Flood Regulation Service Of Multiple Return Periods And Its Response To Land Use Changes-A Case Study In The Yangtze River Basin

An ecosystem’s flood regulation service（EFRS）is important for alleviating flood risk,and increasingly gains attention from the international community.The EFRS and the relationship between EFRS supply and demand are not only subject to land use changes,but also vary across flood return periods.However,previous studies mostly only considered the EFRS under a single flood return period,and lacked a comprehensive method evaluating the EFRS and its supply-demand relationships across different flood return periods.Moreover,there is a lack of systematic understanding of the impact of non-structural measures,e.g.,land uses,and structural measures,e.g.,dam construction,on the multiple flood return-period EFRSs.In this regard,a comprehensive framework is proposed that integrates multiple flood return periods to assess the basin-scale EFRS supply and demand.Based on basic data such as rainfall,land use/cover,soil,dams,flood protection levels,and floodwater depth of different flood return periods,the EFRS supply is calculated as the rainwater retention and flood water regulation capacity,the EFRS demand is estimated as the floodwater volume,and the concept and evaluation method of expected annual EFRS supply（demand）are proposed on the basis of the EFRS supplies（demands）under different return periods.The framework also considers the EFRS supply flowing from the headwaters toward the downstream,and evaluates the impacts of land use/cover changes and dam construction on the EFRSs.The present study takes Yangtze River Basin（YRB）,China as research area to conduct an empirical study.The main conclusions are as follows:1.This research investigated the variation of EFRS supply and demand under different flood return periods.First,the local EFRS supplies remained relatively constant across different return periods,only slightly increasing by 4.15%from 25-year to 200-year return periods.Second,the EFRS demand increased dramatically from 25-to 200-year return periods,with an increase of 146.98%from 3.18×10¹¹m³ to 7.86×10¹¹m³,and high demand density（>60%quantile）sub-basins（19–64）gradually spread from downstream to midstream.In addition,the number of sub-basins with insufficient supply increased dramatically from lower to higher return periods（43–81）and their distributions gradually expanded.Under the 200-year return period,these unsatisfied sub-basins occupied the lower reaches,middle reaches and the east of upper reaches of YRB.The results showed that the EFRS demand and the relationship between supply and demand of EFRS varied with flood return periods.2.This research put forward the concept and evaluation method of expected annual EFRS supply and demand.Based on the results of different return periods,the concept of expected annual EFRS was proposed that was the integration of EFRS under different flood return periods,and this research established evaluation models for expected annual EFRS supply and demand,respectively.The results showed that the expected annual local EFRS supply in YRB in 2015 was 1.78×10¹⁰ m³,and the number of high expected annual supply density（>60%quantile）sub-basins was 43.The expected annual EFRS demand was 1.71×10¹⁰ m³ in YRB in 2015,and there were 43 high expected annual demand density（>60%quantile）sub-basins,being concentrated in the middle and lower reaches.In terms of the expected annual EFRSs that integrated the four return-period EFRS demands and local supplies,47（or 43.12%）sub-basins were found to be unsatisfied（the demand is larger than the supply）.Therefore,the expected annual supply and demand integrating multiple flood return-period results could be more representative than that of a single flood return period,and its quantity and spatial pattern were both in between the results of high and low return periods.3.This research quantified the potential delivery of EFRS supplies across sub-basins in YRB.On the basis of local EFRS supply,the relationship between upstream and downstream of YRB was modeled according to the flow direction and coding rules of river networks for China’s water environment,and the local EFRS supplies of upstream sub-basins were accumulated on the downstream sub-basins to obtain the upstream EFRS supply.First,the sub-basin-level supply density of upstream EFRS supply was significantly higher than the local EFRS supply.The high supply density（>60%quantile）sub-basins have greatly increased from 40（local EFRS supply）to 77（upstream EFRS supply）under 25-year return period,and increased form 49（local EFRS supply）to 81（upstream EFRS supply）under 200-year return period.The number of high expected annual supply density（>60%quantile）sub-basins would increase from 43 to 79 if the expected annual upstream EFRS was included.Furthermore,the unsatisfied sub-basins could be largely alleviated with upstream EFRS supplies.The number of unsatisfied sub-basins could be reduced from 43 to 16 under 25-year and from 81 to 34 under 200-year return periods.There were still 17 unsatisfied sub-basins remained if the upstream EFRS supply was considered in comparing the EFRS supply and demand.The results revealed that the upstream EFRS supply was of great significance in regulating flood risk,but the upstream EFRS supply could not completely solve the flood risk especially under high return periods.4.This research comparatively analyzed the roles of non-structural measures（land uses）and structural measures（dam construction）in mitigating flood hazards.In terms of non-structural measures,through the establishment of a transfer matrix of land use and expected annual EFRS in YRB from 1980 to 2015,the contribution of the mutual conversion between different land uses to the net supply loss/increase was quantified,and on this basis,the relative contribution of the increase or decrease in the area of land use was evaluated.The relative contributions of the increase or decrease in area of paddy fields,dry lands,woodlands,shrubs,sparse woods,other forest lands,water bodies（rivers and lakes）to EFRS supply were positive,while the relative contributions of grasslands（mainly transformed by water bodies）,urban lands,and unused lands were negative.When the farmlands in 1980 were converted to forest lands and water bodies in 2015,the converted areas reached 4125.87 km²and 5578.29 km²,respectively,and their contributions to the net increase in supply were 1.42×10⁷ m³and 4.73×10⁸ m³,respectively.This indicates that the land use policies implemented in China such as“Grain for Green Program”and“Returning farmland to lake”have effectively increased the EFRS supplies via protecting forest lands and water bodies.In terms of structural measures,the dams’share to the total EFRS supplies increased from 7.86%in 1980 to15.31%in 2015.Although the dams’proportion of the total supplies was outweighed by part of land use types,the relative contribution of dam changes to the expected annual EFRS supply（including dams）change was greater than that of land use changes,with a relative contribution of as high as 94.85%,compared with 5.15%from land use changes.Therefore,it is critical to rationally regulate dams becasuse they plays an important role in enhancing flood hazard mitigation;meanwhile,green infrastructure and nature-based solutions should be considered as they still dominate the EFRS supplies.This research developed a multiple return-period framework to estimate the variations of the EFRS supply and demand across different flood return periods and explored the expected annual EFRS supply and demand by integrating the different values and their associated probabilities.This research also analyzed how the spatio-temporal pattern of expected annual EFRS supply and demand were affected by the non-structural measures（land uses）and structural measures（dam construction）.The relevant conclusions could provide a certain scientific basis for the comprehensive management of flood risk in the YRB and the proposed framework could be readily applied in other basins and shed light on a comprehensive understanding and management of the EFRSs.