Influence of different relief data sources and scales on flood estimations from Nash’s Instantaneous Unit Hydrograph

Water is a natural resource indispensible to life and essential to regional economic development. Due to climate change, anthropic interferences, and rapid population growth, the occurrence of flood-related natural hazards in watersheds has increased. Watershed flood-related studies allow the estimation of peak streamflow and direct surface runoff hydrograph resulting from single or multiple rainfall events. However, such estimations are directly dependent on existing streamflow historical series, which might be troublesome in developing countries due to the lack of streamflow gauging stations. In this context, indirect flood estimation methods stand out. Among the different flood estimation methods presented in the literature, the Unit Hydrograph (UH), Instantaneous Unit Hydrograph (IUH), and Geomorphological Instantaneous Unit Hydrograph (GIUH) have caught researchers’ attention. The Nash’s IUH (NIUH) is one of the most widely used IUH models. Several geomorphological approaches have been developed for NIUH, thus relating its parameters to watershed and drainage network physical characteristics. During the characterization of watersheds in geoprocessing softwares, the main information is that of relief obtained from a Digital Elevation Model (MDE) which can be obtained from topographic maps or from radar images (e.g. SRTM) and sensor images (e.g. ASTER). The main objective of this study was to evaluate the applicability and reliability of different relief data sources and scales for determination of geomorphological parameters required to estimate floods from NIUH. This study took into account data sets from five experimental watersheds with different physiographical characteristics, which have hydrological monitoring. The DEMs analysed in this study were obtained from topographical maps in the 1:50,000 scale, SRTM images with 30 and 90-meter spatial resolution, TOPODATA database, ASTER images. For the smallest watershed, an in situ topographic survey was also carried out for DEM derivation. Four geomorphological approaches for the NIUH were selected; two of them were based on the GIUH theory, whereas, the others were adjusted from empirical studies conducted in different watersheds. Based on the results obtained for the analysed watersheds, the main conclusions were: a) the main watercourse’s slope and Horton’s and Schumm’s ratios are the most sensitive parameters to relief data sources and scales; b) flat watersheds are the most susceptible to altimetry errors, which increase as the watershed area decreases; c) it is not possible to identify any combinations of geomorphological approach-DEM that better or worse describe all the analyzed watersheds when assessing watershed slope or drainage area independently; d) the geomorphological approaches which do not depend on streamflow speed information presented satisfactory results when compared to those based on GIUH theory; and e) the combination of different approaches enables to satisfactorily estimate the behavior of direct surface runoff hydrographs and their peak streamflow and time in all the considered watersheds.Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPqA água é um recurso natural indispensável à vida e primordial ao desenvolvimento econômico de uma região. Devido às mudanças climáticas, associadas às ações antrópicas e ao crescimento populacional, a ocorrência de problemas relacionados a cheias em bacias hidrográficas tem aumentado. O estudo de cheias em bacias hidrográficas permite a quantificação da magnitude das vazões de pico e do hidrograma de escoamento superficial direto oriundos de um ou mais eventos de chuva. No entanto, tais estimativas dependem de dados de séries históricas, o que pode ser problemático nos países em desenvolvimento devido a existência de um número insuficiente de seções com monitoramento fluviométrico, tornando a modelagem hidrológica de cheias uma ferramenta imprescindível. Diferentes métodos para estimativa de cheias vêm sendo apresentadas e utilizadas na literatura, com destaque para a do Hidrograma Unitário (HU), a do Hidrograma Unitário Instantâneo (HUI) e a do Hidrograma Unitário Instantâneo Geomorfológico (HUIG). Um modelo de HUI amplamente utilizados é o de Nash (HUIN), para o qual diversas propostas geomorfológicas vêm sendo desenvolvidas, estabelecendo relações para seus parâmetros a partir da caracterização física da bacia hidrográfica e da rede de drenagem. Durante a caracterização de bacias hidrográficas em softwares de geoprocessamento, a principal informação é a do relevo, obtida a partir de um Modelo Digital de Elevação (MDE), que pode ser obtido a partir de cartas topográficas, ou de imagens de radar (ex. SRTM) e sensor (ex. ASTER). O principal objetivo deste estudo foi avaliar a aplicabilidade e confiabilidade de diferentes fontes e escalas de informação do relevo visando à modelagem de cheias através do modelo de HUIN fundamentado em parâmetros geomorfológicos, tomando como base cinco bacias hidrográficas experimentais de diferentes características fisiográficas e dotadas de monitoramento hidrológico. Os MDEs utilizados foram obtidos de cartas topográficas na escala 1:50.000, de imagens SRTM com 30m e 90m, de imagens do banco de dados TOPODATA, de imagens ASTER, e somente para a menor bacia, de dados de um levantamento planialtimétrico. Foram selecionadas quatro propostas geomorfológicas para o HUIN, sendo duas delas baseadas na teoria do HUIG, e as outras duas em estudos empíricos realizados em diferentes bacias hidrográficas. Com base nos resultados obtidos para as bacias analisadas, as principais conclusões deste estudo foram: a) os parâmetros mais impactados pela fonte e escala da informação do relevo são a declividade do curso d’água principal e as razões de Horton e de Schumm; b) as bacias planas são mais suscetíveis a erros altimétricos e estes aumentam conforme a área da bacia diminui; c) não é possível observar uma combinação de proposta-MDE que descreva melhor ou pior o conjunto de bacias analisadas, nem levando em consideração a declividade, nem o tamanho da área das bacias; d) as propostas geomorfológicas que não dependem de informação da velocidade do escoamento apresentaram bons resultados em relação as baseadas na teoria do HUIG; e) a combinação de diferentes propostas permite estimar de forma satisfatória o comportamento do hidrograma de escoamento superficial direto e o tempo e a vazão de pico nas bacias estudadas

