentos e suas relaÃÃes com o regime pluviomÃtrico à a base para o planejamento e a gestÃo eficaz dos recursos solo e Ãgua em uma bacia hidrogrÃfica. Nesse contexto, o presente estudo tem como objetivos identificar os principais processos que influenciam a geraÃÃo do escoamento superficial e a produÃÃo de sedimento em pequenas bacias hidrogrÃficas, e calibrar parÃmetros de modelos sedimentolÃgicos empÃricos, bem como a razÃo de aporte de sedimentos (SDR) para a regiÃo semiÃrida tropical do Brasil. A Ãrea de estudo à a Bacia Experimental de Iguatu (BEI), composta de uma bacia de 16,74 km2, trÃs pequenas microbacias aninhadas (de 1 a 3 ha) e trÃs parcelas de erosÃo de 20 m2. Uma das microbacias foi mantida com Caatinga em regeneraÃÃo hà 35 anos, outra submetida ao manejo de raleamento da Caatinga e na Ãltima foi realizado o desmatamento seguido de queimada e cultivo de capim. O perÃodo de estudo foi de seis anos (2009 a 2014). As coletas para quantificaÃÃo do escoamento superficial e da produÃÃo de sedimentos foram realizadas a cada evento de chuva erosiva, no acumulado de 24 horas. O escoamento superficial nas microbacias foi quantificado atravÃs de calhas Parshall e as produÃÃes de sedimentos foram mensuradas atravÃs de torres e fossos coletores de sedimentos em suspensÃo e arraste. Na bacia de 16,74 km2, o escoamento superficial foi mensurado por meio de um vertedor e a produÃÃo de sedimentos atravÃs de um turbidÃmetro. AnÃlises de agrupamento foram utilizadas para determinaÃÃo de regimes de chuvas e grupos de eventos similares de chuva-deflÃvio. Com dados das parcelas de erosÃo e das microbacias foram calibrados e validados os fatores C e os coeficientes da EquaÃÃo Universal de Perdas de Solo Modificada (MUSLE). A partir dos dados medidos na bacia foram determinadas as razÃes de aporte de sedimentos. A ocorrÃncia de veranicos e a formaÃÃo de fendas no solo foram determinantes no controle da geraÃÃo de escoamento e consequentemente da produÃÃo de sedimentos. A ocorrÃncia de veranicos possibilitou o secamento do solo com a formaÃÃo de fendas, que agem como caminhos preferenciais para o fluxo de Ãgua, gerando elevadas abstraÃÃes inicias durante o inÃcio da estaÃÃo chuvosa. As mudanÃas da cobertura vegetal apresentaram pouca influÃncia sobre o escoamento acumulado, indicando que as caracterÃsticas e condiÃÃes do solo, como umidade e presenÃa de fendas, explicam melhor a geraÃÃo de escoamento em solos expansivos. Maiores perdas por escoamento em todas as microbacias estudadas foram para chuvas do Regime II caracterizado por maiores alturas pluviomÃtricas, intensidades e ocorrÃncias. Quanto aos valores dos fatores C e dos coeficientes de ajuste âaâ e âbâ da MUSLE, para as coberturas estudadas, mostraram-se apropriados e recomendados de acordo com os Ãndices estatÃsticos empregados. Os valores da razÃo de aporte de sedimentos para eventos pluviomÃtricos individuais variaram de 0,08 a 1,67%, com mÃdia de 0,68%. Na bacia, eventos extremos podem causar elevada desagregaÃÃo de partÃculas de solo, mas podem nÃo dispor de energia de transporte suficiente para arrastÃ-las, gerando baixo SDR.

