Browsing by Author "Bárdossy, András (Prof. Dr. rer. nat. Dr.-Ing. )"

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Open Access
Discrete-continuous downscaling model for generating daily precipitation time series
(2008) Yang, Wei; Bárdossy, András (Prof. Dr. rer. nat. Dr.-Ing. )
This work was aimed to improve the generation of daily precipitation time series with information from atmosphere. The motivation is to develop a conditional stochastic downscaling model to well describe temporal and spatial behavior of local precipitation, in particular, the extreme rainfall events. Thereafter, the generated daily precipitation can be applied to integrate with other water-related models for climate impact studies. Global warming has been concerned since the end of last century. The increase of the temperature may result in various consequences, which may further cause the changes in agricultural production, water resources distributions and so on. Therefore, the impact studies must be carried out to understand the influence of the changing climate and predict its possible consequences in order to mitigate and adapt to the changing climate. In the field of hydrology, detailed information such as the situation of landuse, the condition of local climate and others is always required to describe the hydrological processes. Precipitation, especially, is of great concern due to its spatial and temporal variability. Basically, precipitation is a product of atmospheric motions and physical processes in the atmosphere on one hand and an important driving force in the land-atmosphere interactions on the other hand. It is the result of atmospheric movement. Therefore, it is reasonable to derive information about precipitation from the atmospheric studies. The global climate models (GCMs) do produce daily precipitation time series. However, due to their coarse resolutions and incomplete understanding of climate science, the outputs of GCMs can not properly describe the processes in detail for the local regions. A certain method is required to match the mismatch between two different scales and this method is “Downscaling”. In this thesis work, three statistical downscaling methods were explored: a CP- and Regression-based downscaling approach, a CP- and Copula-based downscaling approach and a multi-site weather generator. The first two methods were developed based on the circulation patterns. The circulation patterns can be obtained either from professional knowledge (subjective classification) or from statistical characteristics derived from the observations (objective classification). The scheme proposed and investigated here is based on fuzzy-rule logic. It is a method that works on the concept of fuzzy sets, describing the atmospheric circulations using imprecise statements. The circulation patterns are useful to capture the information at a large-scale, however, they are weak in capturing the continuity of the whole natural system. The additional predictors are therefore required. A combined term, moisture flux, is introduced into the pure CP-based downscaling model. It describes convey of the water vapor by the wind field. It was proved to be highly correlated to the local rainfall events in terms of rainfall probabilities and rainfall amounts and enhance the performance of the CP-based downscaling model. A CP- and Copula-based downscaling model is a further development of the previous downscaling model. The relationship between daily precipitation and moisture flux was described by a joint distribution based on the concept of copula instead of regression method. By using the concept of copula, the dependence is represented by a copula function that couples one-dimensional uniform distribution functions to a multivariate joint distribution. All the marginal distributions of studied variables are uniformed in the space (0, 1). Therefore, any appropriate marginal distribution is allowed to be selected. Furthermore, the copula function is able to represent the various dependence structures between the different quantiles of the variables, which makes it possible to fully reproduce the dependence structure identified from the observations. The CP-based downscaling model is suitable for the regions located in the higher latitudes, where the Coriolis force is quite dominant in forming anticyclones and cyclones. For other regions near to the equator, where the Coriolis force is weaker, the same methodology does not work properly anymore. To downscale daily precipitation for those lower latitude regions, a multi-site weather generator was developed. The developed model is a stochastic statistical downscaling model. It is able to simultaneously generate the reasonable daily rainfall time series. The models have been successfully applied to the different river basins located in the contrasting climate zones. The critical CPs for specific river basins were identified and they could be used to explain the large rainfall events. The generated daily precipitation are comparable to historical observations and can be used as input to other water-related model for the river basin management studies under the impact of climate change.
