Research On Kilometer-scale Evapotranspiration Of A Hilly Plantation In North China

Research on kilometer-scale evapotranspiration has become an important branch of related disciplines such as ecology and hydrology.The mountainous area in northern China has been an important area for the construction of forestry ecological projects in China.Water deficit has become the key factor which restricts sustainable development of plantation and ecological engineering in this region.Conducting observation and research on kilometer-scale evapotranspiration of hilly plantation in northern China can not only reveal the temporal and spatial characteristics of water consumption,but also provide basement for further research on water capacity and accurate measured data for the construction and verification of remote sensing models.So far,there is no reliable and solid method for direct observation of kilometer-scale evapotranspiration of non-uniform landscape such as mountain forests.The two-wavelength?OMS?method has feasibility in observation of evapotranspiration at the kilometer-scale,and it has a broad potential in application,however,there is some uncertainties in OMS method such as data screening,temperature and humidity interaction coefficient?RTq?solving,and universality of similar function,modifying and optimizing the characteristics of the underlying surface in combination with the actual observation are needed.For this reason,we conducted the field campaign based on the original observation data of OMS method,during the main growth seasons of 2016-2018 from April to October at hilly plantation which composed of Quercus variabilis,Platycladus orientalis and Robinia pseudoacacia in this area.In order to improve the accuracy of the OMS system,the research corrected the aerodynamic parameters of the underlying surface,determined the two-wavelength signal saturation line and correction factor,solved the temperature and humidity interaction coefficient?RTq?and optimized the similar functions in the area.Finally,spatial and temporal variation and influence mechanism of kilometer-scale evapotranspiration over the vegetation are discussed based on the optimized OMS.The main findings of this thesis are as follows:1.Based on the combination of optical transmission theory and inner scale effect,methods for determining the saturation line and the correction factor of OMS signals are optimized.Strong and weak saturation lines of OMS in this campaign are 0.152 and 0.409,respectively,and the correction factor is 0.9978.The amount of effective data increased by13.7%.Zero plane displacement is 11.32 m,and roughness length is 2.82 m in the site.There is no significant difference in refractive index of temperature structure parameters?C_T²?,while difference in refractive index of humidity structure parameter?Cq²?is significant when RTq were assumed to be different values.Difference of Cq²is the smallest when RTq=0.6.Research suggests that the results of two-wavelength bichromatic method are closer to eddy covariance.Based on results from eddy covariance,the overall accuracy of the bichromatic method is improved by about 9.2%compared with assuming RTq is a constant.2.Similar functions constructed in the observation area can be expressed as the following equations:f_T?28?5.45?1-7.38z/L?^-2/3,f_q?28?5.51?1-6.92z/L?^-2/3?z/L?27?0?;and,f_T?28?4.87(1?10?2.15?z/L?^2/3),f_q?28?5.50(1?10?1.33?z/L?^2/3)?z/L?29?0?.Compared with eddy covariance,the function constructed showed an accuracy of 22.3%higher than that of the Andreas function in near-neutral,and energy balance closure of the constructed function is0.92.3.On daily scales,kilometer-scale evapotranspiration in the area is consistent with variations of net radiation.The maximum of evapotranspiration is close to 0.2 mm at about12:30 in sunny days.Evapotranspiration shows strong fluctuations in overcast days,and the maximum of evapotranspiration is approximate to 0.13 mm.The maximum value is about0.15 mm in cloudy days.The maximum of evapotranspiration usually appeared in 1-3 days after rain stopped.4.There is a lot more precipitation in July and August than other months,and evapotranspiration is greater than other months.In July and August 2016,precipitation is184.83 mm and 126.51 mm,respectively;in July and August 2017,precipitation is 146.01mm and 125.63 mm,respectively;in July and August 2018,precipitation is 175.44 mm and145.03 mm,respectively.Evapotranspiration in July is close to 98.37 mm,101.33 mm,and91.36 mm,respectively;and in August is approximately 85.12 mm,71.35 mm,and 78.27 mm,respectively.Total precipitation during the observation period of 2016-2018 is 571.93 mm,558.83 mm,and 564.46 mm,respectively.Evapotranspiration during the growth season is357.95 mm,314.22 mm,and 328.87 mm,respectively.Evapotranspiration is larger than precipitation in April,May,October,which means a seasonal drought.5.Source area of evapotranspiration is mainly distributed on one side of the optical path in the dry period.The maximum boundary of the source area in the upwind direction extends to 0.58 km,and the source area is about 0.73 km²,while source area distributed on both sides of the two-wavelength optical path in wet period,and the extension of the 90%source boundary in the upwind direction is up to 0.92 km,and the source area is about 1.62 km².Due to the difference of meteorology,there is the largest area of 1.88 km²of source in July.6.Effect of net radiation on kilometer-scale evapotranspiration is greater than that of the vapor pressure deficit.The influence of net radiation on month-scale is greater than the daily scale.Friction velocity within a certain range(0.15-1.0 m·s^-1)contributes to the exchange of hydrology-energy flux with increasing,so that the evapotranspiration rate was affected.The change in soil water content at 5 cm affects the evapotranspiration process.The greater the rate of change,the stronger the evapotranspiration.8.Taking the data of 2018 as an example,the value of energy closures of the two sets eddy-covariance systems are 0.73 and 0.69.The value is 0.82 when the two sets were combined and expanded,and the value of OMS is 0.90.For example,the evapotranspiration of the hundred-meter scale was 75.53 mm and 69.30 mm in July,respectively.The evapotranspiration is 78.36 mm when joint the two sets and expansion,and the evapotranspiration of the OMS was 91.35 mm.The joint and expansion in two sets of eddy covariance system has a higher energy closure and evapotranspiration,and the OMS observations are closer to the expanded results.It is shown that the observation of single eddy covariance system over non-uniform landscape may underestimate the actual evapotranspiration due to the difference of terrain conditions,while OMS method and observation base on the network can make up the deficiencies and show a well spatial representation to some extent.