Advancing frequency analysis of nonstationary hydrological extremes for reducing flood risk in a changing climate

Floods are the most frequent type of natural disaster all over the world. Flood control and sustainable water resources development under the current and changing climate are of vital importance globally, especially for Norway, a country whose prosperity is largely dependent on the flood level control and water use efficiency. Flood frequency analysis (FFA) is one of the cornerstones in the planning, design and management of hydraulic projects for flood control and water usages. The basic assumption of traditional FFA methods is that the hydrological data used are stationary, independent and identically distributed over time. However, in the past decades this stationarity assumption has been severely challenged because global climate change and/or large-scale human activities have altered the statistical characteristics of hydrological processes. Hydraulic structures (e.g. Dams, Bridges, etc.) design using conventional methods based on the assumption of stationarity will not provide the assumed level of protection (increase dam failure risk) or production of hydropower over the design life (reduce efficiency and effectiveness) since non-stationarity will cause the return period of the design discharge uncertain and change over the project life. This project aims to expand current knowledge of non-stationary FFA and bring new statistical tools to the analysis of hydrological extremes to improve policy and construction by incorporating original thinking and scientific renewal. The key user groups (stakeholders) are the hydropower industry, river basin management society, and authorities (both national and local), who are responsible for planning, design and operation of hydropower reservoirs, for adaptation to climate change in planning of land use and infrastructure at various levels, and for flood forecasting, prevention, protection, floodplain mapping and mitigation, etc. The project is coordinated by Prof Chong-Yu Xu (UiO) in cooperation with the national experts/partners in Norway, e.g. NVE, NORCE, and Statkraft, which include universities, research institutions, government agency, as well as hydropower company. International experts/partners are Texas A&M Univ from USA, University of Malawi from Africa, National Institute of Hydrology from India; Wuhan University, Three Gorges Corporation (the world largest hydropower company) from China. During the project year of 2020.10 ? 2021.9, all planned activities that do not require cross continental travel have been carried out according to the plan and resulted in fruitful results as summarized in the following. (1) The partners and involved students jointly published 19 relevant articles in international peer-reviewed journals on nonstationary flood frequency analysis, variation of hydrological extremes (extreme rainfall, extreme runoff) due to climate change and human influences, and hydrological modeling of climate change impact. (2) The partner members are active in disseminations, which include more than 10 presentations and invited talks organised by institutions and international organisations in more than 6 countries, including The EGU General Assembly 2021. (3) Publication of two popular articles in international journals and websites. (4) Graduation of Master and PhD students who have been benefited from joint supervision and contributed to the project with other funding sources. Due to the influence of COVID-19, there are some deviations for the activities requiring cross continental travel, e.g. the planned annual meeting/workshop and regional training course in Malawi in September 2021 on flood control and water resources management with trainees from southern African countries will be postponed to September 2022. We are now preparing the details of the training course with the support and help of the University of Malawi.

This proposed study follows the Norway's strategy shift toward sustainable development and global environmental protection. It will also contribute to The New Scientific Decade 2013-2022 of IAHS by supporting societies to adapt to changing conditions by considering the uncertainties and feedbacks between natural and human-induced hydrological changes. Frequency analysis of hydrologic extreme variables is vital to hydrological planning, design and sustainable development of water resources. In recent years, the increase of the frequency and intensity of storms, floods and droughts caused by climate change and intensified human activities has challenged the usefulness of traditional frequency analysis methods that assume hydrological extreme variables are stationary, independent and identically distributed over time. There have been methods proposed for the frequency analysis of hydrological variables under non-stationary conditions; however, no single method will revolutionize the way to solve the problem of non-stationarity. Moreover, due to knowledge and data limitation, uncertainty involved in extrapolating frequency curves is huge. Therefore, there is a need for developing more consistent nonstationary frequency analysis methods that can account for transient nature of a changing environment. On the basis of assessment of existing theories and methods by incorporating original thinking and scientific renewal, this project will develop an integrated frequency analysis approach incorporating nonstationarity and uncertainty for hydrological design and sustainable water resources management. The research will improve our understanding in the limits of existing methods, expand the knowledge of frequency analysis in nonstationary environment, and provide an integrated approach to serve regional planning of water project construction, efficient operation and maintenance of existing hydropower systems and disaster reduction.