| Evapotranspiration is the key link of hydrological cycle,and also the link of water and energy transfer among different circles.As a major ecological construction area in China,a series of projects have been implemented to improve the ecological environment on the Loess Plateau.But the uneven spatial and temporal distribution of rainfall and shortage of water resources still restrict the development of the area.The evapotranspiration is the major part of the water use on the Loess Plateau,and the precise estimation of the evapotranspiration is of significance for the understanding of the water and emerge process of the ecosystem.In this study,the study objective is a typical shrub system of Loess Plateau,and the evapotranspiration process in the study area was observed by using the eddy covariance sytem based on the Chunhua Ecohydrology Experimental Base,and the soil water transport model was established by Hydrus-1D to simulate the evapotranspiration process.The intra-year characteristics of the two data are analyzed.The daily putential evapotranspiration calculated from the observed meteorological data by the station is compared with pan evaporation,to reveal the evapotranspiration pattern of shrub system in Loess Plateau gully area and its relationship with environmental factors.The applicability of two generalized nonlinear evapotranspiration complementary models is verified in the study area.The main research results obtained are as follows:(1)The slopes of the linear fitting equations for turbulent energy and effective energy from 2020 to 2022 were 0.66,0.58 and 0.61,respectively and the eddy covariance spstem might have underestimated turbulent energy.The energe balance ratios from 2020 to 2022 were 0.79,0.73 and 0.76,respectively,and the energy closure was in a reasonable range.The trend of sensible heat fluxes during the study period is roughly opposite to th at of latent heat fluxes,and the intrayear fluctuation of soil heat fluxes is small.The ratio of energy components to net radiation has seasonal variation characteristics,and the ratio of latent heat to net radiation is more stable in June-September.The annual average value of the Bowen ratio during the study period was 1.27.(2)The root mean square error of soil water content at different depths simulated by the Hydrus-1D-based soil water transport model in the calibration period of 2020 ranged from 0.0090.033,with the best simulation of soil water content at 120cm depth;the best simulation of soil water content in the 2021 validation period is at 40cm depth,and the largest fluctuations of soil water content is at the surface layer;The simulated soil water content at each depth in the validation period of 2022 is poorer than in 2021.The average daily evapotranspiration in 2020,2021 and 2022 simulated by Hydrus-1D is 2.36mm/d,2.79mm/d and 2.48mm/d,respectively.(3)Under sufficient sunshine,intra-day evapotranspiration peaks around 12:00 a.m.on all three typical sunny days,but may be followed by the "evapotranspiration siesta";the evapotranspiration process is stable in the early morning.Net radiation and soil heat flux are the main factors controlling evapotranspiration during typical sunny days;the evapotranspiration observed by the eddy covariance method and the evapotranspiration simulated by Hydrus-1D have similar trends in 2020 and 2021,and Hydrus-1D underestimates evapotranspiration in most of the summer time of 2022.The average daily evapotranspiration for all months in 2022 was almost higher than that in the same period in the previous two years,with the highest increase in June.(4)The potential evapotranspiration process calculated based on observation meteorological data has a single-peak pattern within the year,and the average daily potential evapotranspiration in June,July and August from 2020 to 2022 shows an increasing trend year by year.The average daily pan evaporation during the observation period of each year was 2.46 mm/d,2.61 mm/d and 2.12 mm/d.The actual evapotranspiration was similar to the potential evapotranspiration within the year during the study period,and the total actual evapotranspiration was lower than the total potential evapotranspiration.In periods of abundant rainfall,the actual evapotranspiration is close to the potential evapotranspiration.The net radiation,temperature,and water vapor pressure difference are characterized by significant seasonal variations during the study period.Actual evapotranspiration and potential evapotranspiration both showed the strongest positive correlation with net radiation.(5)The validation results of the complementary relationships indicated the existence of asymmetric evapotranspiration complementary relationships in the study area.The parameter a of the B2015 model was 1.05 after the calibration.In the H2018 model,the parameter α was 1.12 and b was 2.4 after the calibration.Both of two models fitted the observed values well at the calibration period of 2020 to 2021,but the H2018 model performed slightly better than the B2015 model.The MAE between the simulated and measured daily evapotranspiration during the validation period for the B2015 model and H2018 model were 0.458 mm/d and 0.472 mm/d,respectively.Throughout the study period,the simulated evapotranspiration of the H2018 and B2015 models,and measured evapotranspiration processes had similar trends.The deviation of simulated values in summer is larger and the deviation is smaller in winter.The average daily evapotranspiration simulated by the B2015 model and H2018 model during the validation period were 2.48 mm/d and 2.49 mm/d,respectively,and both B2015 and H2018 had good applicability in the study area. |
