民勤绿洲地下水位异常变化对极端干旱气候和灌溉活动的响应机制

With the increasing frequency of extreme dry climate events, groundwater has been overexploited, and a series of eco-environmental problems such as soil salinization, land desertification and oasis wetland degradation occurred in Minqin Oasis, which is mainly irrigated by groundwater. It is of extreme importance to understand thoroughly the variation characteristics of groundwater level and their response relationship to irrigation activities at different time scales under extreme drought events. However, the current related study is relatively weak in this area. In this project, the characteristics of the groundwater level temporal and spatial variation in past 40 years in Minqin Oasis would be analyzed firstly. Based on them, 30-50 long-term monitoring wells represented of typical areas were selected, and the anomalous change series of groundwater level of the wells were obtained and their change characteristics, influencing factors and influencing degree were analyzed on the time scales of annual, crop growth period, and crop water demand critical period, with the method of Percentile, Empirical Mode Decomposition and Principal Component Regression. The thresholds of extreme ago-climate dry events were determined by percentile method with considering the different water demand characteristics in different crop growth periods, and the change trends of the extreme ago-climate dry events were conducted by cumulative anomaly method and the like. In the end, the quantitative response relationship between the anomalous groundwater level and the extreme dry events - irrigation activities on the time scale of annual, crop growth period and crop water demand critical period in different areas were tentatively established. On balance, this project is of great significance to the sustainable utilization of groundwater resources and maintenance of ecological environment in Minqin Oasis.

以地下水灌溉为主的民勤绿洲，极端干旱气候事件频发，地下水严重超采，并引发一系列生态环境问题。深入认识极端干旱事件条件下不同时间尺度地下水位异常变化特征及其对绿洲灌溉活动的响应关系对水资源利用与生态保护至关重要。针对此关键问题，本项目拟在分析近40年民勤绿洲地下水位时空变化特征的基础上，划分典型区，选择30-50个代表性长期监测孔，采用经验模态分解法逐级分解不同特征尺度上的地下水位波动特征并识别异常变化序列，采用主成分回归法分析地下水位不同波动系列的主控因素和影响程度；结合作物生育期需水不同的特征采用百分位法确定农业极端干旱事件阈值并分析其变化特征；基于上述分析，采用系统动力学模型初步建立不同典型区地下水位异常变化在年、作物生长季及作物需水临界期时间尺度上对极端干旱事件和灌溉活动的量化响应关系。本研究对民勤绿洲应对极端干旱气候风险、地下水资源可持续利用和生态环境维持具有重大现实意义。

水资源天然匮乏叠加大规模灌溉活动导致石羊河流域民勤绿洲地下水严重超采，地下水位快速下降，并引发一系列生态环境问题。深入认识地下水位异常变化特征及其对绿洲灌溉活动和气候变化尤其是极端气候事件的响应关系对水资源利用与生态保护至关重要。 .本项目在分析气候水文变化背景的基础上，结合作物生育期需水不同的特征采用百分位法确定极端干旱事件阈值，采用不同阈值划分了小、中、强降水事件，分析了近几十年极端干旱和极端降水事件变化特征；采用趋势分析法分析了区域地下水位时空变异规律，并采用广义线性模型分析了治理前后气候变化及人类活动对地下水位的影响程度变化；最后采用集合经验模态分解法将地下水埋深系列分解为不同特征尺度的地下水分量并考察各分量对降水、极端降水、开采活动、水库渠系渗漏的相应关系。.主要研究成果如下。.①民勤气温呈显著升高趋势，升高率为0.53℃/10a；降水量呈不显著增加趋势，出山口径流量和蒸发量呈波动变化特征。.②民勤1970s主要以极端降水事件为主；1980s和1990s除1994年发生了近60年最大的一次强降水事件之外，极端强降水事件较少，中小强度降水事件偏多，极端干旱事件并不突出；2000s和2010s降水集中度增加，极端降水事件增多增大，极端干旱事件也可能增大，于2006和2008年发生了近60年次强的两次极端降水事件，也经历了2004、2009年和2013年的极端干旱事件。.③区域地下水位主要呈缓慢下降-快速下降-基本稳定/略有回升过程，重点治理前地下水位平均下降速率为55cm/a；2008-2019年，治理后地下水位下降趋缓，湖区略有回升，区域总体下降速率减小至30cm/a以内。人类活动对地下水位动态的影响程度从重点治理前的88.1%减小到治理后的32.9%。 .④除长期人类灌溉开采活动影响外，湖区和泉山灌区北部地下水埋深主要受干旱胁迫下的波动开采量变化影响；环河灌区、坝区及泉山灌区南部距离水库较近，地下水埋深对地表水灌溉的响应较大；环河灌区地下水埋深对降水入渗及蒸发排泄响应较好。.⑤地下水位埋深增大至6米以上时，地下水补给的滞后性增强，但对极端降水的响应较快；地下水埋深大于15米后，极端降水事件也很难补给地下水；极端降水强度和包气带厚度共同影响地下水的补给响应。.本研究可为民勤绿洲地下水可持续利用、应对极端干旱气候风险和生态环境维持提供科学参考。