Hydrologic Unit Code (HUC) 2

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）是一套符合国家划定标准与分辨率规范的综合性聚合水文单元数据集。其界定了地表水汇至某一单点的面状范围，但沿海或湖滨区域除外——此类区域可能存在多个出水口，具体要求遵循《国家流域边界数据集（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的水文原则划定，不偏向任何行政边界、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。部分区域还包含额外的子分区，对应14位和16位水文单元（Hydrologic Unit，HU）。最低精度方面，美国本土的水文单元勾绘比例尺为1:24000，夏威夷、太平洋流域及加勒比地区为1:25000，阿拉斯加为1:63360，所有数据均符合《国家地图精度标准》（National Map Accuracy Standards，NMAS）。在有合作方及对应数据的区域，更高分辨率的边界数据正在研发中，后续将整合回WBD数据集。WBD数据以多边形及对应边界线的数据集形式提供，边界线用于界定多边形的范围。WBD多边形属性包含水文单元代码（Hydrologic Unit Code，HUC）、面积（以英亩和平方千米为单位）、名称、下游水文单元代码、流域类型、非贡献区以及径流修正信息。HUC用于标识该水文单元在美国的位置及其层级。WBD边界线属性包含每条边界对应的最高层级水文单元、线源信息以及径流修正信息。