USGS Streamgages Linked to the Medium Resolution NHD

The locations of approximately 23,000 current and historical U.S. Geological Survey (USGS) streamgages in the United States and Puerto Rico (with the exception of Alaska) have been snapped to the medium resolution National Hydrography Dataset (NHD). The NHD contains geospatial information about mapped surface-water features, such as streams, lakes, and reservoirs, etc., creating a hydrologic network that can be used to determine what is upstream or downstream from a point of interest on the NHD network. An automated snapping process made the initial determination of the NHD location of each streamgage. These initial NHD locations were comprehensively reviewed by local USGS personnel to ensure that streamgages were snapped to the correct NHD reaches. About 75 percent of the streamgages snapped to the appropriate NHD reach location initially and 25 percent required adjustment and relocation. This process resulted in approximately 23,000 gages being successfully snapped to the NHD.

This dataset contains the latitude and longitude coordinates of the point on the NHD to which the streamgage is snapped and the location of the gage house for each streamgage. A process known as indexing may be used to create reference points (event tables) to the NHD reaches, expressed as a reach code and measure (distance along the reach). Indexing is dependent on the version of NHD to which the indexing is referenced. These data are well suited for use in indexing because nearly all the streamgage NHD locations have been reviewed and adjusted if necessary, to ensure they will index to the appropriate NHD reach.

Flow characteristics were computed from the daily streamflow data recorded at each streamgage for the period of record. The flow characteristics associated with each streamgage include:

First date (year, month, day) of streamflow data
Last date (year, month, day) of streamflow data
Number of days of streamflow data
Number of days of non-zero streamflow data
Minimum and maximum daily flow for the period of record (cubic feet per second)
Percentiles (1, 5, 10, 20, 25, 50, 75, 80, 90, 95, 99) of daily flow for the period of record (cubic feet per second)
Average and standard deviation of daily flow for the period of record (cubic feet per second)
Mean annual base-flow index (BFI) computed for the period of record (fraction, ranging from 0 to 1)
Year-to-year standard deviation of the annual base-flow index computed for the period of record (fraction)
Number of years of data used to compute the base-flow index (years)

The streamflow data used to compute flow characteristics were copied from the NWIS-Web historical daily discharge archive (http://waterdata.usgs.gov/nwis/sw) on June 15, 2005.

美国地质调查局（U.S. Geological Survey, USGS）旗下约23000个当前及历史溪流流量监测站（streamgage）的位置，覆盖美国本土与波多黎各（阿拉斯加除外），已吸附匹配至中等分辨率的国家水文数据集（National Hydrography Dataset, NHD）。国家水文数据集（NHD）包含已测绘的地表水体要素（如溪流、湖泊、水库等）的地理空间信息，构建了可用于判定NHD网络上任意关注点上下游关系的水文网络。

最初通过自动化吸附匹配流程确定每个溪流流量监测站在NHD中的位置，随后由USGS当地工作人员对这些初始匹配位置开展全面审核，确保监测站被正确吸附匹配至NHD河段。初始匹配阶段约75%的监测站已准确匹配至对应NHD河段，剩余25%则需要进行调整并重新匹配。最终约23000个监测站均成功吸附匹配至NHD网络。

本数据集包含每个溪流流量监测站吸附匹配至NHD的点位经纬度坐标，以及监测站房的位置。可通过一种名为“索引”的流程生成指向NHD河段的参考点（事件表），其表示形式为河段代码与沿河段的距离测量值。索引流程依赖于所引用的NHD版本。由于几乎所有溪流流量监测站的NHD位置均经过审核并在必要时完成调整，因此本数据集非常适合用于索引流程，可确保匹配至正确的NHD河段。

基于各溪流流量监测站在观测期内记录的日径流数据，计算得到对应的流量特征参数。每个监测站关联的流量特征包括：
1. 径流数据的起始日期（年、月、日）
2. 径流数据的终止日期（年、月、日）
3. 径流数据总天数
4. 非零径流量天数
5. 观测期内日径流量的最小值与最大值（单位：立方英尺每秒）
6. 观测期内日径流量的百分位值（1、5、10、20、25、50、75、80、90、95、99，单位：立方英尺每秒）
7. 观测期内日径流量的平均值与标准差（单位：立方英尺每秒）
8. 观测期内计算得到的年均基流指数（Base-Flow Index, BFI，取值范围为0至1的分数值）
9. 观测期内计算得到的年基流指数的逐年标准差（分数值）
10. 用于计算基流指数的有效数据年数（单位：年）

用于计算流量特征的径流数据，于2005年6月15日从NWIS-Web历史日径流档案（http://waterdata.usgs.gov/nwis/sw）中拷贝获取。