Assessment of urban flood mitigation capacity on the Qinghai-Tibet Plateau: the case of Lhasa City

Precipitation on the Qinghai-Tibet Plateau has shown an overall increasing trend in recent years, both the intensity and frequency of extreme precipitation events have increased．Continued expansion of plateau cities have affected hydrological cycles, this coupling effect exacerbates the risk of urban flooding．Historical and future changes in flood mitigation capacity of Lhasa city were quantitatively assessed here, by coupling FUTURES urban expansion simulation model and urban flood mitigation capacity assessment model, to integrate different climate change and urban expansion scenarios．It was found that from 2000 to 2015 overall flood mitigation capacity of Lhasa city had decreased by 42.70%．In particular, Chengguan and Dulongdeqing districts each decreased 4.63% and 8.55%, respectively, but mitigation capacity of Dazi district increased 1.13%．Future flood mitigation capacity in Chengguan District was found to have decreased by an average of 15.01% and 29.52% under RCP4.5 and RCP8.5 scenarios, while in Duilongdeqing District it decreased by 16.16% and 31.95%．The new urban land in Chengguan and Duilongdeqing districts has had their mitigation capacity reduced by more than 50% compared to original natural surface．Measures to protect water bodies in urban expansion will likely reduce further degradation of mitigation capacity．Future urban expansion will lead to significant reduction in future flood mitigation capacity in Lhasa city, while climate change will further amplifies its degradation．In Lhasa city the trend of urban construction to expand into water bodies and grasslands must be curbed in the future, and urban land use efficiency must be improved．Construction of green infrastructure within the city will need to be strengthened, standard of flood control and drainage facilities will need to be improved to cope with possible extreme precipitation events due to climate change．