Collaborative Research: Changes in hyporheic exchange and nitrous oxide generation due to streambed alteration by macro-roughness elements

巨砾和大卵石是山地河流中普遍存在的粗糙元素，它们的存在具有重要的生态和水文形态功能。它们通常用于恢复项目，以提高许多水生物种的栖息地质量，因为它们通过诱导侵蚀和沉积区来影响河流形态，其大小和范围取决于泥沙动员流中的巨石/鹅卵石密度、沉积物输入和流动条件。然而，对于粗糙度元素引起的河床变化在调节潜流交换中的作用知之甚少，潜流交换是指水流进出河床沉积物形成地下潜流带的过程。低渗带促进重要的生化转化，影响河流和孔隙水的化学性质，在当地和全球范围内都具有影响，因为它是河流生物活性的主要驱动力，也是温室气体一氧化二氮（N2O）的主要来源。包括河流恢复在内的水资源可持续管理（美国每年10亿美元的产业）将受益于潜流过程的知识，以预测在当地和流域尺度上调节流量、气候和土地利用对生态系统、营养循环和溶质运输的影响。该项目的科学目标是了解、量化和模拟形成流过程中巨石和鹅卵石密度和相对沉积物供应对低流量条件下潜流交换、化学和N2O生成的影响。这将通过水槽和现场实验来解决，并通过分析和数值模拟来支持，并通过以下假设进行测试：卵石或鹅卵石引起的河床形态变化(1)在调节河段尺度上低渗交换的数量和化学方面起主要作用；(2)改变有氧呼吸、硝化和反硝化速率，并增加N2O的表观产生和释放速率。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Boulders and large cobbles are ubiquitous roughness elements in mountain rivers and their presence provides vital ecological and hydromorphological functions. They are routinely used in restoration projects to increase habitat quality for many aquatic species because they impact on stream morphology by inducing erosional and depositional areas, whose size and extent depend on the boulder/cobble density, sediment inputs, and flow conditions during sediment mobilizing flows. However, little is known about the role of roughness element induced-streambed changes in regulating hyporheic exchange, which is the process moving stream water into and out of stream bed sediments to form the subterraneous hyporheic zone. The hyporheic zone promotes important biochemical transformations, which affect stream and pore water chemistries with implications at both local and global scales because it is a primary driver of bioactivity in streams and a key source of the greenhouse gas nitrous oxide, N2O. Sustainable management of water resources including river restoration (a $1B/year industry in the US) would benefit from knowledge of hyporheic processes to predict the effects of regulated flows, climate, and land use on ecosystems, nutrient cycles, and solute transport at the local and watershed scales. The scientific goal for this project is to understand, quantify, and model the effects of boulder and cobble density and relative sediment supply during forming flows on hyporheic exchange, chemistry, and N2O production during low flow conditions. This will be addressed with flume and field experiments, supported by analytical and numerical modeling, and tested with hypotheses that boulder or cobble-induced changes in streambed morphology (1) have a primary role in regulating both the quantity and chemistry of hyporheic exchange at the reach scale, and (2) alter aerobic respiration, nitrification, and denitrification rates and increase the rates of apparent production and release of N2O.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.