Streamflow and Stream Chemistry - Small Watersheds - Location, Photographs, Equipment - Small Watershed 3. Dead Run at Franklintown. BES ID 536-

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：Gary Fisher，美国地质调查局（U.S. Geological Survey, USGS）（负责溪流流量监测）；Ed Doheny，美国地质调查局（U.S. Geological Survey, USGS）（负责溪流流量监测）；Ken Belt，美国林务局（U.S. Forest Service）（负责每周站点巡检）

I. 美国地质调查局溪流测站流量数据采集
BES溪流化学样品采集于由美国地质调查局建设维护的测流站点，此类站点主要由长期生态研究（Long-Term Ecological Research, LTER）基金资助，但部分站点的资金来源包括美国地质调查局、马里兰州环境部、巴尔的摩县DEPRM以及巴尔的摩市DPW。

各测站的水位测量精度可达约0.01英尺，采用浮子式水位计或压力传感器以15分钟（或更短）的间隔进行记录。上述水位数据将通过水位-流量关系（stage-discharge relationships，详见下文）转换为流量数据。洪峰水位记录仪（crest-stage recorders，浮塞式水位计floating cork gages）可标记高水位线，在洪水导致记录水位计失效时，作为高水位数据的备份来源。每个测站均会安装静水井（stilling wells），可设置于河道内或通过管道连接至河道，为水位传感提供水流平稳的测量环境。

通用流量测量与流量率定方法
在流量（单位时间的体积流量）测量中，作业人员将使用涉水杆（wading rod）与螺旋桨式流速仪（propeller current meter，通常为微型（侏儒型）流速计mini (pygmy) velocity meter）在测站附近的测流断面上开展多点流速测量。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），再将各子断面的流量（单位：立方英尺每秒）求和，得到该测流断面的总流量测量值。通常会在断面上布设足够多的流速测点，使每个子断面的流量占总流量的比例不超过10%，但高流量工况下可能无法满足该要求。每一次总流量测量结果与同步的水位读数共同构成水位-流量率定曲线（stage-discharge rating curve）的一个数据点。随着时间推移，针对一系列水位开展流量测量，即可构建完整、准确且时效性强的水位-流量率定曲线（图表）。

美国地质调查局野外作业团队每六周到访所有站点一次，完成数据下载、设备检查与校准，并开展低流量测量以校验水位-流量率定曲线是否发生偏移，同时观测测流断面是否出现影响高流量率定的潜在变化。指派一名美国地质调查局水文专家负责BES站点，以保障工作连续性并加深对站点的熟悉程度。美国地质调查局的上述巡检工作，将由BES野外作业团队在林务局水文专家的指导下开展的每周巡检予以补充（详见下文第二部分：溪流测站校准与流量率定质量控制与质量保证（Quality Assurance and Quality Control, QAQC）：每周巡检）。

高流速无法在河道内直接测量，美国地质调查局作业团队将通过测站附近的桥梁开展流速测量。在径流事件期间，需反复开展流量测量，直至确定率定曲线的高流量段。对于无法直接测量的极端高流量，可采用坡度-面积曲线法进行估算，该方法基于测流断面几何形态、河道糙率以及由退水曲线（hydrograph）遗留碎屑所指示的水力坡线（hydraulic grade line）高点的实测高程。

大型流域的数据采集间隔为15分钟，小型流域则为5分钟。数据通过电子方式下载并在美国地质调查局办公室进行处理，针对校准问题、回水修正（backwater corrections，由碎屑与冰堵引发）、数据缺失、率定曲线偏移等情况开展调整。水位-流量率定曲线低流量段的小幅调整可采用