Global Watersheds

A new data set of watersheds and river networks is available, being derived primarily from the TerrainBase Global DTM (Digital Terrain Model) with additional information from the CIA World Data Bank II. These data are useful in hydrologic modeling, and can serve as a basemap for routing continental runoff to the appropriate coast, and therefore into the appropriate ocean or inland sea in a variety of methods. Using this data set, the runoff produced in any grid cell, when coupled with a routing algorithm, can easily be transported to the appropriate water body and distributed across that water body as desired. The data set includes watershed and flow direction information, as well as supporting hydrologic data at 5 minute, 1/2 degree, and 1 degree resolutions globally.

The dataset is composed of nine spatial layers, each delineated at 3 resolutions. These layers include:

Land/Sea Mask -- Determined from TerrainBase DTM elevation data after conversion to GIS format. Some manual correction was also performed in land areas with elevations below sea level.

Flow Direction Data -- Derived from the filled digital elevation model produced from the land/sea mask and the TerrainBase DTM using GIS. Elevation data conditioned before filling by burning in rivers from the CIA World Data Bank II with subsequent manual corrections for discrepancies between model coastlines. Flow direction data in Antarctica altered to ensure valid flow direction information.

Flow Accumulation Data -- Derived from the flow direction data in GIS after correction.

Rivers Delineation -- Created from flow accumulation data based on threshold values using GIS. Greenland and Antarctica removed from river delineation.

55 Large Watersheds Delineation -- Derived from flow direction and flow accumulation data using GIS. Watershed selection conducted as combination of largest watersheds and those rivers suggested by Russell and Miller [1990] for use in global climate modeling studies.

Internally Draining Regions -- Defined as those large regions of Africa, central Eurasia, and Australia which were internally draining. Derived using GIS based on the original elevation data and closed depressions.

19 Large-Scale Drainage Regions -- Derived as watersheds from the flow direction data in conjunction with coastal cells in GIS. The 19 basins were originally selected as land-water body pairings for use in the National Center for Atmospheric Research (NCAR) Climate System Model (CSM).

19 Large-Scale Drainage Regions Including Water Bodies -- 19 large-scale drainage regions including water bodies were derived by computing nearest neighbor data values for grid cells without data values in GIS. Artificial divisions have been included in some areas as they are common geographic modeling divisions, or otherwise create a better separation of drainage regions.

Lakes Delineation -- Created in GIS by gridding water bodies from the CIA World Data Bank II.

Runoff Data -- Runoff data was taken from Perry et al. [1996] and UNESCO [1974] for the 55 rivers selected in this study. ASCII data file data (tabular) with associated geographic and political reference information.

Technical reports describing data analysis and integration are online at [http://www.ngdc.noaa.gov/seg/cdroms/graham/graham/graham.htm].

本数据集为全新的流域与河网数据集，主要基于TerrainBase全球数字地形模型（Digital Terrain Model, DTM）构建，额外补充了美国中央情报局（CIA）世界数据库II（CIA World Data Bank II）的相关信息。该数据集可应用于水文模拟，亦可作为底图，通过多种方法将大陆径流汇至对应海岸，进而注入相应的大洋或内陆海域。依托本数据集，任意网格单元产生的径流可结合汇流算法，便捷地输送至对应水体，并可按需在该水体范围内进行扩散。本数据集包含流域与流向信息，以及全球范围内5弧分、0.5度和1度三种分辨率的配套水文数据。

本数据集包含9个空间图层，每个图层均覆盖上述三种分辨率，具体图层如下：
- 陆地海洋掩码：基于TerrainBase DTM高程数据转换为地理信息系统（Geographic Information System, GIS）格式后生成，并针对海拔低于海平面的陆地区域进行了人工校正。
- 流向数据：基于陆地海洋掩码与TerrainBase DTM，通过GIS工具生成填洼后的数字高程模型；在填洼前，已通过导入CIA世界数据库II的河道数据对高程数据进行河道烧录修正，并随后针对模型海岸线的偏差开展人工校正。南极洲的流向数据已进行调整，以确保其有效性。
- 汇流累积量数据：经校正后的流向数据通过GIS工具计算得到。
- 河网提取结果：基于阈值法，利用汇流累积量数据通过GIS工具生成，其中格陵兰与南极洲区域已被移除。
- 55个大型流域提取结果：基于流向与汇流累积量数据，通过GIS工具生成；流域选取综合了大型流域以及Russell与Miller[1990]提出的适用于全球气候模拟研究的河道相关流域。
- 内流区：指非洲、欧亚大陆中部以及澳大利亚的大型内流区域，基于原始高程数据与闭合洼地通过GIS工具提取得到。
- 19个大型流域分区：基于流向数据结合海岸格网单元，通过GIS工具提取得到流域；这19个流域最初是为美国国家大气研究中心（National Center for Atmospheric Research, NCAR）气候系统模型（Climate System Model, CSM）选取的陆-水体配对分区。
- 包含水体的19个大型流域分区：通过计算GIS中无数据网格单元的最近邻数据值生成；部分区域设置了人工分区，这类分区是地理建模中常见的划分方式，或可更好地实现流域分区的分离效果。
- 湖泊提取结果：通过将CIA世界数据库II的水体数据进行网格化处理，在GIS工具中生成。

径流数据：径流数据取自Perry等人[1996]与联合国教育、科学及文化组织（United Nations Educational, Scientific and Cultural Organization, UNESCO）[1974]的相关成果，对应本研究选取的55条河道；数据以ASCII格式存储为表格文件，附带相关地理与行政参考信息。

有关本数据集分析与整合的技术报告可访问以下链接获取：[http://www.ngdc.noaa.gov/seg/cdroms/graham/graham/graham.htm]