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Skill of a discharge generator in simulating low flow characteristics in the Rhine basin

Kersbergen, A.M. (2016) Skill of a discharge generator in simulating low flow characteristics in the Rhine basin.

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Abstract:Low flows are important to consider in water management as low flows can have societal and economic impact: by e.g., navigation problems, lack of irrigation water for agriculture, salt intrusion, lack of cooling water and bloom of algae. Synthetic time series can be used for low flow frequency analysis and for gaining information about the development and characteristics of low flows. The Generator of Rainfall And Discharge Extremes (GRADE), consisting of a weather generator, a hydrological model and a hydraulic model, has given satisfactory results for simulating peak flows with large return periods in the Rhine basin, and it is expected that there is potential to apply the combination of models also for low flow analysis. In this research the skill of the hydrological model, the skill of the weather generator and the skill of the combination of weather generator and hydrological model for simulating low flows in the Rhine basin are evaluated and a first start is made with the improvement of skill of the hydrological model. Low flows are defined as discharges under the monthly thresholds determined by the Dutch National Committee of Water allocation (LCW) and split into thresholds in the growing season and thresholds throughout the entire year. For seven discharge locations at the outflow of seven mayor sub-basins in the Rhine (Lobith (Lower Rhine), Andernach (Middle Rhine), Cochem (Moselle), Frankfurt (Main), Rockenau (Neckar), Rekingen (East Alpine sub-basin) and Untersiggenthal (West Alpine sub-basin)) analyses are conducted on the low flow discharges, the low flow events (duration and cumulative discharge deficit below the threshold), lake levels, snow covers, groundwater levels and the meteorology. After the evaluation of the skill has been decided to improve the performance of the hydrological model in part of the East Alpine sub-basin by recalibration. Five parameters, including the snowfall correction factor, have been selected and a Monte Carlo simulation has been performed for four sub-catchments. The results from the evaluation of the skill show that discharges are mainly underestimated in the historical simulations by the hydrological model. This causes more low flow events and more severe events. In the Alps most underestimation takes place in the summer. The simulation of snow plays a role in this. Although a conceptual hydrological model is used, variations in processes like snow, lake levels and groundwater are captured well. The synthetic series of the weather generator simulates periods of dry weather, but less persistent dry periods (especially in the summer), which makes that less low flows occur and there is a decrease of extreme severities of low flow events compared to the observations, especially for events with the growing season thresholds. In the West Alpine sub-basin the snowfall from the weather generator is less than with observed weather, causing more low flows in summer. Comparing the synthetic simulations with the observations gives a good skill of the model for discharges at Rockenau, return periods of duration at Lobith and Andernach and the return periods of duration and severities in the growing season at the Alpine locations. This skill is however based on the compensation of two errors. The skill of the hydrological model for simulating low flow characteristics has been improved by the recalibration. There is less underestimation of flow and thus there are less false alarms. The performance on the other analyses has improved or stayed the same. Synthetic weather series are a useful tool in low flow risk assessment, when both the weather generator and the hydrological model give acceptable results. In this study is shown that models made for simulating peak flows are not necessarily acceptable for low flows. By tracing the important processes in the model (in this case snow) and with focus on low flows, improvements in the skill are possible.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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