Modeling grazing effects on coupled water and heat fluxes in Inner Mongolia grassland
Modeling grazing effects on coupled water and heat fluxes in Inner Mongolia grassland
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DOI：
10.1016/j.still.2010.04.005
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发表时间：
2010-08
影响因子：
6.5
通讯作者：
Ying Zhao;S. Peth;R. Horn;J. Krümmelbein;B. Ketzer;Yingzhi Gao;J. Doerner;C. Bernhofer;X. Peng
中科院分区：
文献类型：
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作者：
Ying Zhao;S. Peth;R. Horn;J. Krümmelbein;B. Ketzer;Yingzhi Gao;J. Doerner;C. Bernhofer;X. Peng
过度放牧是半干旱地区草原退化的主要原因。为了评价土壤水热通量对放牧的响应，对内蒙古草原生态系统中4个不同放牧强度的站点2004~2006年的土壤、植物和气象参数进行了观测。放牧强度为：(1)1979年以来未放牧，(2)1999年以来未放牧，(3)中度放牧，(4)重度放牧。与其他处理相比，重度放牧降低了总孔容和大孔容。利用基于过程的水力模型Hydrus-1D模拟了放牧引起的土壤孔隙结构变化对土壤水热通量的影响。考虑到立地的边界条件，我们将蒸散量划分为绿植和枯草的动态覆盖面积指数的函数，采用与根长密度相关的根系生长模型，并用Shaw模型估算截留量。此外，采用三种模拟方法对土壤水力参数对模型结果的不确定性进行了评估：(I)实验室水力特性(LDP)；(Ii)神经网络(NN)分析；(Iii)逆优化(逆向)。在前人标定的基础上，对Hydrus-1D模型进行了验证，模拟结果与土壤水分和温度的实测值吻合较好，为评价放牧对土壤水分和能量平衡的影响提供了依据。在所用的三种方法中，反演法的模拟效果最好，而LDP法比神经网络法更能准确地反映土壤的结构性功能。模型结果表明，由于土壤结构和地表盖度的变化，放牧增加了土壤热通量。两个非放牧地和适度放牧地的水分收支组成没有显著差异，而重度放牧显著减少了17~7 mm的截留量和121~74 mm的蒸腾，增加了88~128 mm的蒸发量。结果表明，内蒙古草原集约化放牧使土壤功能退化，植物有效水减少，从而降低了草地生产力，增加了风蚀和水蚀的风险。
Overgrazing is a major cause of grassland degradation in semi-arid regions. To evaluate how soil water and heat fluxes respond to grazing, investigations on soil, plant and meteorological parameters were conducted at four sites with different grazing intensities through three growing periods (2004–2006) in a steppe ecosystem of Inner Mongolia. The grazing intensities were (1) ungrazed since 1979, (2) ungrazed since 1999, (3) moderately grazed, and (4) heavily grazed. In comparison to other treatments, heavy grazing had decreased total pore and macropores volumes. The impacts of these grazing-induced changes of soil pore structure on water and heat fluxes were simulated by the processed-based hydraulic model HYDRUS-1D. To account for the site-specific boundary condition, we partitioned evapotranspiration as a function of dynamic cover area index of green and dead plant materials, used the root growth model related with root length density, and estimated interception using the SHAW model. Furthermore, the uncertainty of soil hydraulic parameters on model results was evaluated using three simulation approaches: (i) laboratory-derived hydraulic properties (LDP), (ii) neural network (NN) analysis, and (iii) inverse optimization (Inverse). On the basis of previous calibrations, HYDRUS-1D was validated with a good agreement between modeled and measured soil moisture and temperature, which provided a basis to evaluate the grazing effects on water and energy balance. Of the three approaches used, the Inverse expressed the best simulation, and the LDP was better than the NN due to more precise reflection of soil structural functions. Model result showed that, due to the changed soil structure and soil surface coverage, grazing increased soil heat fluxes. There was no significant difference on water budget components between the two ungrazed sites and moderate grazing, while heavy grazing significantly decreased interception from 17 to 7mm and transpiration from 121 to 74mm, and increased evaporation from 88 to 128mm. We conclude that intensive grazing in Inner Mongolia grassland deteriorated soil functions and reduced plant available water, and consequently reduced grassland productivity and enhanced the risks for wind and water erosion.