基于Gash修正模型的元宝槭林分结构对降雨分配的影响

Influence of stand structure of Acer truncatum forest on rainfall redistribution based on modified Gash model

林冠截留是森林生态系统降雨分配的起始点,准确模拟林冠截留是探究森林水文过程的基础。为了研究林分结构中林分密度对林冠截留的影响规律及Gash修正模型的适应性,以北京市八达岭森林公园2种林分密度的元宝槭纯林为研究对象,对其林冠截留过程进行了模拟分析,并针对元宝槭林分结构特征探讨了Gash修正模型的适应性。研究结果表明:(1) 2013年生长季内,元宝槭密林(3 665株/km^2)穿透降雨量、树干茎流量和林冠截留量实测值分别为365.50、38.19、69.82 mm,模拟值分别为386.45、33.86、53.21 mm,相对误差分别为5.73%、4.33%和23.80%;元宝槭疏林(1 090株/km^2)的穿透降雨量、树干茎流量和林冠截留量实测值分别为380.20、19.04、74.25 mm,模拟值分别为368.37、13.71、91.43 mm,相对误差分别为3.12%、27.99%和23.13%。疏林条件下,更多的降雨穿透林冠,进入了森林生态系统的下一水文效应层,Gash修正模型的模拟结果亦呈现基本一致的特征,模型对元宝槭林的模拟精度基本可以满足需要。(2)对Gash修正模型中的平均降雨强度(R)、树干茎流系数(Pt)、饱和林冠持水能力(S)、郁闭度(c)、饱和树干持水能力(St)、平均饱和林冠蒸发速率(E)共6个关键参数的进行敏感性分析,各参数对元宝槭密林林冠截留量模拟的影响程度大小依次为R>S>E>St>c>Pt,对于元宝槭疏林的影响程度大小依次为S>R>E>c>St>Pt。通过分析可以得出,选择适宜的时间尺度计算平均降雨强度和饱和林冠持水能力,是提升Gash修正模型模拟精度的关键,适当疏伐可以为林分的生长提供更好的水分条件。

Canopy interception is the starting point of rainfall distribution in forest ecosystem,and accurate simulation of canopy is the basis of exploring forest hydrological process.In order to study the influence of stand density on canopy interception and the adaptability of Gash modified model in stand structure,the Acer truncatum forests with two kinds of forest density in Badaling Forest Park of Beijing were taken as the research object.The simulation analysis of the canopy interception process was carried out,and the adaptability of the Gash correction model was discussed for the stand structural characteristics of the A.truncatum forest.The results showed that:(1) In the growing season of 2013,the measured values of the throughfall,stem flow and canopy interception of A.truncatum forest with the density of 3 665 strain/km^2 were 365.5 mm,38.19 mm and 69.82 mm,and the simulated values by the modified Gash model were 386.45 mm,33.86 mm and 53.21 mm.Compared with the measured values,the relative errors were 5.73%,4.33% and 23.80% respectively.The throughfall,stem flow and canopy interception of A.truncatum forest with the density of 1 090 strain/km^2 were 380.2 mm,19.04 mm and 74.25 mm,and the simulated values by the modified Gash model were 368.37 mm,13.71 mm and 91.43 mm.And the relative errors were 3.12%,27.99% and 23.13% compared with the measured values.Under the sparse forest conditions,more rainfall penetrated the canopy and entered the next hydrological effect layer of the forest ecosystem.The simulation results of the Gash modified model also showed basically consistent characteristics.The simulation accuracy of the model to the A.truncatum forest can basically meet the needs.(2) Sensitivity analysis of six key parameters in the Gash modified model was carried out,such as the average rainfall intensity(R),the drainage partitioning coefficient(Pt),the canopy storage capacity(S),the canopy cover fraction per unit area(c),the trunks storage capacity(St),the average saturated canopy evaporation rate(E).The influence of the six parameters on the A.truncatum forest with high density was R>S>E>St> c>Pt.And it was S>R>E>C>St>Pt on the A.truncatum forest with low density.It can be concluded from the analysis that it is the key to improve the simulation accuracy of the modified Gash model by selecting the appropriate time scale to calculate the average rainfall intensity and the canopy storage capacity.Proper thinning can provide better water conditions for stand growth.