东北春玉米根系吸水过程的控制机制及其参数方案改进对陆-气通量模拟的影响

Strengthening research on processes of root water uptake (RWU) and its control mechanism for typical vegetations is expected to play an important role in further understanding terrestrial water cycling and improving the simulation performance of land surface models (LSMs) for water and heat fluxes. Considering the dissimilarity of RWU between different growing stages of plant under different water stress conditions and shortcomings of RWU parameterization schemes in main existing LSMs, based on the Jinzhou Maize Agricultural Ecosystem Observation Station, the large farmland soil moisture control experimental field and root observational systems, integrating minirhizotron method and sap flow observation technology, root distribution and biomass as well as some parameters related to RWU at different growing stages of maize under different soil water contents will be dynamically observed. RWU process and its control mechanisms are expected to be revealed with a deep analysis of observation data. In addition, three measures are planned to conduct in order to improve RWU parameterization scheme in CLM4CNcrop model in this study. Firstly, simulating dynamically the root distribution will be achieved through deeply investigating the relationship between root distribution and biomass. Secondly, with two soil water availability threshold values, one that enable potential transpiration when part of the root system experiences water stress and another that no water uptake is allowed when water availability is less than, confirmed by observations, response function of RWU to soil moisture is optimized. Thirdly, a hydraulic redistribution scheme is incorporated into the model to modify the parameterization of vertical water transport among different soil layers. Afterwards, through evaluating quantitatively the effects of improvement measures on simulation results of land-atmosphere fluxes, the impact mechanism of RWU on water and heat exchange between land and atmosphere will likely be revealed further. Above all, this study will contribute to understand the interaction between land and atmosphere in agroecosystem and to improve the simulation performance of LSMs as well as climate models.

加强典型植被根系吸水过程及其控制机制研究对深入理解陆地水循环过程，改善陆面过程模型对水热通量的模拟具有重要作用。针对不同水分胁迫条件下植物不同生长阶段根系吸水过程可能存在的差异，以及现有主要陆面过程模型根系吸水方案存在的不足，以东北春玉米为研究对象，基于锦州玉米农田生态系统野外观测站、大型农田土壤水分控制试验场、大型根系观测系统，结合微根管和茎流观测技术，在不同土壤水分条件下对玉米不同发育期根分布和生物量以及根系吸水相关参数进行动态观测，通过对实测资料的深入分析揭示根系吸水过程及其控制机制；通过实现根分布动态模拟、土壤水对根系吸水影响函数的优化和引入根系水分再分配作用对CLM4CNcrop模型根系吸水方案进行改进；通过定量评价模型改进对陆气通量模拟的影响，揭示根系吸水作用对陆气水热交换影响的内在机制，为深入了解农田陆气相互作用规律、提高陆面过程模型乃至气候模式模拟能力提供参考。

本项目对不同水分胁迫下东北春玉米生理特征和根系吸水过程的干旱响应规律进行研究；通过确定最优根分布模型，优化土壤水对根系吸水影响函数和引入根系水分再分配作用对CoLM模型进行改进，利用2007-2009年涡度相关资料研究根系吸水参数方案优化对水热通量模拟的影响，得以下结论：1）玉米根长密度（RLD）与土壤湿度（Ws）有较好的时空对应关系，不同发育期干旱胁迫使RLD最大值所在土壤深度（60cm）比对照（CK）（40cm）增大，CK和拔节期干旱处理（BJ）的RLD在最大值深度以下随深度增加而减小，抽雄干旱处理（CX）的RLD在80cm以下仍保持较大值。BJ和CX比CK处理的d50 和d95 (累积根比例为50%和95%的土壤深度)分别增大45%、59%和8%、41%，证明一定干旱条件下玉米根系向深层土壤生长。2）根系吸水（RWU）受Ws和辐射影响显著，除生育末期以外的正常水分条件下，RWU与辐射呈显著对数关系，随干旱持续该关系减弱，当恢复供水后，对数关系随之恢复。在干旱条件下，RWU随干旱程度加重而减小，其日峰值由正午提前至9-10时，干旱后复水在植株补偿性生长作用下，RWU高于正常水平，但日峰值仍提前。3）在Schenk、Zeng和Jackson 建立的3种根分布参数方案（M1、M2、M3）中，M2和M3模拟精度基本一致，仅在玉米吐丝期以后模拟精度较高，而M1能够对玉米不同发育期根分布模拟精度较高；4）根分布对CoLM模型水热通量模拟的影响研究表明：d50敏感性强于d95；根分布对Ws的影响在极端干旱条件下很小，在一定Ws范围内随Ws增大而减小；植物蒸腾受根分布影响最大，其次是土壤蒸发，而叶片蒸发不受影响；5）分别利用Li与Zheng and Wang提出的两种根系吸水函数对CoLM模型进行改进，与Li方案相比，Zheng方案对CoLM模型性能改进能力较强，模拟精度受决定根系累计吸水效率的参数影响显著，但对决定根系吸水分布的参数不敏感，在降水偏少年份对Ws、土壤温度、感热和潜热的模拟性能明显改善。6）引入根系水分再分配作用后，CoLM模型模拟性能并未得到改善，表明该过程在玉米农田对水热交换过程模拟没有影响。