Alexander Richard B, Boyer Elizabeth W, Smith Richard A, Schwarz Gregory E, Moore Richard B
J Am Water Resour Assoc. 2007 Feb;43(1):41-59. doi: 10.1111/j.1752-1688.2007.00005.x.
Knowledge of headwater influences on the water-quality and flow conditions of downstream waters is essential to water-resource management at all governmental levels; this includes recent court decisions on the jurisdiction of the Federal Clean Water Act (CWA) over upland areas that contribute to larger downstream water bodies. We review current watershed research and use a water-quality model to investigate headwater influences on downstream receiving waters. Our evaluations demonstrate the intrinsic connections of headwaters to landscape processes and downstream waters through their influence on the supply, transport, and fate of water and solutes in watersheds. Hydrological processes in headwater catchments control the recharge of subsurface water stores, flow paths, and residence times of water throughout landscapes. The dynamic coupling of hydrological and biogeochemical processes in upland streams further controls the chemical form, timing, and longitudinal distances of solute transport to downstream waters. We apply the spatially explicit, mass-balance watershed model SPARROW to consider transport and transformations of water and nutrients throughout stream networks in the northeastern United States. We simulate fluxes of nitrogen, a primary nutrient that is a water-quality concern for acidification of streams and lakes and eutrophication of coastal waters, and refine the model structure to include literature observations of nitrogen removal in streams and lakes. We quantify nitrogen transport from headwaters to downstream navigable waters, where headwaters are defined within the model as first-order, perennial streams that include flow and nitrogen contributions from smaller, intermittent and ephemeral streams. We find that first-order headwaters contribute approximately 70% of the mean-annual water volume and 65% of the nitrogen flux in second-order streams. Their contributions to mean water volume and nitrogen flux decline only marginally to about 55% and 40% in fourth- and higher-order rivers that include navigable waters and their tributaries. These results underscore the profound influence that headwater areas have on shaping downstream water quantity and water quality. The results have relevance to water-resource management and regulatory decisions and potentially broaden understanding of the spatial extent of Federal CWA jurisdiction in U.S. waters.
了解源头对下游水体水质和水流状况的影响对于各级政府的水资源管理至关重要；这包括近期法庭关于《联邦清洁水法》（CWA）对汇入较大下游水体的高地地区管辖权的裁决。我们回顾了当前的流域研究，并使用水质模型来研究源头对下游受纳水体的影响。我们的评估表明，源头通过对流域内水和溶质的供应、运输及归宿的影响，与景观过程和下游水体存在内在联系。源头集水区的水文过程控制着地下水库的补给、水流路径以及整个景观中水体的停留时间。高地溪流中水文和生物地球化学过程的动态耦合进一步控制着溶质向下游水体运输的化学形态、时间和纵向距离。我们应用空间明确的质量平衡流域模型SPARROW来考虑美国东北部整个河网中水和养分的运输与转化。我们模拟了氮的通量，氮是一种主要养分，是溪流和湖泊酸化以及沿海水体富营养化的水质问题关注点，并完善模型结构以纳入溪流和湖泊中氮去除的文献观测结果。我们量化了从源头到下游通航水域的氮运输，在模型中源头被定义为一级常年溪流，包括来自较小的间歇性和临时性溪流的水流和氮贡献。我们发现，一级源头对二级溪流的年均水量贡献约为70%，氮通量贡献约为65%。在包括通航水域及其支流的四级及更高级河流中，它们对平均水量和氮通量的贡献仅略微下降至约55%和40%。这些结果强调了源头地区对塑造下游水量和水质的深远影响。这些结果与水资源管理和监管决策相关，并可能拓宽对美国水域中联邦CWA管辖权空间范围的理解。