Open Access
Impacts of the existing water allocation scheme on the Limarí watershed : Chile, an integrative approach
(2015) Kretschmer, Nicole; Bárdossy, András (Prof. Dr. rer. nat. Dr.-Ing. )
The research is motivated by an interest in evaluating the special Chilean water management framework, relating to the 1981 Water Code legislation, introduced by the military government. This law mainly strengthened private property rights and increased private autonomy in water use. In particular, it is of interest to assess the impacts of this legislation in the context of the current highly stressed water availability situation in central and northern Chile, combined with intensive and increasing agricultural demands. The reason to look at this region first is to test a catchment with a more or less vivid water market. The purpose of this research is to investigate the influence of water rights on water management practices under the present situation as well as changing situations. Here changing situation refer on one hand to improvement and extension of infrastructure, on the other hand to different use of the water in magnitude and further time and space. The latter one mainly based on the water market. The main objective of this study is to investigate if the proposed WRAP Modelling System (Water Rights Analysis Package) which is used in the whole state of Texas, is able to model the consequences of the allocation scheme in the present, as well changing situations, incorporating the Chilean legal framework, here especially the allocation according to water rights. The main changes are subject to i. new legislation to incorporate in the allocation of water resources ii. further development, like new reservoirs in the upstream sub-catchments, iii. water right transfers as well as iv. different operation policies WRAP was chosen to investigate the impacts of the water management practices. It combines detailed information describing water resource development, management, allocation and use with natural river system hydrology represented by naturalized streamflows, assuming that the hydrological pattern of a catchment stays the same in the future (Wurbs, 2011). Beside the development of the spatial configuration of the system, which has been defined as a set of control points (CP) that represent pertinent sites in the river basin, geospatial data, time series data, census data, operational data sheets of the organisations as well as information and data about the water rights of each stakeholder have been statistically and spatially pre-processed in order to be able to estimate agricultural water demand, understand the legal system in general and of the basin under study in particulary. Further information of the Food and Agricultural Organisation (FAO), monitored data of the National Water Authority (DGA), elaborated data of the National Centre of Natural Resources (CIREN) and historical and actual regional as well as local studies were consulted to elaborate all the needed information to model the system. With this information and preprocessed data the WRAP modelling system was implemented, to quantify the impacts of decision making and its consequences on the whole system. Model results include water supply reliabilities (including reliability indices) as well as flow and storage frequency statistics developed from the simulation results representing long-term probabilities or percent-of-time estimates. Furthermore shortage metrics have been developed by the model and evaluated for each scenario. The model includes the following frequency statistics for concisely summarizing modelling results: (a) volume and period reliability tables for water supply diversion, (b) frequency tables for naturalized, regulated and unappropriated flows, reservoir storage volumes, as well as instream flow shortages and (c) reservoir storage-reliability tables. After all the different scenario simulations and analysis of the results it can be stated that the WRAP modelling system is applicable for the questions under study based on the legal Chilean water management framework. Flexibility is provided for adaption of a broad range of modelling approaches. A huge variety of management records can be combined in many different ways to be able to model any application. Ingenuity is required from the modeller to achieve the incorporation of sometimes quite complex allocation rules, apply different target options, demands, administrate a variety of users and include new developments within a multiple and multipurpose reservoir-river management system. Although some simplification of the independent sub-catchments was necessary, the achieved results show that the consequences of allocation decisions, including water transfer and future development are simulated in a satisfactory manner and can therefore be much better understood. The model system is adequate to serve as a basis for decision making within the chilean legal framework.
Open Access
Sediment dynamics in the floodplain of the Mekong Delta, Vietnam
(2011) Nguyen, Nghia-Hung; Bárdossy, András (Prof. Dr. rer. nat. Dr.-Ing. )
The Mekong Delta is one of the largest and most intensively used estuaries in the world. It experiences annual widespread flooding, which provide the basis of livelihood for about 17 million people in the Mekong Delta, but they also pose a considerable hazard when extreme events exceed protection levels. Especially since the Delta in Vietnam is intensively used for agriculture, the pristine natural floodplains have been altered to channel networks, dike rings, paddy fields and aquaculture ponds. Sediment dynamics play an important role in carrying contaminants, bacteria, nutrients, heavy metals, phytoplankton, pesticides, etc. They are the primary source for the productivity of biota in floodplains as well as a sustainable agroecosystem within the Delta. However, little is known about the dynamics of these suspended sediments, including multi-processes erosion, deposition, and suspension in the complex channel network of the Delta. In particular, quantitative analyses are lacking, mainly because of lacking data about the inundation processes in the floodplains. In 2008, therefore, a comprehensive in-situ monitoring scheme to monitor floodplain inundation as well as the dynamics of suspended sediment was established in a study area of the North-Eastern part of the Delta, the Plain of Reeds. The in-situ monitoring system is equipped with twenty-one water level probes and seven water quality monitoring stations. They all have a robust design, autonomous power supply, and the ability to gather reliable data over a long period of time with a high temporal resolution. The water quality parameters include suspended sediment concentration, pH, EC (electrical conductivity) and water temperature. Sediment traps were installed additionally and used in combination with recorded suspended sediment loads to investigate deposition and erosion in the floodplains. This in-situ data collection was complemented by high-resolution inundation maps derived from the TerraSAR-X satellite. Hence, the floodplain hydrology of the Delta could be described quantitatively for the first time. In detail, the present study investigates:(1) spatial and temporal floodplain inundation processes, (2) anthropogenic influence on floodplain hydrology, (3) the tidal influence on flow regime, (4) suspended sediment dynamics, and (5) the erosion and deposition processes during flood season. These processes were analysed quantitatively, providing insights into basic control parameters, dynamics of the processes and the quantity and timing of human interference. The practical contribution of this study is to fill the knowledge gaps in water resources management in order to foster a sustainable future development of the Delta. The scientific contributions are the new ideas in the in-situ monitoring system for floodplain inundation study. In particular, it provided insights into the sedimentation process and enabled an estimation of the key parameters for deposition and erosion as a basis for future model based quantitative estimations of floodplain sedimentation.