| Populus is extraordinarily popular in China which has greatest plantation area in the world, so recently hydrological circulation and effect of Populus plantation get more consideration. Our central objective was to study sustainability of Populus based on water balance in water crisis region, which located in the warm temperate and sub-humid climate zone.The energy balance is an important and objective method for estimation reliability of energy and materials exchange between ecosystem and atmosphere. This study analyses energy balance and energy components for Populus plantation using three years'data (2006-2008) measured by eddy covariance method. The results of energy closure and energy partition were following:The high energy closure caculated from half-hourly data or daily data indicated high data quality and reliability of energy partition. The energy imbalance mainly happened at night wihich induced to data uncertainty at daily and yearly scale. We found the energy bclosure was best in November and worst in April at monthly scale. Energy closure was related to the friction velocity and atmosphere stability. Energy closure got better and closed to 1.0 when friction velocity and atmosphere stability increased. Energy closure got worse during rainy days and was far away from 1.0 when rainfall quantity increased. Energy closure was indirectly influenced by LAI which was related with friction wind. The bowen ratio and albedo were reasonably determined when energy balance ratio closed to 1.0, which were two important parameters for energy partition. Daily flux of net radiation(Rn),latent heat flux(LE),sensible heat flux(H),soil heat flux(G) had remarkably seasonal variation. Seasonal flux of Rn,LE,H,G was exposed to variation due to leaf area index and rainy days. LE dominated the energy flux and had greater energy flux than H after leaf expansion within at the end April and at the beginning of May. H was greater than LE during leafless period (November to April). Difference of energy partition in growing season and in dormant season was mainly controlled by leaf area index (LAI) and albedo. VPD and soil volume moisture (VWC) had influence on change of latent heat flux (LE) and sensible heat flux (H) during the short growth period of Populus. When contrasting the energy partition in different ecosystem, the LE/Rn of Populus plantation was much greater than that of ecosystem including natural or artificial forest ecosystem. Agriculture ecosystem had the highest LE/Rn. The LE/Rn of the coniferous forest was lower than that of broadleaf ecosystem.We found that seasonal evapotranspiration (AET) and transpiration (T) were exposed to the extremely fluctuation and variability due to the changes of seasonal precipitation and leaf area index (LAI) Populus plantation at Beijing Daxing site had more water consumption with a mean value 1.60±0.04 mm/day from 2006 to 2008, compared with those in the other sites using eddy covariance system. Populus transpiration of mean value 1.78±0.03 mm/day from 2006 to 2009 measured by sapflow probes was as great as those at the other sites. The annul water budget in 2006 and 2008 were deficit and profit in 2007 and 2009. The main supply of PPT and consumption of AET from 2006 to 2009 were 482 and 599,661 and 561,662 and 672,414 and 517mm, respectively. The dry period between March and June required irrigation drawing groundwater to balance drawback of monthly water budget. In comparision with the different sites, the main water supply and consumption of Populus plantation at Daxing site in sub-humid climate district need more water supply for growth than those at the other sites, where there was the same precipitation as Daxing site. The main reason was that multi-years mean temperature (12.2℃) with an increasing trend was higher than those at other sites. Populus plantation under the same plant area and stand age condition simultaneously needed more water for AET resulting from increasing temperature. The continuously increasing water use of Populus plantation conflicted with main water supplying (PPT). Irrigation which was helpful for increasing water supply in the system alleviated the contradiction between water supply and consumption. However the irrigation drawing groundwater aggravated the severity of water crisis due to low water table.We concluded that Populus plantations may not be sustainable in Beijing area due to water shortage including low groundwater recharge and water stress. The hydrological impact of Populus plantation should be taken into consideration involving with water resource management and wood production.
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