Shrub encroachment effect on the evapotranspiration and its component—A numerical simulation study of a shrub encroachment grassland in Nei Mongol, China
WANGQi-Dan1, YANGWen-Xin1, HUANGJie-Yu1, XUKun1, WANGPei1,2,*, 1School of Natural Resources, Faculty of Geographical Science, Beijing Normal University, Beijing 100875, Chinaand 2State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China 通讯作者:* 通讯作者 Author for correspondence (E-mail:peiwang@bnu.edu.cn) 版权声明:2017植物生态学报编辑部本文是遵循CCAL协议的开放存取期刊,引用请务必标明出处。 基金资助:国家自然科学基金(41671019、91425301和41301014)和北京市本科生基金项目
关键词:灌丛化;生态水文效应;蒸散发;二源模型;数值模拟 Abstract Aims Shrub encroachment is a common global change phenomenon occurring in arid and semi-arid regions. Due to the difficulty of partitioning evapotranspiration into shrub plants, grass plants and soil in the field, there are few studies focusing on shrub encroachment effect on the evapotranspiration and its component in China. This study aims to illustrate shrub encroachment effect on evapotranspiration by the numerical modeling method. Methods A two-source model was applied and calibrated with the measured evapotranspiration (ET) by the Bowen ratio system to simulate evapotranspiration and its component in a shrub encroachment grassland in Nei Mongol, China. Based on the calibrated model and previous shrub encroachment investigation, we set three scenarios of shrub encroachment characterized by relative shrub coverage of 5%, 15% and 30%, respectively, and quantified their effects caused by shrub encroachment through localized and calibrated two-source model.Important findings The two-source model can well reconstruct the evapotranspiration characteristics of a shrub encroachment grassland. Sensitivity analysis of the model shows that errors for the input variables and parameters have small influence on the result of partitioning evapotranspiration. The result shows that shrub encroachment has relatively small influence on the total amount of ET, but it has clear influence on the proportion of the components of evapotranspiration (E/ET). With shrub coverage increasing from 5% to 15% and then 30%, the evapotranspiration decreased from 182.97 to 180.38 and 176.72 W·m-2, decreasing amplitude values of 0.34% and 0.44%, respectively. On average, E/ET rises from 52.9% to 53.9% and 55.5%, increasing amplitude values to 2.04% and 3.25%. Data analysis indicates that shrub encroachment results in smaller soil moisture changes, but clear changes of ecosystem structure (decreasing ecosystem leaf area index while increasing vegetation height) which lead to the decrease of transpiration fraction through decreasing canopy conductance. The research highlights that, with the shrub encroachment, more water will be consumed as soil evaporation which is often regarded as invalid part of evapotranspiration and thus resulting in the decrease of water use efficiency.
