National Watershed Boundary Dataset (WBD): Hydrologic Unit Code 6-Digit Basins of Washington State

The Watershed Boundary Dataset (WBD) is a comprehensive aggregated collection of hydrologic unit data consistent with the national criteria for delineation and resolution. It defines the areal extent of surface water drainage to a point except in coastal or lake front areas where there could be multiple outlets as stated by the "Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)" “Standard” (http://pubs.usgs.gov/tm/11/a3/). Watershed boundaries are determined solely upon science-based hydrologic principles, not favoring any administrative boundaries or special projects, nor particular program or agency. This dataset represents the hydrologic unit boundaries to the 12-digit (6th level) for the entire United States. Some areas may also include additional subdivisions representing the 14- and 16-digit hydrologic unit (HU). At a minimum, the HUs are delineated at 1:24,000-scale in the conterminous United States, 1:25,000-scale in Hawaii, Pacific basin and the Caribbean, and 1:63,360-scale in Alaska, meeting the National Map Accuracy Standards (NMAS). Higher resolution boundaries are being developed where partners and data exist and will be incorporated back into the WBD. WBD data are delivered as a dataset of polygons and corresponding lines that define the boundary of the polygon. WBD polygon attributes include hydrologic unit codes (HUC), size (in the form of acres and square kilometers), name, downstream hydrologic unit code, type of watershed, non-contributing areas, and flow modifications. The HUC describes where the unit is in the country and the level of the unit. WBD line attributes contain the highest level of hydrologic unit for each boundary, line source information and flow modifications.流域边界数据集（Watershed Boundary Dataset, WBD）是一套符合国家级区划与分辨率标准的综合性汇整水文单元数据集合。其界定了地表水汇流至某一节点的区域范围，但沿海或湖滨区域除外——此类区域可存在多个出水口，具体遵循《国家流域边界数据集（Watershed Boundary Dataset, WBD）联邦标准与规程》中的“标准”条款（http://pubs.usgs.gov/tm/11/a3/）。流域边界仅基于科学水文原则划定，不偏袒任何行政边界、专项工程、特定项目或机构。本数据集覆盖全美范围的12位码（第6级）水文单元边界。部分区域还包含代表14位码与16位码水文单元（Hydrologic Unit, HU）的额外细分区划。最低精度要求下，美国本土48州的水文单元采用1:24000比例尺划绘，夏威夷、太平洋流域及加勒比地区采用1:25000比例尺，阿拉斯加州采用1:63360比例尺，所有划绘均符合《国家地图精度标准》（National Map Accuracy Standards, NMAS）。在具备合作方与对应数据的区域，更高分辨率的边界正处于开发阶段，后续将整合汇入WBD数据集。WBD数据以多边形及定义多边形边界的对应线要素数据集形式提供。WBD多边形要素的属性字段包含水文单元代码（Hydrologic Unit Code, HUC）、面积（以英亩与平方千米为单位）、名称、下游水文单元代码、流域类型、非汇水区以及径流修正信息。水文单元代码可用于标识该单元在美国的地理位置以及所属层级。WBD线要素的属性字段包含每条边界对应的最高层级水文单元、线要素来源信息以及径流修正信息。