| Evapotranspiration is the sum of vapor transported from land surface to atmosphere.As a link between hydrological and ecological processes of soil-plant-atmosphere continuum(SPAC),evapotranspiration is very important to the stability of terrestrial ecosystems.In arid and semi-arid regions,where as much as 90%of the effective precipitation is consumed by evapotranspiration,variations in evapotranspiration processes may weaken the stability of a grassland ecosystem plagued by water stress.Located in the north China farming-grazing transitional zone,ecologic environment of desert steppe is fragile in Yanchi County.Excessive conversion to farmland,overgrazing and artificial disturbance greatly decreased ecological function of this grassland ecosystem and produced serious changes in evapotranspiration processes.Due to inadequate previous research on the characteristics of actual evapotranspiration of desert steppe in Yanchi,little background information of evapotranspiration limited anticipate effects of ecological restoration project.Thus,studies on the evapotranspiration characteristics of desert steppe ecosystem within a fragile ecological environment is of great significance to regional agriculture and animal husbandry development,ecological restoration and reconstruction,and the construction of eco-barriers.Eddy covariance method,ecosystem process model of BESS and crop coefficient approach were employed.The study,based on meteorological data,observed flux data and remote sensing images,aims to analyze multi time scale characteristics of evapotranspiration,discuss the main driving environmental factors and reveal the internal mechanisms of desert steppe in Yanchi.This study obtained the following results and conclusions:(1)From May to September in 2016,the monthly energy balance closure ratio was at a high level,and the quality of flux data observed by eddy covariance was reliable in study area.The average daily dynamics of net radiation,sensible heat flux,latent heat flux and soil heat flux all presented "single peak",whose appearance were inconsistent.There was a significant diurnal difference in energy distribution.At different stages of growing season,the distribution of sensible heat flux and latent heat flux was decided by soil moisture,so latent heat flux dominates the energy distribution under the wetting soil.(2)The average daily ET was 1.91 mm·d-1 and ET had obvious differences in different stages of growing season in 2016.Precipitation changing the soil moisture,daily ET of a continuous sunny day after the rain showed a ladder-like decreasing trend.In growing season,precipitation was completely returned to the atmosphere in the form of evapotranspiration,so the groundwater deficit was obvious.On the daily scale,net radiation,soil moisture at 10 cm depth and vapor pressure deficit were all positively correlated with ET.Through soil moisture content,precipitation controlled the response of evapotranspiration to environmental factors.(3)BESS model could be used to simulate the actual evapotranspiration(ET)of desert steppe in Yanchi.From 2004 to 2016,the average daily ET was 0.55 mm·d-1 in spring,1.11 mm d-1 in summer,0.72 mm d-1 in autumn,and 0.17 mm·d-1 in winter,respectively.What's more,the annual ET was 205.74?273.41 mm in the last 13 years.The annual ET and it's participations significantly increased with an evident inter-annual fluctuation.The distribution pattern of precipitation strongly affected the inter-annual variation of ET.Evaporation ratio was much higher than transpiration ratio,thus evaporation from land surface was the main water outflow.(4)In 1954?2016,the average potential evapotranspiration(PET)reached 1202.75 mm in Yanchi.The annual PET increased at a rate of 1.97 mm·a-1(P<0.05)with a mutation phenomenon in 1997.Hurst index(0.87)indicated that the potential evapotranspiration would keep up with the current downward trend in the future.Because of climate fluctuation,the potential evapotranspiration had significant oscillation characteristics on an inter-decadal(15.75 years and 31.50 years)scales.PET had a strong response to the rising surface temperature.Thus the inter-annual variation of PET mainly came from the change of annual mean temperature,wind speed and hours of sunshine.(5)The correlation analysis between daily crop coefficient and environmental factors indicated that temperature(T),relative humidity(RH),surface soil temperature(ST)and moisture(SM),vapor pressure deficit(VPD)and wind speed(WS)were the major explanatory variables for daily crop coefficient.Daily crop coefficient would be expressed as a function of T,SWC,RH.Based on potential evapotranspiration of study area,the crop coefficient could be used to simulate actual evapotranspiration in some degree. |
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