Keywords:shrub encroachment;eco-hydrological effect;evapotranspiration;two-source model;numerical simulation -->0 PDF (3372KB)元数据多维度评价相关文章收藏文章 本文引用格式导出EndNoteRisBibtex收藏本文--> 王芑丹, 杨温馨, 黄洁钰, 徐昆, 王佩. 灌丛化的蒸散耗水效应数值模拟研究——以内蒙古灌丛化草原为例. 植物生态学报, 2017, 41(3): 348-358 https://doi.org/10.17521/cjpe.2016.0236 WANGQi-Dan, YANGWen-Xin, HUANGJie-Yu, XUKun, WANGPei. Shrub encroachment effect on the evapotranspiration and its component—A numerical simulation study of a shrub encroachment grassland in Nei Mongol, China. Chinese Journal of Plant Ecology, 2017, 41(3): 348-358 https://doi.org/10.17521/cjpe.2016.0236 灌丛化是当今干旱半干旱草原的一种常见全球变化现象, 是指在干旱半干旱草原生态系统中出现草本植物退化, 原生灌木或木本植物的植株密度、盖度和生物量增加的现象(van Auken, 2000; 陈蕾伊等, 2014)。灌丛化的发生对于区域生态过程具有较大影响, 包括植物多样性的改变、碳循环等过程的变化(高琼和刘婷, 2015)。灌丛化会导致木本植物多度的增加, 进而影响到这些地区植被的组成, 改变生态系统的结构和功能(李宗超和胡霞, 2015), 进而会对生态系统的水文过程产生影响(Li et al., 2013; 彭海英等, 2014)。灌丛化的生态水文效应研究是生态学、水文学等学科研究的前沿与热点。在干旱半干旱生态系统中, 蒸散发是水量和能量平衡中关键的组分, 然而灌丛化的蒸散耗水效应由于受到野外土壤、灌丛及草本的蒸散耗水难于拆分的限制,目前研究还较少。蒸散发一般包括土壤蒸发和植物蒸腾部分, 将其拆分为土壤蒸发与植被蒸腾是很有必要的。它一方面有助于我们提高对水分从土壤—植物—大气连续体传输机制的认知; 另一方面, 在水文学中, 植被蒸散的水也被称为“绿水”, 对农业系统有着重要的作用。土壤蒸发的水分也被称为“白水”, 是一种水资源无效损耗(Yamanaka, 2009)。因此, 探讨灌丛化的蒸散耗水效应, 对灌丛化组分进行拆分研究对于水资源的管理有着积极意义。 在我国, 关于灌丛化的生态水文学效应研究较少。Li等(2013)对内蒙古灌丛草地下垫面灌丛斑块及草地斑块的土壤水分动态进行了长期观测, 并指出内蒙古太仆寺旗站点的灌丛斑块与草地斑块土壤水分动态无显著季节差异。Peng等(2013)对不同程度的人类干扰下灌丛化梯度的生物量、群落特征、水分可利用性等进行了比对研究。Zhang等(2013)对不同灌丛化梯度下草地的土壤有机质、土壤全氮、土壤全磷、土壤孔隙度、土壤质地、土壤饱和导水率进行了对比研究。高琼和刘婷(2015)曾研究解释我国半干旱区灌丛化形成的机理。张宏等(2001)研究了灌丛化空间分布对土壤异质性的响应。蒸散发是水文及能量循环中的重要组分, 然而在野外环境条件下, 土壤、灌丛和草地的蒸散耗水往往混杂在一起, 难以拆分。因此, 针对灌丛化的蒸散耗水效应研究相对较少。在前人的研究中, 有许多拆分蒸散发的方法, 其中包括物理分析方法、化学同位素分析方法(Scott et al., 2006; Moran et al., 2009)、模型分析法(Brenner & Incoll, 1997; Reynolds et al., 2000)。同位素方法可以较好地拆分蒸散发(Wang et al., 2015, 2016), 但是难以用来情景化分析。模型分析法由于可以运用的时间尺度较长, 且可适用的空间广度相对较大, 已得到了广泛的应用。二源模型尽管相对简单, 但仍能较为准确地把握土壤蒸发与植被蒸散的通量, 而且在复杂多变的气候条件下, 二源模型已被验证可以提供合理的蒸散发组分的估算(Anderson et al., 2004; Li et al., 2008), 并且模型较为适合情景化效应分析。因此, 二源模型对于准确量化并拆分陆地生态系统蒸散发具有较好的实用性。本文以中国内蒙古草原灌丛化区域为例, 利用已有的二源模型对蒸散发进行估算并拆分(Wang & Yamanaka, 2014), 针对内蒙古地区的情况并结合前人研究, 对已有二源模型进行相关参数本地化率定, 通过波文比能量收支平衡方程计算出的潜热通量(lET)与模型输出值进行对比验证, 使其能够准确重建内蒙古灌丛化草地蒸散发动态。在此基础上结合先前灌丛化的调查, 进行灌丛化的情景模拟, 进一步量化分析灌丛、草地、土壤三者对蒸散发的贡献度, 从而对灌丛化的蒸散耗水效应进行阐述, 旨在为草地的退化管理、修复等提供一定的科学依据。
灌丛化会引起植被群落结构的一系列变化, 最为显著的是灌丛盖度的变化, 导致生态系统植被冠层结构的改变, 进而引起LAI、冠层高度、根深等一系列的变动。本研究参照先前对内蒙古太仆寺旗草地灌丛化的实际调查结果(彭海英, 2012; Peng et al., 2013), 以此为依据, 进行了灌丛化程度的情景设置: 即轻度灌丛化(灌丛覆盖度为5%)、中度灌丛化(15%, 即本站点实际灌丛盖度)和重度灌丛化(30%)。利用已率定好的二源模型, 根据不同灌丛化程度, 在模型中改变其权重f, 因此在不同的灌丛化情境下, 在模型中相应改变了变量的LAI、θ (不同深度灌丛加权平均)和ZV这3个关键变量, 具体数据如表2及图1所示。可以看出, 灌丛化引起了生态系统结构变化, 表现为LAI降低, 冠层高度增加, 灌丛化引起的土壤水分变动并不显著(图1)。 显示原图|下载原图ZIP|生成PPT 图1不同灌丛化情景下草原生态系统加权平均叶面积指数、植被高度、土壤体积含水量的变化。 -->Fig. 1Weighted mean leaf area index, vegetation height and volumetric water content in soil of grassland ecosystem under three shrub encroachment scenarios. -->
2 研究结果
2.1 蒸散发的模拟