Forestry Best Management Practices (BMPs) include guidelines, recommendations, and protocols utilized to protect forest water quality from nonpoint source pollutants (NPSP). Sediment is the most common NPSP associated with forest operations, and BMPs are implemented primarily to reduce erosion and potential sediment delivery to streams. Skid trails, stream crossings, forest roads, decks, and harvest areas are major forest operational features that have the potential to erode and deliver sediment to streams. These five features are also common BMP categories evaluated by states across the southeastern U.S. Although BMPs are designed to minimize erosion and sediment delivery, the exact relationship between BMP implementation rates, erosion rates, and potential sediment delivery is largely unexamined. Specifically, the inherently intuitive but unverified concept that increasing levels of BMP implementation decreases erosion and sediment delivery associated with forest harvesting. This hypothesis was tested in this project at five operational features located within three physiographic regions, including the Mountains, Piedmont, and Coastal Plain, across clearcut harvest sites in the southeastern U.S. First, BMP implementation rates, audit questionnaires, and state guidelines were compared across 13 states in the southeastern region at 116 clearcut harvest sites. Overall, BMPs were implemented at an average rate of 90.1% in the southeastern United States, thus demonstrating that BMPs are currently being implemented consistently at high levels throughout the region. Across all regions, average BMP implementation rates were highest at harvest areas (95.6%), followed by decks (92.7%), haul roads (91.9%), stream crossings (88.2%), and skid trails (82.9%). Average BMP implementation rates for Mountain stream crossings (83.9%) and skid trails (76.1%) were significantly lower than rates calculated in the Piedmont and Coastal Plain, and had the lowest implementation rate for any feature in any region. These findings indicate that skid trails and stream crossings, especially in the Mountains, may benefit the most from enhanced BMP implementation and resources. In the second manuscript, the relationship between BMP implementation, estimated erosion, and potential sediment delivery were examined in three regions of Virginia and North Carolina. This study is one of the only forestry studies that have presented sediment delivery ratios by operational feature and physiographic region. BMP implementation rates and sediment delivery ratios were poorly correlated, however, a significant inverse relationship existed between BMP implementation and the total sediment mass delivered to streams (Spearman ρ = -0.2206, p-value = 0.0027). Generally, as BMP implementation increased, erosion rates and the amount of sediment delivered to streams also decreased. Additionally, this study demonstrated that most of the erosion generated by clearcutting operations in the southeast is trapped in either the harvest area or in Streamside Management Zones (SMZs) prior to reaching the stream. In the third manuscript, BMP implementation rates and erosion estimates were categorized into three BMP levels (BMP−, BMP-standard, BMP+) which represent low, moderate, and high levels of BMP implementation, respectively. Skid trails and haul roads generally had the highest erosion estimates, regardless of BMP level and physiographic region. Non-parametric correlation analyses indicated that significant inverse relationships existed between BMP implementation rates and erosion estimates at skid trails (Spearman ρ = -0.589, p-value < 0.0001), haul roads (Spearman ρ = -0.388, p-value < 0.0001), and harvest areas (Spearman ρ = -0.2305, p-value = 0.0169), while decks and stream crossings were more poorly correlated with erosion estimates. This reinforces the need for BMP audit questions that specifically address ground cover and bare soil, water control structures, gradients, and stabilization to better address potential erosion and sedimentation. Clearcut areas, erosion and sediment estimates, and sediment removal efficiencies were presented for the Mountains, Middle/Lower Coastal Plain, and Piedmont/Upper Coastal Plain for the fourth, fifth, and sixth manuscripts, respectively. Regardless of BMP level and physiographic region, a combination of harvest areas, skid trails, and haul roads were responsible for over 95% of potential sediment delivery. Increasing site-wide BMP implementation from BMP− to BMP+ could reduce sediment delivery by 70% in all physiographic regions. High levels of BMP implementation were most effective at reducing potential erosion and sediment delivery from skid trails and haul roads throughout the southeast. Findings from these studies demonstrate that current BMPs are highly effective at mitigating sediment. In the southeastern U.S., increasing levels of BMP implementation effectively reduce both potential erosion and sedimentation associated with forest harvesting. Generally, both estimated erosion and sedimentation associated with clearcutting in the region is much lower than rates associated with other land uses such as development or agriculture, especially when BMPs are implemented at standard or high levels. However, several opportunities exist to improve the effectiveness of BMPs in the southeastern U.S. Skid trails, haul roads, and stream crossings consistently had the lowest BMP implementation rates and highest estimated erosion rates and sediment delivery ratios. While these features only represent a small proportion of total clearcut area in the southeast, they are responsible for a disproportionate amount of sediment delivery and should receive more attention and resources during the pre-harvest planning and closure processes. Conclusively, this project addresses several knowledge gaps pertaining to water quality impacts resulting from harvesting operations in the southeastern U.S. For instance, this is only the third project that has presented sediment delivery ratios associated with forest operations in the southeastern U.S., and the first to do so for the Mountains and Coastal Plain regions. On average, SMZs and harvest areas trap 66-96% of sediment on-site before it can be deposited into streams. Additionally, this project provides one of the first and most comprehensive regional comparisons of state BMP manuals, audits, and programs in-field using a third-party approach. Several significant differences existed among state BMP programs and protocol, and states may need to design specific BMP guidelines and audit protocol for major physiographic regions to address the challenges and variation of on-site conditions inherent of each region. Additionally, this project presents one of the only regional-scale estimates of sediment and efficiencies of increased levels of BMP implementation at mitigating sediment associated with forest operations in the southeastern U.S. Conclusively, this project provides forest managers, state and federal agencies, and policymakers with a robust assessment on the effectiveness of forestry BMPs in the southeast. / Doctor of Philosophy / Forestry Best Management Practices (BMPs) are used throughout the southeastern U.S. to minimize the