Riscos de inundação em bacias regularizadas: Estudo de caso da cheia do rio Mondego, Portugal

Hydrological studies of flooding in watersheds with a high hazard of inundation are important for mitigation and prevention methodologies. The Mondego river flooding, which occurred in Portugal in 2001, with 460.1 mm total month precipitation around the city of Coimbra, generated great economic and social losses. This study proposed to calculate the flood Return Period (RP) that occurred in 2001 using a methodology developed for the analysis of flood risk in regularized basins. The information used in this analysis was data of precipitation and reservoir average level. The results showed that the 2001 Mondego River flood has an RP of 439.4 years. The combination of precipitation and reservoir data proved to be important to understand the dynamics of floods in a regularized basin. In this sense, the proposed methodology can be applied to any basin that is heavily influenced by upstream reservoirs.Estudos hidrológicos de cheias em bacias hidrográficas com históricos alarmantes de inundações são de valiosa importância para aplicar metodologias de prevenção e mitigação. Para a cheia de 2001 no rio Mondego, em Portugal, estimou-se 460,1 mm mensal próximo à cidade de Coimbra, gerando grandes prejuízos econômicos e sociais. Esse estudo propôs calcular o Tempo de Retorno (TR) da cheia ocorrida em 2001, utilizando uma metodologia desenvolvida para a análise de risco à inundação em bacias regularizadas. As informações utilizadas nessa análise foram dados de precipitação e nível médio dos reservatórios. Os resultados indicaram que a cheia no rio Mondego, ocorrida em 2001, possui TR de 439,4 anos. A combinação de dados de chuva e de reservatório se mostrou importante para compreender a dinâmica de inundações em uma bacia regularizada. Nesse sentido, a metodologia utilizada neste trabalho pode ser aplicável a qualquer bacia que sofra grande influência de barramentos a montante e com vazões bem regularizadas

Applicability of geomorphological approaches combined with the modified Clark’s model for flood hydrograph estimation

