Contributing Area - Multiple Flow Direction (Partial) derived from 1" SRTM DEM-H

CA_MFD_PARTIAL is contributing area in m2 computed using multiple flow directions on hillslopes and ANUDEM-derived flow directions in channels. The contributing area was computed on 1 degree tiles with 200 cell (about 5 km) overlaps so the areas in channels do not account for catchments beyond that size (hence the use of PARTIAL in the name). The primary purpose of this product was to calculate topographic wetness index (TWI; Gallant and Wilson, 2000) for which full contributing areas in channels are not necessary. Do not use this product to represent contributing areas of catchments larger than 5 km across.\n\nThe CA_MFD_PARTIAL product was derived from the Hydrologically enforced Digital Elevation Model (DEM-H; ANZCW0703014615), which was derived from the 1 arc-second resolution SRTM data acquired by NASA in February 2000. \n\nThis data is available in tiled format at 1 arc-second and 3 arc-second resolution.\n\nThe 3 arc-second resolution product was generated from the 1 arc-second CA_MFD (partial) product and masked by the 3” water and ocean mask datasets.\n\nLineage: Source data\n\n1.\t1 arc-second SRTM-derived Hydrologically Enforced Digital Elevation Model (DEM-H; ANZCW0703014615).\n2.\tDEM-H Flow direction grid\n3.\t1 arc-second CA_MFD (partial) product\n4.\t3 arc-second resolution SRTM water body and ocean mask datasets\n\n\nCA_MFD_PARTIAL calculation\nCA_MFD_PARTIAL was calculated from DEM-H to provide the contributing area for the calculation of TWI, following the methods described in Gallant and Wilson (2000). It is derived from a slope-weighted multiple flow algorithm for flow accumulation, but uses the flow directions derived from the interpolation (ANUDEM) where they exist. In this case, they are the ANUDEM-derived flow directions only on the enforced stream lines, so the flow accumulation will follow the streams.\n\nThe varying area of grid cells with latitude due to the geographic projection of the data was accounted for in the calculation of contributing area.\n\nThe CA_MFD_PARTIAL calculation was performed on 1° x 1° tiles, with 200 cell (about 5 km) overlaps at tile edges. This overlap ensures that contributing area for channels with catchments up to 5 km across is correctly captured, but the area of larger catchments will not be included. This product is therefore not suitable for use where the contributing areas for larger catchments are needed. The primary purpose of this product was to support calculation of Topographic Wetness Index (TWI), which is designed to represent patterns on hillslopes and does not need full catchment contributing area in channels.\n\nA full contributing area product will be developed in the future.\n\nThe 3 arc-second resolution version was generated from the 1 arc-second CA_MFD (partial) product. This was done by aggregating the 1” data over a 3 x 3 grid cell window and taking the maximum of the nine values that contributed to each 3” output grid cell. The 3” CA_MFD data were then masked using the SRTM 3” ocean and water body datasets.\n\n\nReferences\nGallant, J.C. and Wilson, J.P. (2000) Primary topographic attributes, chapter 3 in Wilson, J.P. and Gallant, J.C. Terrain Analysis: Principles and Applications, John Wiley and Sons, New York.

CA_MFD_PARTIAL是指在坡面采用多流向法、在河道采用ANUDEM衍生流向所计算的汇水面积（单位：平方米）。该汇水面积基于1°分幅瓦片计算，瓦片边缘设置200个单元格（约5千米）的重叠区域，因此河道汇水面积未包含超出该范围的流域范畴，这也是其名称中“PARTIAL（部分）”的由来。本数据集的核心用途为计算地形湿度指数（Topographic Wetness Index, TWI; Gallant和Wilson, 2000），此类计算无需河道区域的完整汇水面积。请勿使用本数据集表征跨度超过5千米的流域汇水面积。

CA_MFD_PARTIAL数据集源自经水文修正的数字高程模型（Hydrologically enforced Digital Elevation Model, DEM-H; ANZCW0703014615），而DEM-H则由美国国家航空航天局（NASA）2000年2月获取的1弧秒分辨率SRTM数据生成。

本数据以分幅形式提供，分辨率包含1弧秒与3弧秒两种。

3弧秒分辨率的数据集由1弧秒分辨率的CA_MFD_PARTIAL数据集生成，并通过3弧秒分辨率的水体与海洋掩膜数据集进行掩膜处理。

数据溯源：源数据
1. 1弧秒SRTM衍生的水文修正数字高程模型（DEM-H; ANZCW0703014615）
2. DEM-H流向栅格
3. 1弧秒分辨率CA_MFD（部分）数据集
4. 3弧秒分辨率SRTM水体与海洋掩膜数据集

CA_MFD_PARTIAL计算方法
CA_MFD_PARTIAL遵循Gallant与Wilson（2000）所述方法，基于DEM-H生成，用于为TWI计算提供汇水面积数据。其采用坡度加权多流向算法进行流量累积，但在已有流向数据的区域（仅在修正后的河道流线处使用ANUDEM衍生的流向），则直接采用该预设流向。在此规则下，流量累积将沿河道行进。
计算过程中已考虑数据地理投影导致的栅格单元面积随纬度变化的校正。
本数据集的计算基于1°×1°分幅瓦片，瓦片边缘设置200个单元格（约5千米）的重叠区域。该重叠区域可确保跨度不超过5千米的河道流域汇水面积被正确捕获，但更大流域的汇水面积将无法被完整包含。因此本数据集不适用于需要大流域汇水面积的应用场景。本数据集的核心用途为支撑地形湿度指数（TWI）的计算——该指数旨在表征坡面地形格局，无需河道区域的完整流域汇水面积。
未来将开发完整汇水面积数据集。
3弧秒分辨率版本由1弧秒分辨率的CA_MFD_PARTIAL数据集生成：先通过3×3栅格窗口对1弧秒数据进行聚合，取每个3弧秒输出栅格对应9个输入值的最大值，再通过SRTM 3弧秒分辨率海洋与水体数据集对生成的3弧秒CA_MFD数据进行掩膜处理。

参考文献
Gallant, J.C. 和 Wilson, J.P. (2000) 主要地形属性，收录于Wilson, J.P. 和 Gallant, J.C. 所著《地形分析：原理与应用》（Terrain Analysis: Principles and Applications）第三章，约翰威立出版公司，纽约。