impacts that harvesting has on soil and water resources. Eroded soil that is eventually deposited into streams as sediment is the most important pollutant that BMPs address in forestry. Common BMP guidelines utilized to minimize sediment include leaving riparian buffers along streams, providing ground cover, minimizing slopes on roads, and using water control structures to divert runoff from road systems. The exact relationship between forestry BMPs, erosion, and sediment delivery is largely unexamined. Objectively, this study was designed to provide a better understanding of this relationship, and to present estimates of erosion and sediment delivery resulting from clearcut harvesting on over 100 harvest sites across 13 states with diverse conditions and topography in the southeastern U.S. Forestry BMPs are being implemented at an average rate of 90.1% across the southeast. BMPs are implemented at higher rates in the Coastal Plain, followed by the Piedmont and Mountains. Generally, harvest areas had the highest BMP implementation rates, followed by decks, haul roads, stream crossings, and skid trails, respectively. This relationship was consistent across most regions and states. Logging decks, which are areas where wood was transported for processing and loading onto logging trucks, were generally located distantly from streams and followed most state-approved BMPs. Whereas skid trails, which are low-standard temporary roads trafficked during primary transport and require water diversion structures such as waterbars, occupied a much larger area and were generally located on much steeper slopes. Mountain skid trails leading to stream crossings are concerning because of their low BMP implementation rates and high erosion potential. These features would greatly benefit from increased ground cover and water control structures. Forestry BMPs reduce both erosion and sedimentation associated with clearcutting. The highest level of BMP implementation reduces potential sediment by over 70% in all regions, and the highest BMP level is most effective at reducing sediment from skid trails and forest roads. Average sedimentation rates, especially at the highest BMP level, calculated for Mountains, Piedmont, and Coastal Plain clearcuts are considerably lower than sedimentation rates associated with agricultural and developmental land uses. Clearcut forestland, which is generally the most erodible time period during a forest's cycle, only makes up 1-2% of total forestland in the southeast annually. Both erosion and sedimentation rates should decrease further in the years following harvesting as the site revegetates and trees begin to reestablish. This study verifies that BMPs are highly effective at reducing erosion and sediment while subsequently providing estimates of erosion and sediment delivery based on ranges of BMP implementation that state forestry agencies can use to better quantify the effectiveness of their BMPs.

No description available.

Forest roads and stream crossings can be important sources of sediment in forested watersheds. The purpose of this research was to compare trapped sediment and forestry best management practice (BMP) effectiveness from haul road stream crossing approaches and ditches. The three studies in this dissertation provide a quantitative assessment of sediment production and potential sediment delivery from forest haul roads in the Virginia Piedmont and Ridge and Valley regions. Sediment production rates were measured and modeled to evaluate and compare road and ditch segments near stream crossings with various ranges of road attributes, BMPs, and management objectives. Sediment mass delivered to traps from 37 haul road stream crossing approaches ranged from <0.1 to 2.7 Mg for the one year collection. Collectively, five approaches accounted for 82% of the total sediment mass trapped. Approaches were categorized into Low, Standard, and High road quality rankings according to road attributes. Seventy-one percent (5 of 7) of Low ranked approaches delivered sediment to traps at rates greater than 11.2 Mg ha-1 yr-1. Nearly 90% of Standard or High road quality approaches generated less than 0.1 Mg of sediment over one year. Among approaches with less than 0.1 Mg of trapped sediment, road gradients ranged from 1% to 13%, bare soil ranged from 2% to 94%, and distances to nearest water control structures ranged from 8.2 to 427.0 m. Such a wide spectrum of road attributes with relatively low levels of trapped sediment indicate that contemporary BMPs can mitigate problematic road attributes and reduce erosion and sediment delivery. Three erosion models, USLE-forest, RUSLE2, and WEPP were compared to trapped sediment data from the 37 forest haul road stream crossing approaches in the first study. The second study assessed model performance from five variations of the three erosion models that have been used in previous forest operations research, USLE-roadway, USLE-soil survey, RUSLE2, WEPP-default, and WEPP-modified. The results suggest that these soil erosion models could estimate erosion and sediment delivery within 5 Mg ha-1 yr-1 for most approaches with erosion rates less than 11.2 Mg ha-1 yr-1, while model estimates varied widely for approaches that eroded above 11.2 Mg ha-1 yr-1. Based on the results from the 12 evaluations of model performance, the modified version of WEPP consistently performed better compared to all other model variations tested. However, results from the study suggest that additional field evaluations and improvement of soil erosion models are needed for stream crossings. The soil erosion models evaluated are not an adequate surrogate for informing policy decisions. The third study evaluated sediment control effectiveness of four commonly recommended ditch BMPs on forest haul road stream crossing approaches. Sixty ditch segments near stream crossings were reconstructed and four ditch BMP treatments were tested. Ditch treatments were bare (Bare), grass seed with lime fertilizer (Seed), grass seed with lime fertilizer and erosion control mat (Mat), rock check dams (Dam), and completely rocked (Rock). Mat treatments had significantly lower erosion rates than Bare and Dam, while Rock and Seed produced intermediate levels. Findings of this study suggest Mat, Seed, and Rock ditch BMPs were effective at reducing erosion, but Mat was most effective directly following construction because Mat provided immediate soil protection measures. Any BMPs that reduce bare soil can provide reduction in erosion and even natural site condition, including litterfall and invasive vegetation can provide erosion control. However, ditch BMPs cannot mitigate inadequate water control structures. Overall, forest roads and stream crossings have the potential to be major contributors of sediment in forested watersheds when roads are not designed well or when BMPs are not properly implemented. Forestry BMPs reduce stormwater runoff velocity and volume from forest roads, but can have varying levels of effectiveness due to site-specific conditions. Operational field studies provide valuable information regarding erosion and sediment delivery rates, which helps guide BMP recommendations and subsequently enhances water quality protection. / Ph. D.

