半干旱区退耕草地蒸散发估算，分离及其检验的同位素示踪及数值模拟

Partitioning of evapotranspiration (ET) into soil evaporation (E) and vegetation transpiration (T) is important not only for improving water use effciency in semi-arid region but also for predicting the response of ecosystem water/energy budgets to climatic change. Isotopic composition of water can provide useful information to progress in a better quantification of this partition. These studies will combines field measurements with a modelling work in order to understand the coupled water, heat and isotopic transfer within the soil-plant-atmosphere-continuum (SPAC) with special emphasis on water availability in semi-arid. Field measurements of δ18O and δD among different water pools (i.e., water vapor, soil water, stem water and leaf water) will be made in semi-arid agriculture and grassland ecosystem. A simple Iso-SPAC (SPAC with isotopic tracers) model will develop and use to make progress in understanding of partition evapotranspiration. The partitioning results were compared by separate, simultaneous field work for isotope measurements in the liquid and gas phases with keeling plot method. Sensitivity analysis for partitioning results will conduct to assess the possible errors caused by the input variables and assumed parameters. The principal factor controlling variation of partitioning results will be analysis, the role of soil moisture and plant cover regulation on the partitioning results will be discussed. Under the future climate scenarios (downscales from regional climate models output), validated Iso-SPAC will be used to evaluating the climate change impacts on ecohydrological response and giving some useful suggestions for water resources management in semi-arid region.

在我国的半干旱区农田及退耕草地生态系统,如何提高干旱区水分利用效率及气候变化情境下,退耕草地否会出现 "生境干旱化"等问题是干旱区水资源管理的主要内容。分离蒸散发为土壤蒸发及植物蒸散有助于科学回答以上问题。同位素方法能够提供更好地辅助分离二者并加深对水分在土壤-植物-大气连续体（SPAC）运移的理解。本项目将选取内蒙古太仆寺旗北京师范大学实验站，选取退耕草地，农田，原生草地，通过现场观测，同位素取样，耦合同位素运移数值模拟等方法，构建和发展一个耦合同位素蒸馏过程的模型（Iso-SPAC）来分离蒸散发并取得进展。分离的结果将通过模型，同位素方法以及小型蒸渗仪多种方法比较，分析各种方法的异同点。比较各不同类型的下垫面水分利用效率。结合我国干旱区未来的气候变化情景（选取已有区域气候模型预测并降尺度），探讨气候变化情境下，对我国干旱区农田及退耕草地生态系统影响。提出相应的建议和对策。

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。蒸散发的拆分是生态水文学研究的难点和热点,模型模拟是研究蒸散发的主要方法。本文在原有基础上,建立和提高了Iso-SPAC模型,该模型应用于诸多不同下垫面特征的典型生态系统（草地、 农田、灌丛化草地）进行蒸散发拆分,并用涡动或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草地、农田、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。蒸散占比重（T/ET）均随着植被生长逐渐增加,人为的割草、灌溉等活动对T/ET的影响均得到了较好的模拟。草地、农田、灌丛化草地生态系统,均值及生长季波动分别为75±24%、0.81±0.21%、58±17%。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)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)均值分别为52.9%、53.9%和55.5%。灌丛化从初期到中期、中期发展至后期, 蒸散发降幅(均值±波动)分别为0.34% ± 0.51%和0.44% ± 0.71%, E/ET升幅分别达2.04% ± 1.58% 及3.25% ± 2.59%。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。