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灌丛化是干旱半干旱草原一种常见的全球性变化现象,由于野外土壤、灌丛和草本的蒸散耗水难于拆分的限制,关于灌丛化蒸散耗水效应的研究较少。该文将已有的二源模型应用于我国内蒙古灌丛化草原估算其蒸散发,并用波文比系统观测结果对模型进行了率定。研究结果表明改进的模型可以较好地重建灌丛化草地的蒸散发特征;敏感性分析结果表明模型输入变量及参数对蒸散发组分拆分结果产生的误差较小。在此基础上进行了灌丛化的情景模拟,研究其耗水效应。结果表明:灌丛化对蒸散发总量影响较小,而对蒸散发组分影响较大。灌丛化初期盖度5%、中期盖度15%及后期盖度为30%的情境下,对应的生长季内蒸散发(ET)平均值分别为182.97、180.38和176.72 W·m~(–2);土壤蒸发(E)占蒸散发比率(E/ET)平均值分别为52.9%、53.9%和55.5%。灌丛化从初期到中期、中期至后期,蒸散发降幅平均值分别为0.34%和0.44%,E/ET升幅分别达2.04%及3.25%。该研究结果表明在内蒙古太仆寺旗站点灌丛化导致的土壤水分差异并不明显,但随着灌丛化加剧,灌丛逐渐替代草本,改变了原有的生态系统结构,植被叶面积指数变小,导致冠层导度降低。研究结果强调我国半干旱草原区灌丛化加剧对生态系统总蒸散耗水量影响不大,但其土壤蒸发无效损耗快速增加会导致系统水分利用效率降低。
Shrubification is a common global change phenomenon in arid and semi-arid steppe. Due to the difficulty in splitting the evapotranspiration of the soil, shrubs and herbs in the field, there are few researches on water consumption effect of shrub evapotranspiration. In this paper, the existing two-source model is applied to the evapotranspiration in the shrub steppe of Inner Mongolia, China. The model is calibrated with the observation results of the Bowen ratio system. The results show that the improved model can better reconstruct the evapotranspiration characteristics of shrub grassland. The sensitivity analysis results show that the error of model input variables and parameters on the resolution of evapotranspiration components is small. Based on this, a bush-like scenario simulation was carried out to study the water consumption effect. The results showed that shrubification had little effect on the total amount of evapotranspiration but greater impact on the components of evapotranspiration. Under the initial shrub stage with 5% coverage, 15% middle cover and 30% late cover, the average values of ET during the growing season were 182.97, 180.38 and 176.72 W · m ~ (- 2). Soil evaporation (E) accounted for 52.9%, 53.9% and 55.5% of the average evapotranspiration (E / ET), respectively. From the initial to mid-term, middle-to-late stage, the mean values of evapotranspiration decline were 0.34% and 0.44% respectively, and the E / ET increases were 2.04% and 3.25% respectively. The results showed that there was no significant difference in soil water content caused by shrubbing at Taipusi flag site in Inner Mongolia. However, as shrubs intensified, shrubs gradually replaced herbs and changed the original ecosystem structure. Leaf area index of vegetation became smaller, Resulting in reduced canopy conductance. The results emphasize that the intensification of shrubification in the semi-arid steppe region has little effect on the total evapotranspiration of the ecosystem, but the rapid increase of ineffective soil evaporation loss will lead to the decrease of water use efficiency of the system.