Streamflow - Longitudinal Sites Along The Gwynns Falls - Boundary Station 4. Gwynns Falls at Route 1/Carroll Park BES ID 398-

Participants Gary Fisher, U.S. Geological Survey (oversees stream flow monitoring) Ed Doheny, U.S. Geological Survey (oversees stream flow monitoring) Ken Belt, U.S. Forest Service (oversees weekly station checks) I. Collection of Flow Data at USGS Stream Gage Stations BES stream chemistry samples are collected at gaging stations built and maintained by the U.S Geological Survey and which are funded mainly by LTER funds. However a number of the stations are funded by other sources, including the USGS, Maryland Department of the Environment, Baltimore County DEPRM, and Baltimore City DPW. Stage at each gage site is measured to about 0.01 ft accuracy and is recorded at 15 minute (or shorter) intervals using either a float gage or pressure transducer. These stage records are converted to flow records using stage-discharge relationships (see below). Crest-stage recorders (floating cork gages) indicate high water marks and provide a backup source of high water in case recording gages are compromised in flooding conditions. Stilling wells are installed at each site, either in the channel or via piping to the channel, to provide quiescent locations for sensing of stage. General Approach to Discharge Measurement and Flow Ratings In a measurement of discharge (volumetric flow per unit time), a number of velocity measurements are taken across a transect located near the gage using a wading rod and propeller current meter (usually a mini (pygmy) velocity meter). Velocities (ft/sec) for each sub section are multiplied by their respective sub section areas (sq. ft.) and these discharges (cubic feet per second) are then summed to get a total discharge measurement for the section. Generally, an attempt is made to include enough velocity measurements across the transect such that no more than 10 % of the total discharge is contained in each sub section, although this may not be achievable for higher flows. Each total discharge measurement, along with the concurrent stage reading constitutes a point on a stage-discharge rating curve. Over time, discharge measurements are obtained for a range of stages so that a complete, accurate and current stage-discharge rating curve (graph) can be constructed. USGS field crews visit all sites about every six weeks to download data, check and calibrate equipment, and to do a low flow discharge measurements to check the stage-discharge rating for shifts, and to observe the channel section for potential changes in the high flow rating. A single USGS hydrologist is assigned to the BES stations to provide continuity and to maximize familiarity with the stations. These USGS visits are augmented by weekly visits by BES field crews who do a number of checks under the guidance of a Forest Service Hydrologist (see below, II. Stream Gage calibration & Flow Rating QAQC: Weekly Checks). High flow velocities cannot be measured in-stream, and are metered by USGS crews from bridges near the gaging station. Discharge measurements during runoff events are repeatedly conducted until the high flow portion of the rating is defined. Extremely high flow discharges that cannot be measured are estimated by a slope area curve method using cross section geometry, channel roughness and surveyed elevations of high points of the hydraulic grade line indicated by debris left by the receding limb of the hydrograph. Data are recorded at 15 minute intervals at the larger watersheds and at 5 minute intervals at small watersheds. Records are downloaded electronically and processed at the USGS office where adjustments are made for calibration problems, backwater corrections (due to debris and ice buildup), missing records, shifts in the rating, etc. Small changes in the low flow portion of the stage-discharge rating are handled using

参与人员包括：Gary Fisher与Ed Doheny，均任职于美国地质调查局（U.S. Geological Survey, 下文简称USGS），负责径流监测工作；Ken Belt任职于美国林务局（U.S. Forest Service），负责每周站点巡检工作。
一、USGS水文测站流量数据采集
BES水化学样品采集工作依托美国地质调查局建设并维护的测流站点开展，此类站点主要由长期生态研究（Long-Term Ecological Research, LTER）基金资助，不过部分站点的资金来自其他渠道，包括USGS、马里兰州环境部、巴尔的摩县规划与资源管理局以及巴尔的摩市公共工程部。
各测站的水位测量精度可达约0.01英尺，采用浮式测尺或压力传感器进行数据采集，记录间隔为15分钟（或更短）。通过水位-流量关系（详见下文）将水位记录转换为流量记录。洪水位记录仪（浮式浮塞测尺）可记录最高水位标记，作为洪水情况下测录设备失效时的高水位数据备份来源。各站点均设置静水井，可设于河道内或通过管道引至河道，为水位传感提供稳定的静水环境。
二、流量测量与流量率定通用方法
在单位时间体积流量的测量中，采用涉水杆和旋桨式流速仪（通常为微型（侏儒型）流速仪）在测站附近的断面开展多次流速测量。将各子断面的流速（单位：英尺/秒）乘以对应子断面的面积（单位：平方英尺），再将所得流量（单位：立方英尺/秒）求和，得到该断面的总流量测量值。通常会在断面上布置足够多的流速测点，确保每个子断面承担的流量不超过总流量的10%，但高流量工况下可能无法达到这一标准。每一次总流量测量值与同步获取的水位读数共同构成水位-流量率定曲线的一个数据点。随着时间推移，针对一系列水位值开展流量测量，即可构建完整、准确且时效性强的水位-流量率定曲线（图表）。
USGS野外作业人员约每六周对所有站点进行一次巡检，内容包括数据下载、设备检查与校准、低流量测量以校验水位-流量率定曲线是否发生偏移，以及观察河道断面是否存在影响高流量率定的潜在变化。指定一名USGS水文专员负责BES站点的相关工作，以保障工作连续性并最大化对站点的熟悉程度。USGS的巡检工作由BES野外作业人员的每周巡检予以补充，后者在美国林务局水文专员的指导下开展多项检查工作（详见下文第二部分：水文测站校准与流量率定质量保证与质量控制：每周巡检）。
高流速无法在河道内直接测量，需由USGS作业人员在测站附近的桥梁上开展测量。在径流事件期间需重复开展流量测量，直至明确率定曲线的高流量段。无法直接测量的极端高流量，可采用比降-面积曲线法进行估算：该方法依托断面几何形态、河道糙率，以及通过洪水退去后遗留碎屑所标记的水力梯度线高点的实测高程完成计算。
大型流域的数据记录间隔为15分钟，小型流域则为5分钟。数据通过电子方式下载至USGS办公室进行处理，针对设备校准问题、回水修正（因碎屑与冰堵导致）、数据缺失、率定曲线偏移等情况进行调整。水位-流量率定曲线低流量段的小幅调整则采用