The Characteristics Of Potential Evapotranspiration And Its Changing Trend In The Headwater Area Of The Yellow River

The headwater area of the Yellow River is an important ecological barrier in my country and one of the Chinese water towers.In the context of global warming and increasing human activities,the meteorological and hydrological conditions in the source area of the Yellow River have undergone significant changes in the past 50 years,and problems such as runoff reduction and increasing frequency of extreme climates have emerged.Potential evapotranspiration(Ep)is an important parameter to measure the loss of surface water,and it is also one of the important indicators to indicate extreme meteorological events.Studying the characteristics and changing trends of potential evapotranspiration at multiple scales has an important decision-making reference for water resources management in the Yellow River Basin and for responding to risks and challenges brought by climate change.This article relies on the National Natural Science Foundation of China project"Arid Area Surface-Groundwater System Interface Dynamics and Water Cycle Research",based on multi-source data(topographic data,land use,meteorological data,hydrological data,socio-economic data and Coupled Model Intercomparison Project Phase 5(CMIP5)downscaling climate change model data)in the headwater area of the Yellow River,using the Penman-Monteith(PM)method Analyzed the temporal and spatial evolution of potential evapotranspiration in the source region of the Yellow River in the past 50 years.As the future climate change model data(CMIP5)is difficult to meet the parameter requirements of the PM method,in order to predict the future trend of potential evapotranspiration in the headwater area of the Yellow River,taking the PM method as a reference,select the HS method with higher accuracy from 6 representative simple algorithms(Priestly-Taylor(PT method),Doorenbos-Pruitt(DP method),Hargreaves-Samani(HS method),Rohwer(RO method),Thornthwaite(TH method)),Blaney-Criddle(BC method))and modify it.Then use the revised HS method to predict the future spatiotemporal characteristics of potential evapotranspiration in the source region of the Yellow River under RCP4.5 and RCP8.5scenarios,and then an extreme climate index system was constructed to provide early warning of extreme climate events that may be encountered in the region in the future.Finally,put forward suggestions for response.The achievements mainly include:(1)In the past 50 years(1970-2018),the potential evapotranspiration calculated according to the PM method in the headwater area of the Yellow River has shown an insignificant upward trend on the time scale,with the annual potential evapotranspiration ranging from 661 to 759 mm/year.The monthly potential evapotranspiration is between 12and 105 mm/month,and the daily potential evapotranspiration is between 0.41 and 3.67 mm/d.On the spatial scale,the spatial differentiation of potential evapotranspiration in the source region of the Yellow River is obvious,generally showing a downward trend from northwest to southeast.The high-value centers of the multi-year average and the interdecadal values of the potential evapotranspiration in the headwater area of the Yellow River are located in Xinghai County and Guinan County,and the high-value centers of the potential evapotranspiration in spring,summer and autumn are located in Guinan and Xinghai County,the high-value centers in winter are Maqu and Hongyuan counties.(2)Because the future climate change model data(CMIP5)cannot meet the parameter requirements of the PM method,in order to predict the future trend of potential evapotranspiration in the headwater area of the Yellow River,this paper uses the PM method as a reference,and the accuracy of the potential evapotranspiration calculated by 6 simple algorithms on the temporal and spatial scales is compared by using the meteorological measured data of the past 50 years.It is found that among the six simple algorithms,the HS method has higher accuracy in both time and space scales.By introducing an average relative humidity factor in the HS formula,this paper establish HS improvement formula suitable for this study area:E_hs=0.0023×0.408R_a(T+17.8)(T_max-T_min)^0.5-0.928Ln(RH_mean)+3.677.After the improvement of the HS method,the calculation accuracy and applicability in the headwater area of the Yellow River have been further improved,and it can be used to predict the future potential evapotranspiration in the headwater area of the Yellow River.(3)Using the revised HS method to predict the potential evapotranspiration of the headwater area of the Yellow River in the future(2019-2100)under the two scenarios of RCP4.5 and RCP8.5,it is found that the potential evapotranspiration of the headwater area of the Yellow River will increase significantly in the future.Under the RCP4.5 scenario,the multi-year average potential evapotranspiration in the headwater area of the Yellow River from 2019 to 2100 is about 634?915 mm/year.The high-value center of the spatial distribution of E_p is located in Qingshuihe,Guinan and Hongyuan counties,and the low-value center Located in Dari and Jiuzhi counties.Under the RCP8.5 scenario,the multi-year average potential evapotranspiration in the source region of the Yellow River from 2019 to2100 is about 673?1118 mm/year.The high-value center of the spatial distribution of E_p is located in Zoige County,and the low-value center is located in Dari and Jiuzhi counties.(4)Based on the predicted potential evapotranspiration in the source region of the Yellow River in the future(2019-2100),construct an extreme climate index system to explore the temporal and spatial characteristics of future extreme climate in the source region of the Yellow River.The research results indicate that on the time scale,all extreme weather indexes during the forecast period from 2019 to 2100 show an upward trend under the two scenarios of RCP4.5 and RCP8.5,and the headwater area of the Yellow River will continue to be warm and humid in the future.Spatially,the spatial distribution of extreme meteorological indexes under the two scenarios is significantly different.The headwater area of the Yellow River has significant overall humidification.And the possibility of extreme precipitation and extreme high temperature events in the eastern region is higher than in the western region.In the future,Henan Mongol Autonomous County is most likely to encounter extreme precipitation events,and Guinan County is the most likely to encounter extreme high temperature events,which requires key prevention.Based on the actual situation in the study area,this article recommends:for the future high-risk areas of extreme precipitation and extreme high temperature in the headwater area of the Yellow River,in order to improve the capacity of the source area of the Yellow River to cope with climate change,the research and monitoring of extreme meteorological disasters should be strengthened,a real-time meteorological disaster monitoring and early warning system should be established,and engineering defense facilities should be strengthened.