黑河下游柽柳根系水力提升对林分蒸散的贡献

摘要

准确量化植物根系水力提升(HL)及其生态-水文效应对于陆地生态系统水分循环和全球变化研究具有重要意义.基于2011-2012年黑河下游柽柳林地土壤含水量和涡度协方差观测资料,通过将土壤体积含水量分割为HL和水分损失量(WD),结合涡度协方差测定的潜热通量计算的蒸散量(ET),首次定量黑河下游柽柳根系HL及其对ET的贡献.据估算,柽柳根系HL主要发生在20-60 cm深度,生长季HL大小在0-1.4 mm/d之间变化,平均为0.22 mm/d,WD在0-0.76 mm/d之间变化,平均为0.23 mm/d,HL与WD的年内变化存在同步性,且HL与WD处于正平衡状态,表明HL通过将深层吸收的土壤水或地下水释放在根系吸收层以供植物蒸腾消耗外,还有剩余水分留存在该层内.生长季ET在0.31-5.38 mm/d之间变化,平均为2.82mm/d,但值得注意的是,HL与ET的年内变化存在时间滞后性,HL在5月最高,但ET在7月最大.HL对ET的贡献率在0.06％-108.25％之间变化,平均为19.25％,比例高于100％的时段主要在生长初期,也就是说在蒸散最大的夏季,HL是相对较小的,其原因可能有2个:一是HL受到夏季深层土壤干化的抑制,二是HL受到夏季夜间蒸腾的抑制,究竟是何种原因还有待进一步研究.%Accurately quantifying of hydraulic lift (HL) and its eco-hydrological effects has great significance for the terrestrial ecosystem water cycle and global change research.Based on measurements of the soil water content and latent heat flux by the eddy covariance method during 2011-2012 in the lower Heihe River,the HL was first calculated by separated water depletion (WD) in the soil water content.Its contribution to evapotranspiration (ET),that calculated from latent heat flux,was then determined.The daily variation of HL that largely occurred in 20-60 cm soil depth ranged from 0 to 1.4 mm/d with an average of 0.22 mm/d,and the WD was from 0 to 0.76 mm/d with an average of 0.23 mm/d.The annual variation of HL and WD was synchronized and the magnitude of both was in positive balance,indicating that the water extracted from the deep soil layer or groundwater via HL was expected to meet the transpiration;however,the remainder of the water was retained in the soil layer.The daily variation of ET ranged from 0.31 to 6.42 mm/d with an average of 3.37 mm/d;however,the annual variation of HL and ET has a time-lag effect,with the maximum of HL and ET occurring in May and July,respectively.The ratio of HL to ET ranged from 0.06％ to 108.25％ with an average of 19.25％,which indicated that the HL did not increase with an increase in ET.Two possible reasons for this are that the HL was restrained by the low soil water moisture of the deep layer or by the nocturnal transpiration during the dry season.Further research is required to confirm which one of these is correct.