Due to rising temperatures, rainfall patterns around the world are being affected, causing extreme precipitation events to become more frequent and intense, resulting in an increased probability of severe flash floods. Thailand is no exception to the increased risk of these hazards, which is why Early Warning Systems are being set up. Since flash floods occur within a few hours after the triggering precipitation event, timely and accurate precipitation observations are critical, to enable timely warning and evacuation of inhabitants to mitigate the risk. One solution for this is the use of rain radar, which provides rain data with high spatial and temporal resolutions.

An analysis of this technique was chosen to form the basis of this research in Northeastern Thailand's Lam Takhlong basin. The objective was to investigate the importance of using distributed precipitation data. In this research, the hydrological response of the catchment of interest was studied. The ability of the physically-based, conceptual and distributed CALEROS model to capture this response was assessed. Different modelling strategies and sources of precipitation input were analysed, and the additional value of using distributed precipitation data was determined. The examination of multiple hydrological characteristics in the study area shows great heterogeneity of the catchments response to precipitation. A clear differentiation can be made between the hydrological response of the upstream and downstream part of the catchment. Additionally, the results from the catchment characterisation indicate great spatial variability of the precipitation patterns in the study area. This is confirmed when using the CALEROS model to recreate the catchments hydrology.

Four modelling strategies were used, varying by spatial and temporal constancy. Calibration performed using uniform precipitation showed to be incapable of capturing the discharge trends in the catchment, failing to properly catch the observed peaks as well as the base flow. Runs performed using distributed precipitation maps obtained by Inverse Distance interpolation of rain gauge data showed significantly better results, adequately capturing the trend of the discharge observed in the catchment. Differences in parameterisation only had limited effect on the outcome, making the precipitation input the most important parameter. Runs using synthetic precipitation data demonstrated the importance of properly capturing the precipitation pattern and movement across the catchment, as it greatly influences the timing of occurring discharge peaks.

A comparison between rain gauge data and rain radar data shows that, although data quality of the rain gauges seems acceptable, rain gauges are not capable of properly capturing rainfall patterns, while rain radar does. Precipitation patterns were found to be the most crucial parameter for modelling of flash floods in the area of interest, signifying the importance of using rain radar data for accurate flash flood forecasting.

Rapid urbanization has altered the natural hydrological cycle of the Tub Ma catchment, inducing major floods after heavy rainfall events. For that reason, structural mitigation measures were built, including floodwalls along the channel and a pumping station at the outlet. However, no operational rules were available for operating the river outlet structures and inundation was still common. To prevent or reduce flooding by improving the operational management of the outlet pumping system, an early warning system was required. The objective of this research was to assess whether informed decision-making resulted in an operational pumping strategy that prevents or reduces fluvial flooding. First, the question was asked whether water level monitoring could result in a timely and reliable warning service for an early warning system. The second question was whether timely operation of the outlet pumping station prevented fluvial flooding. For the fulfillment of the data requirements in this thesis, a fieldwork trip was organized first. During a short period of dedicated data survey, water level time series and river geometries were obtained. Water levels from three gauging stations were statistically tested as precursors for fluvial flooding in Tub Ma. Consequently, a hydraulic model was built to assess the flood mitigation capabilities of the outlet pumping station. Lastly, synthetic flood design hydrographs were run through the model for different pumping station operation strategies, with and without notification from a flood early warning system. The results from this thesis demonstrated that monitoring water levels in the upstream tributaries of Tub Ma was a useful information source for an early warning system that forecasts flood waves. This water level based early warning enabled a maximum potential lead time of 13-15 hours for flood mitigation. Nevertheless, connecting flood early warning to the operation of the pumping station did not reduce or prevent flooding. The Tub Ma river had a flow regime of limited baseflow and rapidly rising discharge, and as a result, the discharge capacity of the pumping station ahead of flood waves was physically limited by water level and inflow. Therefore, improved operation of the pumping station was not the solution for preventing or reducing fluvial flooding. On the basis of the results from this research, structural measures were suggested for flood mitigation, like temporary storage, a bypass channel, and measures that slow down the run-off processes. Also, it was recommended to examine the value of a flood early warning system by using 13-15 hours of lead time within a broader scope of disaster risk reduction in Tub Ma.