Dynamic Simulation Of Evapotranspiration In Vegetation Growing Season In Xilin River Basin

Evapotranspiration is an important part of the water cycle and water balance of the SPAC system.It determines the main process of water and soil moisture entering the atmosphere.Estimating and dynamically simulating evapotranspiration on a regional scale is important for terrestrial ecosystem management and analysis of the response of the water cycle to climate change.The Xilin River Basin in Inner Mongolia is located in the transition zone from the semi-humid meadow steppe area in the east to the arid desert steppe area in the northwest,and has a special position in the temperate grassland in northern China.Therefore,the dynamic simulation study of the actual evapotranspiration in the Xilin River Basin has important scientific significance and application value for studying the response of grassland ecosystems to climate change,grassland degradation prevention and restoration,and water resources utilization and management in pastoral areas.In this study,the Xilin River Basin was used as the study area,and the evapotranspiration observation method and the estimation method were combined to study the temporal and spatial variation characteristics of vegetation evapotranspiration in the Xilin River Basin.The main conclusions are as follows:(1)The field experiment were carried out from May to September of 2017 in the Xilin River Basin.A daily evapotranspiration estimation model for vegetation community in Xilin River Basin was established referring to the theory of aerodynamics in the FAO Penman-Monteith model and other classical models,with considering the heat factor and some factors,and adding moisture Factors and vegetation characteristics factors.The model determination coefficient R~2 was 0.7926,and the F value was 305.7,which was much larger than the critical value of 2.63 at the P=0.05 significant level.The results of 20 groups of verifications showed that the consistency index C-index was 0.97,and the mean square error MSE was 0.062.Compared with different P-M crop coefficient(Kc)approach,the relative error of the model in this study was the smallest,with an average of 14.03%,and the simulation effect was good,which met the accuracy requirements.(2)Estimation method of evapotranspiration on the basin scale:Considered the spatial heterogeneity of images,the data was calculated and processed at the pixel level through RS and GIS technology platform,and the actual evapotranspiration from 2000 to 2017 was dynamically simulated.Used the measured data for 2017,the evapotranspiration obtained by the watershed estimation method was compared in point,and their correlation was good.The determination coefficient R~2 reached 0.71,which indicated that the method had good applicability in estimating the evapotranspiration in the study area.(3)The evapotranspiration of the basin during 2000-2017 was highly volatile,and the change trend was basically consistent with the precipitation during the same period.The average annual evapotranspiration was 83.22 mm,the minimum appeared in 2009,and the maximum appeared in 2012.Under the influence of precipitation,the amount of evapotranspiration increased in July,but the trend was not significant(p>0.05).The order of total evapotranspiration and precipitation in the growing season of different years was basically the same,and the evapotranspiration distribution was basically consistent with the characteristics of“wet year>normal year>dry year”.In the dry year,the evapotranspiration curve in the growing season changed gently.In the wet year,the change of evapotranspiration in the growing season was a distinct bell-shaped curve,and the variation of the evapotranspiration curve in the growing season of the flat water year was between the two.(4)The evapotranspiration in the basin in July showed a strip-like distribution characterized by decreasing from southeast to northwest.The evapotranspiration of the low-wet vegetation on both sides of the river,the meadow grassland in the upper reaches,and the forest land and cultivated land in the southeast and southwest were the largest,followed by the typical steppe,cultivated land and construction land in the middle reaches.The minimum evapotranspiration occurred in the downstream Stipa krylovii steppe,saline land and bare river bed in the northwest of the basin.The average rate of change of evapotranspiration in July was-1.61 mm/a,and the average coefficient of variation was 0.42.The variability of evapotranspiration in the basin was less stable in July.The spatial distribution of evapotranspiration during the growing season of the Xilin River Basin was basically consistent with the phenological period of the vegetation in the basin.(5)The relationship between evapotranspiration and environmental factors in the Xilin River Basin indicated that both soil moisture and vegetation cover significantly affected the amount of evapotranspiration in both July and growing seasons.With the increase of soil water content and vegetation coverage,the evapotranspiration of the basin increased significantly.In July,the evapotranspiration of the basin did not respond strongly to temperature,and the response to precipitation was spatially heterogeneous.In the vegetation growing season,the evapotranspiration in most areas was significantly positively correlated with temperature and precipitation.The result indicated that evapotranspiration was closely related to temperature,precipitation,soil water content and vegetation coverage in a certain time scale.Different environmental factors change the water and heat conditions of soil and its ground plants in different ways,which in turn affects soil evaporation and vegetation transpiration.