Die Dissertation erforscht die Unsicherheit, Sensitivität und Grenzen großskaliger Erosionsmodelle. Die Modellierung basiert auf der allgemeinen Bodenabtragsgleichung (ABAG), Sedimenteintragsverhältnissen (SDR) und europäischen Daten. Für mehrere Regionen Europas wird die Bedeutung der Unsicherheit topographischer Modellparameter, ABAG-Faktoren und kritischer Schwebstofffrachten für die Anwendbarkeit empirischer Modelle zur Beschreibung von Sedimentfrachten und SDR von Flusseinzugsgebieten untersucht. Der Vergleich alternativer Modellparameter sowie Kalibrierungs- und Validierungsdaten zeigt, dass schon grundlegende Modellentscheidungen mit großen Unsicherheiten behaftet sind. Zur Vermeidung falscher Modellvorhersagen sind kalibrierte Modelle genau zu dokumentieren. Auch wenn die geschickte Wahl nicht-topographischer Algorithmen die Modellgüte regionaler Anwendungen verbessern kann, so gibt es nicht die generell beste Lösung. Die Ergebnisse zeigen auch, dass SDR-Modelle stets mit Sedimentfrachten und SDR kalibriert und evaluiert werden sollten. Mit diesem Ansatz werden eine neue europäische Bodenabtragskarte und ein verbessertes SDR-Modell für Einzugsgebiete nördlich der Alpen und in Südosteuropa abgeleitet. In anderen Regionen Europas ist das SDR-Modell bedingt nutzbar. Die Studien zur jährlichen Variabilität der Bodenerosion zeigen, dass jahreszeitlich gewichtete Niederschlagsdaten geeigneter als ungewichtete sind. Trotz zufriedenstellender Modellergebnisse überwinden weder sorgfältige Algorithmenwahl noch Modellverbesserungen die Grenzen europaweiter SDR-Modelle. Diese bestehen aus der Diskrepanz zwischen modellierten Bodenabtrags- und maßgeblich zur beobachteten bzw. kritischen Sedimentfracht beitragenden Prozessen sowie der außergewöhnlich hohen Sedimentmobilisierung durch Hochwässer. Die Integration von nicht von der ABAG beschriebenen Prozessen und von Starkregentagen sowie die Disaggregation kritischer Frachten sollte daher weiter erforscht werden. / This dissertation thesis addresses the uncertainty, sensitivity and limitations of large-scale erosion models. The modelling framework consists of the universal soil loss equation (USLE), sediment delivery ratios (SDR) and European data. For several European regions, the relevance of the uncertainty in topographic model parameters, USLE factors and critical yields of suspended solids for the applicability of empirical models to predict sediment yields and SDR of river catchments is systematically evaluated. The comparison of alternative model parameters as well as calibration and validation data shows that even basic modelling decisions are associated with great uncertainties. Consequently, calibrated models have to be well-documented to avoid misapplication. Although careful choices of non-topographic algorithms can also be helpful to improve the model quality in regional applications, there is no definitive universal solution. The results also show that SDR models should always be calibrated and evaluated against sediment yields and SDR. With this approach, a new European soil loss map and an improved SDR model for river catchments north of the Alps and in Southeast Europe are derived. For other parts of Europe, the SDR model is of limited use. The studies on the annual variability of soil erosion reveal that seasonally weighted rainfall data is more appropriate than unweighted data. Despite satisfactory model results, neither the careful algorithm choice nor model improvements overcome the limitations of pan-European SDR models. These limitations are related to the mismatch of modelled soil loss processes and the relevant processes contributing to the observed or critical sediment load as well as the extraordinary sediment mobilisation during floods. Therefore, further research on integrating non-USLE processes and heavy-rainfall data as well as on disaggregating critical yields is needed.