| Water resource scarcity and water environment pollution lead to an increasingly prominent water crisis.Upstream and downstream regions often have conflicts in water utilization and discharge,which will further exacerbate the adverse effects of the water crisis on the coordinated development of regional society,economy,and ecology.Watershed eco-compensation is a flexible and efficient water management method to alleviate the conflict of water utilization between upstream and downstream regions and coordinate the watershed’s resource,ecological and economic development.The definition of responsibility for eco-compensation,the determination of compensation amount,and the construction of eco-compensation measures are indispensable considerations.The single-level eco-compensation study ignores the interaction between eco-compensation links and fails to propose a systematic watershed eco-compensation planning that considers multiple elements and links.The single-level eco-compensation affects the scientificity and integrity of compensation decisions.Ensuring ecological water demand in a watershed is an important prerequisite for achieving ecological runoff maintenance,ecosystem function,self-purification,biodiversity and other water ecological functions.The ensurance of ecological water demand is of great significance for watershed ecological restoration.However,systematic watershed eco-compensation planning considering ecological water demand has not been reported,and the utility of eco-compensation mechanism considering ecological water demand is not clear.In this study,the Daguhe watershed in Qingdao,Shandong Province is selected as the study area.The major pollutants,i.e.,ammonia nitrogen（NH3-N）and total phosphorus（TP）,are used as water quality factors for eco-compensation.The Soil and Water Assessment Tool（SWAT）,interval chance-constrained programming（ICCP）,ensurance of ecological water demand（EW）,and eco-compensation mechanism（EC）are integrated into a model group.Then the model group is established for researching on medium and long-term（2021-2030）eco-compensation mechanisms and planning.The main results include:（1）Under all mechanisms,low available water level leads to low system benefits because low available water level limits the amount of water utilization for production（fisheries,plantations,livestock farming）and the scale of them,which leads to low system benefits.During the planning period,the average system benefit of the ecological water demand-based system eco-compensation mechanism（EWEC）under low available water level is[7.0,11.9]×109 RMB,which is lower than the system benefit under medium and high available levels by[8.0,16.7]×109 RMB.The result implies that a higher water level will lead to higher benefits.（2）A multi-criteria analysis of eco-compensation mechanism is carried out by considering system benefits,production costs,ecological benefits,environmental costs,ensurance of ecological water demand,ecological service value,watershed environmental capacity and other indicators.The results show that EWEC is the optimal eco-compensation mechanism,which can achieve the highest comprehensive benefits of the watershed system during the planning period,balance the water consumption conflicts between upstream and downstream areas,flexibly deal with the relationship between regional development and ecological restoration,ensure ecological water demand and improve the ecological service value of the region.（3）Under the scenario of water quality target enhancement,the system benefits show the pattern of EWEC>EW>EC>base mechanism.When the water quality target is improved from III to II,the system benefits of EWEC will be higher than those of EW by[2.26,3.79]×108 RMB,higher than those of EC by[3.78,5.71]×108 RMB,and higher than those of the base mechanism by[4.15,6.24]×108 RMB.This is because in EWEC,the compensated county-level cities can use ecological compensation bill（ECB）to build ecological restoration measures,and the pollutant treatment capacity and water self-purification capacity are thus improved,with a consequent increase in environmental capacity.At the same time,EWEC achieves ecological benefits through ecological water demand ensurance.The results imply that EWEC mechanism performs best in coordinating economic development and water quality protection.Combined with the results of multi-criteria analysis of eco-compensation mechanism,EWEC mechanism is the optimal mechanism.（4）Combining the circulation of ECB in upstream and downstream regions under the EWEC mechanism,Laixi and Pingdu will lose ECB,and Jimo and Jiaozhou are the surplus side of ECB.Under water available levels,the ECB paid by Laixi to Pingdu is[1.47,5.19]×108 RMB（high level）,[0.57,2.34]×108 RMB（middle level）,[0.62,1.40]×108 RMB（low level）;the ECB to be paid by Pingdu to Jimo is[8.65,21.11]×108 RMB（high level）,[3.73,9.74]×108 RMB（middle level）and[1.54,3.88]×108 RMB（low level）.This is mainly because Pingdu and Laixi have the largest scale of planting land in Qingdao,the two county-level cities have larger non-point source discharges and a high possibility of overdischarge at the section,so they need to compensate other county-level cities.Therefore,they are prone to be the losing side of ECB.This result implies that the regions with large production scale（especially agricultural scale）and large emission volume are easy to be the losing party of ECB.（5）Under the EWEC mechanism,production lands can be converted to wetlands with the ECB to provide additional environmental capacity to expand production and continue to contribute to the growth of system benefits during the planning period.Among the production land,fishery land and planting land have the largest reduction area.19.24km2and 19.44km2 of fishery land will be reduced in 2021 and 2022,while planting land will be reduced from 2023,and 21.04km2 of planting land will be reduced in 2030.In addition,the area of constructed wetlands will exceed the area of natural wetlands.The annual addition of constructed wetlands in 2026 is 34.34km2 and 34.49km2 in 2027;while the addition of natural wetlands in 2026 is 6.12km2 and 6.37km2 in 2027.This is because the pollutant（NH3-N and TP）treatment capacity of constructed wetlands is better than that of natural wetlands,and the construction cost is lower than that of natural wetlands.Therefore,constructed wetlands are recommended for eco-compensation measures. |
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