Study On Evapotranspiration Estimation Method And Water Use Efficiency In Dune Alternated With Meadow Area In Horqin

Evapotranspiration（ET）is the core of the carbon and water cycles in the ecosystem,and water use efficiency（WUE）is the key factor to characterize the coupling relationship between carbon and water cycles.Accurate estimation of ET is the basis for clarifying the WUE of the ecosystem,which is of great significance to reveal the carbon and water cycles mechanisms and eco-hydrological processes.Horqin sandy land is a typical desertification area,dominated by the landform landscape with the alternate distribution of dune and meadow.Due to climate change and human activities,water shortage,fragile ecological environment and other environmental problems are prominent,and water has become the main factor limiting its sustainable development.Therefore,accurate estimation of ET and WUE can provide theoretical and scientific basis for desertification prevention,disaster prevention and mitigation,sustainable development,and rational utilization of water resources in dune alternated with meadow regions.In this study,a typical dune-meadow cascade ecosystem in Horqin sandy land was selected as the research object,which comprises five landscapes:mobile semi-mobile dune,semi-fixed dune,fixed dune,meadow maize and meadow wetland.Based on hydrometeorological data,soil data,eddy covariance data,and field experiments such as vegetation ecological survey and soil evaporation observation by micro-lysimeter during2013-2018,this study tried to achieve following objectives:（1）building an ET model suitable for dune alternated with meadow regions by improving Penman-monteith（PM）and Shuttleworth-Wallace（SW）models;（2）accurately estimating ET and its components for dune-meadow cascade ecosystems;（3）clarifying the proportion of ET components and their driving mechanisms;（4）evaluating WUE for dune-meadow cascade ecosystems and analyze the response mechanisms of ecosystem WUE to environmental factors.The main results were as follows:（1）Ball-Berry（BB）model coupled with SW model（i.e.,SW-BB model）performed better than that Stannard（ST）model and Jarvis-Stewart（JS）model coupled with SW model（i.e.,SW-ST,SW-JS model）in dune-meadow cascade ecosystems.At hourly timescale,the R² and IA of SW-BB model were higher than 0.84 and 0.95,and its RMSE and MAE were lower than 0.048 mm/h and 0.030 mm/h.At daily timescale,the R² and IA of SW-BB model were higher than 0.82 and 0.94,and its RMSE and MAE were lower than 0.57 mm/d and0.43 mm/d.Besides,the SW-BB model showed better performance than SW-ST model and SW-JS model in the extreme drought period.Soil moisture has an important effect on stomatal conductance and ET in dune ecosystems.The PM-BB model showed better applicability in dense vegetation condition,and the SW-BB model performed better than PM-BB model in sparse vegetation condition.（2）The ET and vegetation transpiration（T）in the dune-meadow cascade ecosystems showed an obvious step-change during growing seasons from 2013 to 2018 as mobile semi-mobile dune（A4）<semi-fixed dune（G4）<fixed dune（F4）<meadow maize（BC4）<meadow wetland（C4）,with mean daily ET of 0.99 mm/d,1.57 mm/d,1.70 mm/d,2.27 mm/d,and 2.75 mm/d,and mean daily T of 0.24 mm/d,0.89 mm/d,and 1.05 mm/d,1.64 mm/d,and 2.13 mm/d.Soil evaporation（E）followed the order A4>G4>F4>BC4>C4,with means of 0.75 mm/d,0.68 mm/d,0.65 mm/d,0.63 mm/d,and 0.62 mm/d,respectively.Precipitation was the main source of ET in dune alternated with meadow regions,and precipitation intensity controls ET intensity.（3）T/ET in the dune-meadow cascade ecosystems showed as A4<G4<F4<BC4<C4,with annual variation in the range of 0.14-0.34,0.55-0.58,0.58-0.69,0.68-0.71 and 0.72-0.78,respectively.At hourly timescale,T/ET was primarily controlled by both soil moisture content and stomatal conductance in dune ecosystems,and it was mainly controlled by stomatal conductance in meadow ecosystems.At monthly timescale,T/ET was mainly controlled by both leaf area index（LAI）and water conditions in dune ecosystems,whereas it was mainly controlled by LAI in meadow ecosystems.（4）The ecosystem water use efficiency（WUEe）in the dune-meadow cascade ecosystems showed as A4<G4<F4<C4<BC4,with annual variation in the range of 0.40-0.58 g C Kg^-1H₂O,0.67-0.69 g C Kg^-1H₂O,0.69-0.73 g C Kg^-1H₂O,0.77-0.87 g C Kg^-1H₂O,0.87-0.88 g C Kg^-1H₂O,and the WUEe in meadow ecosystems were significantly higher than that in dune ecosystems.The canopy water use efficiency（WUEc）was largest in mobile semi-mobile dune and smallest in meadow wetland,indicating that the Artemisia halodendron showed better adaptability to arid environment.Precipitation was found to be the key factor affecting WUEe in the dune-meadow cascade ecosystems.In meadow wetland ecosystem with sufficient water supply,precipitation affected WUEe by affecting WUEc.In the dune and meadow maize ecosystems,precipitation affected T/ET by influencing LAI,further affecting WUEe.