Among the techniques available for modeling direct surface runoff (DSR), the Unit Hydrograph and the Instantaneous Unit Hydrograph (IUH) have been widely used. Clark’s IUH (CIUH) stands out, mainly in its distributed version known as ModClark, which has two parameters: time of concentration (tc) and storage coefficient (R). The best procedure is to determine tc and R from observed rainfall-runoff datasets, however, they usually are not available due to a lack of monitoring or insufficient temporal discretization. Thus, geomorphological approaches have been developed to allow the estimation of t c and R from relief information of the watershed, however, their use is little disseminated in literature. In addition, only a few studies have evaluated the influence of spatial discretization of the watershed on the definition of tc and R (parameterization) and its impact on the application of geomorphological approaches for the ModClark. This work aimed to evaluate the ModClark performance for estimating DSR hydrographs in two Brazilians watersheds (Cadeia river watershed and Jaguara creek watershed), using four little-known geomorphological approaches (PST, POA, PD, and PMC) for the determination of its parameters under two parameterization schemes: lumped and semi-distributed. The following findings were evidenced: i) the application of the ModClark presented a satisfactory performance for estimating DSR hydrographs; ii) the characteristics of the selected rainfall-runoff events also had a significant influence on the estimation of the DSR hydrographs; iii) the lumped parameterization scheme provided more adequate parameters for ModClark, thus resulting in more accurate DSR hydrographs; iv) PST gave the most accurate estimated DSR hydrographs for the Cadeia river watershed under both schemes and for the Jaguara creek watershed under the lumped scheme, whereas PMC outperformed the other approaches for Jaguara creek watershed under semi-distributed; and v) for watersheds with geomorphological characteristics similar to those of the watersheds analyzed in this study, PST would be the most suitable geomorphological approach for determining the ModClark’s parameters

Soil loss estimated by means of the RUSLE model in a subtropical climate watershed

Erosion process occurs naturally, shaping the Earth’s surface. Soil loss can cause harmful effects to the environment when intensive anthropic activities occur. Mathematical models have been used as effective and less costly alternatives for identifying sites highly prone to soil loss, especially at the watershed scale. In Brazil, the Revised Universal Soil Loss Equation (RUSLE) is one of the most commonly used soil loss prediction models. The RUSLE requires information on soil erodibility, rainfall erosivity, topography, land use and cover (C), and conservation practices (P) to estimate average annual soil losses. Images derived from remote sensing techniques are generally used to quantify the spatialization of C factor; however, the variation in land use throughout the year is not usually considered. This study aimed to estimate soil losses in an important subwatershed of Candiota river watershed (CRWsub) by using RUSLE, considering land use and rainfall erosivity in different periods of the year. The periods considered were P1 (January, February and March), P2 (April, May and June), P3 (July, August and September) and P4 (October, November and December). Based on the results, the lowest soil losses occurred in P1. Probably, the high vegetation cover in the soil increases its protection against rainfall erosivity. In P3, the heavy rainfall events are predominantly frontal, occurring in the same months as those when the preparation of the soil for later planting takes place; that is, there is no vegetation cover in this period, thus making the soil more prone to erosion. The use of different images to classify and identify land uses is the best way to understand soil losses throughout the year in the study area. It was possible to observe that agricultural areas are generally associated with greater soil losses in the subwatershed. In addition, the land uses were considered to vary quarterly, thereby making it possible to identify the periods most prone to erosion processes throughout the year. Finally, the erosion percentages in the subwatershed can be linked to the tolerance index for different land-uses, soil classes, and slope categories

Hydrological regionalization of maximum stream flows using an approach based on L-moments

<div><p>ABSTRACT The proper design of hydraulic structures depends on estimates of maximum stream flows. The scarce stream flow monitoring in Brazil has led to the use of regionalization methods. The main objective of this study was to develop a tool via regional function to estimate maximum stream flows and their corresponding return periods (RP) with the aid of techniques based on the L-moments method, seeking for adequate hydrologic engineering applications and flood risk management. Annual maximum stream flow historical series were adjusted to traditional 2-parameter probability density functions (PDFs) (Normal, 2-parameter Log-Normal, Gumbel, Gamma) and multiparameter PDFs (GEV and Kappa), based on the L-moments method, which were used in the development of the regional function employing the dimensionless curve method. The regional function’s predictive capability was determined by cross-validation for different RPs. It can be concluded that the approach based on L-moments was successfully used to adjust the regional function. In addition, the regional function: i) was improved when using the aforementioned multiparameter PDFs and ii) was framed as optimum for RP of up to 100 years and considered useful for practical engineering projects and flood risk management.</p></div