图2对比了灌丛化草地波文比观测蒸散及模型模拟值。结果发现实测值与模型输出值有较好的1:1线性拟合关系, 相关性高达0.95。二者季节动态呈现较一致的变化规律。以上波文比实测值与模型模拟值的对比结果说明该模型对于模拟内蒙古灌丛化草原蒸散发季节动态的重建是有效的, 为灌丛化情景模拟奠定了基础。 显示原图|下载原图ZIP|生成PPT 图2波文比实测值与模拟输出蒸散发(潜热)值对比。 -->Fig. 2Comparison of evapotranspiration (express as latent heat flux) between measured by the energy balanced Bowen ratio system and predicted during measurement period. -->
2.2 模型的T/ET敏感度分析
为了评估指定模型参数或测量变量中可能造成的误差, 对模型的输入项进行了敏感性分析(Beven, 1979; Qiu et al., 1998)。对于模型中驱动变量及参数对蒸散发及其组分的影响, 可以定义一个敏感性参数: $$S_i=\frac{\partial O}{\partial p_i}\frac{p_i}{O}\ \ (15)$$ 其中, pi是第i个可以影响输出结果(O, 如lET或T/ET)的驱动参数及变量。其偏导∂O/∂pi的具体算法如下: $$\frac{\partial O}{\partial p_i}=\frac{O_p^*-O_p}{p_i^*-p_i}\ \ \ (16)$$ 其中, Op*是预先假设pi (如pi*)情况下模型预测出的结果, Op是用模型中指定或测量的pi预测出的结果。Si = 0.1意味着pi增加1%会引起输出结果O的0.1%的增加。负的敏感型参数表示输出结果O的减少是由pi的增加引起的, 反之亦然。简言之, Si给予了特定参数在可能变化范围内及与其他参数组合时对模型输出变量(蒸散发及其组分)变化程度的度量。 表3总结了lET和T/ET对模型输入参数的敏感性分析结果。结果表明: rst_min对lET的改变影响最大, 其30%的误差可引起lET平均8.4%的误差, 对T/ET产生2.4%的误差。其他参数对于T/ET的敏感性指数相对较小。对输入变量的敏感性分析表明: 相对湿度(ha)对改变lET的影响最大, 其5%的误差可引起lET多至7%的误差, 对T/ET产生0.6%的误差。这种误差范围在实际精确观测的情况下是可以忽略的。因此, 尽管在指定模型参数和(或)测定参数中可能存在误差, 但是对lET和T/ET的估计值还是十分可信且可接受的。 Table 3 表3 表3指定模型参数及测量参数对蒸散发(ET)和蒸散比率(T/ET)敏感性系数(Si)的平均值±标准偏差(SD) Table 3Mean and standard deviation (SD) of the sensitivity coefficients (Si) of evapotranspiration (ET) and transpiration fraction (T/ET) to the assigned model parameters and measured parameters (mean ± SD)
参数符号 Parameter code
参数名称 Parameter name
lET
T/ET
rst_min
最小气孔阻抗 Minimum stomata resistance
-0.28 ± 0.14
-0.08 ± 0.02
rst_max
最大气孔阻抗 Maximum stomata resistance
-0.01 ± 0.08
0.00 ± 0.01
αV
植被冠层反照率 Albedo of vegetation canopy
-0.17 ± 0.23
-0.02 ± 0.03
αG
地表反照率 Albedo of ground surface
-0.01 ± 0.06
0.01 ± 0.01
CLAI
冠层集聚度 Clumping factor for permittivity of vegetation
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A multiscale remote sensing model for disaggregating regional fluxes to micrometeorological scales. 1 2004
... 在我国, 关于灌丛化的生态水文学效应研究较少.Li等(2013)对内蒙古灌丛草地下垫面灌丛斑块及草地斑块的土壤水分动态进行了长期观测, 并指出内蒙古太仆寺旗站点的灌丛斑块与草地斑块土壤水分动态无显著季节差异.Peng等(2013)对不同程度的人类干扰下灌丛化梯度的生物量、群落特征、水分可利用性等进行了比对研究.Zhang等(2013)对不同灌丛化梯度下草地的土壤有机质、土壤全氮、土壤全磷、土壤孔隙度、土壤质地、土壤饱和导水率进行了对比研究.高琼和刘婷(2015)曾研究解释我国半干旱区灌丛化形成的机理.张宏等(2001)研究了灌丛化空间分布对土壤异质性的响应.蒸散发是水文及能量循环中的重要组分, 然而在野外环境条件下, 土壤、灌丛和草地的蒸散耗水往往混杂在一起, 难以拆分.因此, 针对灌丛化的蒸散耗水效应研究相对较少.在前人的研究中, 有许多拆分蒸散发的方法, 其中包括物理分析方法、化学同位素分析方法(Scott et al., 2006; Moran et al., 2009)、模型分析法(Brenner & Incoll, 1997; Reynolds et al., 2000).同位素方法可以较好地拆分蒸散发(Wang et al., 2015, 2016), 但是难以用来情景化分析.模型分析法由于可以运用的时间尺度较长, 且可适用的空间广度相对较大, 已得到了广泛的应用.二源模型尽管相对简单, 但仍能较为准确地把握土壤蒸发与植被蒸散的通量, 而且在复杂多变的气候条件下, 二源模型已被验证可以提供合理的蒸散发组分的估算(Anderson et al., 2004; Li et al., 2008), 并且模型较为适合情景化效应分析.因此, 二源模型对于准确量化并拆分陆地生态系统蒸散发具有较好的实用性.本文以中国内蒙古草原灌丛化区域为例, 利用已有的二源模型对蒸散发进行估算并拆分(Wang & Yamanaka, 2014), 针对内蒙古地区的情况并结合前人研究, 对已有二源模型进行相关参数本地化率定, 通过波文比能量收支平衡方程计算出的潜热通量(lET)与模型输出值进行对比验证, 使其能够准确重建内蒙古灌丛化草地蒸散发动态.在此基础上结合先前灌丛化的调查, 进行灌丛化的情景模拟, 进一步量化分析灌丛、草地、土壤三者对蒸散发的贡献度, 从而对灌丛化的蒸散耗水效应进行阐述, 旨在为草地的退化管理、修复等提供一定的科学依据. ...
