基于氢氧同位素的农田GSPAC界面水分转化机理与模拟研究

Water transformation between the interfaces of GSPAC (Groundwater-Soil-Plant-Atmosphere Continuum, GSPAC) is the key topic of field water cycle research, and it is the theoretical basis to improve the water use efficiency. The determination of groundwater recharge and evapotranspiration and quantification of their water sources is the challenge in the research of interface water transformation. Combination of experimental observation, isotope tracing technique and simulation method is the feasible approach to figure out above question. In this project, measurements of water balance components were conducted in large lysimeters with different precipitation and water table conditions. The δ2H and δ18O contents in different water samples were measured. The dynamic changes of water balance components and isotopic characteristics in GSPAC were analyzed. The water sources of root uptake and groundwater discharge were quantified with the Bayesian mixture model. Evapotranspiration was partitioned into evaporation and transpiration with water balance and isotopic mass balance methods, and the sources of crop water consumption were determined. Based on the above work, the SWAP and Iso-SPAC models were combined to develop the water-isotope transport model Iso-GSPAC, which could be used to simulate the variations of water flux and contributions of different sources in GSPAC. With the Iso-GSPAC, the influencing mechanism of rainfall and groundwater level on water transformation between GSPAC interfaces were substantially demonstrated with scenarios simulation and sensitivity analysis. Through this research, it can be clearer to understand the dynamic processes of rainfall infiltration-soil moisture change-evaporation/transpiration/groundwater recharge. This study can also provide scientific basis for efficient utilization of agricultural water resources.

GSPAC界面水分转化关系是农田水循环研究的核心问题，是提高水分利用效率的理论基础。确定地下水补给和蒸散发并量化其水分来源是界面水分转化研究的难点，综合运用定位监测、同位素示踪和模型模拟等手段是解决上述问题的有效途径。为此，本项目利用大型测坑群，设置不同降雨和地下水位处理，开展夏玉米农田GSPAC水量平衡要素和氢氧同位素的观测试验，分析水分动态和同位素分布特征，采用贝叶斯混合模型和同位素质量守恒量化根系吸水和地下水补给来源并分割蒸散发，明确作物耗水来源的动态变化规律。基于以上工作，将SWAP和Iso-SPAC模型结合，建立水分和同位素运移的耦合模型Iso-GSPAC，实现GSPAC界面水分转化通量和来源比例的动态模拟，结合情景模拟揭示降雨和地下水位对GSPAC界面水分转化的影响机理。本研究可揭示降雨入渗-土壤水分变化-蒸发蒸腾/地下水补给的动态过程，并可为农业水资源高效利用提供科学依据。

气候变化（温度和CO2升高）和人类活动影响下的降水和地下水位变化改变了地下水-土壤-植被-大气连续体（GSPAC）界面水分转化关系，农业用水和粮食安全面临着严峻挑战。蒸散发的准确区分和植被水分利用来源的定量解析是变化环境下GSPAC界面水分转化研究的关键问题，对农业水文基础理论发展和区域水资源管理具有重要意义。本项目综合运用“五水转化动力过程实验装置”模拟控制实验、农田定位监测试验、同位素示踪和模型模拟等方法，对比分析了不同地下水位和降水频率、增温2°C和CO2倍增（700ppm）情景下多时间尺度（小时、日、五日、生育阶段和生长季）GSPAC水量平衡组成要素的变化规律；定量解析了冬小麦和夏玉米根系吸水来源及其贡献比例；区分了农田蒸散发组分，明确了作物耗水来源的季节变化特征及其驱动因素；引入同位素平衡分馏系数，建立了基于HYDRUS的土壤-水-热-同位素运移耦合模型，模拟了次降雨/灌溉的滞留时间并动态解析了作物水分吸收利用过程。研究发现：1）多年平均气候情景下地下水位下降夏玉米蒸散量（ET）明显增加，产量变幅较小，水分利用效率（WUE）显著降低。增温2℃不同时间尺度的ET、产量和WUE均显著增加，但CO2倍增情景下未发生明显改变。2）50%降水频率下冬小麦耗水来源与根长分布和生长初期土壤含水量均负相关。夏玉米耗水来源与根长分布正相关，CO2倍增刺激深层根系发育且促进了地上部生理生长，但未明显改变ET。3）冬小麦产量和WUE与拔节-灌浆期蒸腾量二次相关，与蒸腾占比无显著相关关系。区别于传统“土壤蒸发无效论”，证明了蒸发在一定范围内可促进作物生长并提高产量。4）耦合同位素信息的HYDRUS模型可更精准刻画农田水分运移过程及其对降雨和地下水位的动态响应，深入认识GSPAC水分转化机理和作物响应规律，为区域适水农业和可持续发展提供决策依据。项目已发表SCI 论文6篇，中文核心2 篇。培养硕士生1 名，在读博士生2 名。