Hydrologic Sciences, UC Davis, Davis, CA, 95616, USA.
Environ Monit Assess. 2018 Jul 11;190(8):458. doi: 10.1007/s10661-018-6841-x.
Creating or recruiting new riparian forests to improve Lower Putah Creek (LPC) ecosystem functions is challenging under the modified stream flow regime developed after historic gravel mining and installation of Monticello Dam upstream. Hydrologic connectivity between riparian trees, shallow groundwater, and the low flow channel is essential towards maintaining these forests and related habitats through the annual summer and multi-year drought periods typical in this Sacramento Valley region of California. Despite increased average summer flows, significant mature cottonwood and willow tree mortality along the LPC riparian areas below the Putah Creek diversion dam in 2014 raised concerns over the soil and hydrologic factors affecting riparian vegetation survival. A forensic analysis was conducted combining annual canopy coverage fractions and tree ring studies with daily soil-water balances, low flow records, and available groundwater level Information from the past few decades to determine the key hydrologic factors affecting riparian tree survival along the LPC. The 2011-2016 drought was linked with greater than prior average soil-water deficits in 2012-2015 and lower initial soil-water storage on March 1 of 2012 and 2014 that would be expected to stress the trees. However, such stress was not apparent in decreased tree ring spacing during this period from mature (40-50 years old), deceased, and living trees. Tree canopy coverage declined dramatically (by as much as 50% as compared to the previous decade average) only in the summer of the 2014 despite a ~ 35% increase in average summer flows from 2011 to 2014. However, the regional water table aquifer levels declined at an average rate of ~ 35 mm/day in 2014 (as compared to ~ 17 mm/day in previous decade) and by several meters overall between 2011 and 2016 suggesting that deceased trees lacked access to the water table aquifer or lateral stream seepage. The increased rates of water table decline and overall depth may be associated with a large increase in adjacent irrigated almond orchard areas in 2014-2016. Knowledge of the dynamic hydrologic factors controlling sustainability of riparian trees should better inform and guide future tree restoration efforts along the LPC.
在历史采砾和上游蒙蒂塞洛水坝建成改变了河流流量模式的情况下，要想在下游普塔溪建立或培育新的河岸林，以改善生态系统功能，具有挑战性。河岸树木、浅层地下水和低流量河道之间的水文连通性对于维持这些森林和相关栖息地至关重要，因为加利福尼亚州萨克拉门托河谷地区每年夏季和多年干旱都会造成影响。尽管夏季平均流量有所增加，但 2014 年普塔溪分流坝以下的 LPC 河岸地区仍有大量成熟的棉白杨和柳树死亡，这让人对影响河岸植被生存的土壤和水文因素感到担忧。为此，我们结合树冠覆盖率年度数据、树木年轮研究、逐日土壤水分平衡、低流量记录以及过去几十年的地下水水位信息，对影响 LPC 河岸树木生存的关键水文因素进行了法医分析。2011-2016 年干旱与 2012-2015 年土壤水分亏缺大于平均水平以及 2012 年 3 月 1 日和 2014 年 3 月 1 日初始土壤水分储量较低有关，这预计会给树木带来压力。然而，在这一期间，从成熟（40-50 岁）、死亡和存活树木的年轮间距来看，并没有明显的压力迹象。尽管 2011 年至 2014 年夏季平均流量增加了约 35%，但 2014 年树木树冠覆盖率却急剧下降（与前十年平均水平相比，下降了多达 50%）。然而，在 2014 年，区域地下水位含水层的平均下降速度为每天约 35 毫米（而前十年为每天约 17 毫米），在 2011 年至 2016 年期间总体下降了数米，这表明死亡的树木无法获得地下水位含水层或侧向溪流渗漏的水源。地下水位下降速度和整体深度的增加可能与 2014-2016 年相邻灌溉杏仁果园面积的大幅增加有关。了解控制河岸树木可持续性的动态水文因素，应能更好地为未来 LPC 河岸的树木恢复工作提供信息和指导。