Interrelationships between plant functional types and soil moisture heterogeneity for semiarid landscapes within the grassland/forest continuum: A unified conceptual model. 1 1999
Soil water and temperature dynamics in shrub-encroached grasslands and climatic implications: Results from Inner Mongolia steppe ecosystem of north China. 1 2013
Application of a two-source model for partitioning evapotranspiration and assessing its controls in temperate grasslands in central Japan. 2014
Partitioning evapotranspiration in a temperate grassland ecosystem: Numerical modeling with isotopic tracers. 1 2015
... 在我国, 关于灌丛化的生态水文学效应研究较少.Li等(2013)对内蒙古灌丛草地下垫面灌丛斑块及草地斑块的土壤水分动态进行了长期观测, 并指出内蒙古太仆寺旗站点的灌丛斑块与草地斑块土壤水分动态无显著季节差异.Peng等(2013)对不同程度的人类干扰下灌丛化梯度的生物量、群落特征、水分可利用性等进行了比对研究.Zhang等(2013)对不同灌丛化梯度下草地的土壤有机质、土壤全氮、土壤全磷、土壤孔隙度、土壤质地、土壤饱和导水率进行了对比研究.高琼和刘婷(2015)曾研究解释我国半干旱区灌丛化形成的机理.张宏等(2001)研究了灌丛化空间分布对土壤异质性的响应.蒸散发是水文及能量循环中的重要组分, 然而在野外环境条件下, 土壤、灌丛和草地的蒸散耗水往往混杂在一起, 难以拆分.因此, 针对灌丛化的蒸散耗水效应研究相对较少.在前人的研究中, 有许多拆分蒸散发的方法, 其中包括物理分析方法、化学同位素分析方法(Scott et al., 2006; Moran et al., 2009)、模型分析法(Brenner & Incoll, 1997; Reynolds et al., 2000).同位素方法可以较好地拆分蒸散发(Wang et al., 2015, 2016), 但是难以用来情景化分析.模型分析法由于可以运用的时间尺度较长, 且可适用的空间广度相对较大, 已得到了广泛的应用.二源模型尽管相对简单, 但仍能较为准确地把握土壤蒸发与植被蒸散的通量, 而且在复杂多变的气候条件下, 二源模型已被验证可以提供合理的蒸散发组分的估算(Anderson et al., 2004; Li et al., 2008), 并且模型较为适合情景化效应分析.因此, 二源模型对于准确量化并拆分陆地生态系统蒸散发具有较好的实用性.本文以中国内蒙古草原灌丛化区域为例, 利用已有的二源模型对蒸散发进行估算并拆分(Wang & Yamanaka, 2014), 针对内蒙古地区的情况并结合前人研究, 对已有二源模型进行相关参数本地化率定, 通过波文比能量收支平衡方程计算出的潜热通量(lET)与模型输出值进行对比验证, 使其能够准确重建内蒙古灌丛化草地蒸散发动态.在此基础上结合先前灌丛化的调查, 进行灌丛化的情景模拟, 进一步量化分析灌丛、草地、土壤三者对蒸散发的贡献度, 从而对灌丛化的蒸散耗水效应进行阐述, 旨在为草地的退化管理、修复等提供一定的科学依据. ...
Numerical modeling the isotopic composition of evapotranspiration in an arid artificial oasis cropland ecosystem with high-frequency water vapor isotope measurement. 1 2016