Total Basin Run Off (ESA POLAR+ 4D Greenland Experimental Dataset)

The total monthly runoff (2017-2021), both surface and basal, of five drainage basins in Greenland; NEGIS, Northern Lakes, Southern Lakes, Store, and Watson. The run off is estimated by summing the Surface Melt, Firn Retention, Supraglacial Change and Subglacial Runoff into monthly basin scale estimates.<br> 1. Surface water was computed from the chosen RCM on a monthly basis by summing the surface melt and the liquid precipitation (rain fall).<br> 2. Firn Retention is computed by subtracting the RCM runoff from the RCM surface melt in the snow/firn-covered part of the drainage basins<br> 3. Supraglacial change volume estimates were determined via the following methodology by Lancaster University.<br> Supraglacial lake extent were delineated using RT algorithm to classify water within S2 images. Lake depths were calculated using a radiative transfer model (section 2.2.6.1 ATBD). Lake depth was determined using Sentinel 2 red band imagery (Band 4). Total lake volume was calculated by multiplying lake depths by lake area for each individual lake polygon and summing over the basin area.<br> Volume uncertainty was determined via comparison with ICESat-2 transects. The uncertainty is represented by the maximum percentage difference between lake depths integrated over ICESat-2 transects and the corresponding S2 derived lake depth transect. For the red band, it was found that lake volumes were being underestimated by up to 33.5%.<br> Some of the meltwater in the supraglacial lakes will not leave the ice sheet at the end of the melt season but remain frozen over the winter. It is very difficult to estimate how much. In this experimental dataset we assume that it is 25% of the observed September water volume that is frozen and stored during winter.<br> 4. Subglacial meltwater flux maps were computed Greenland-wide, and integrated over the drainage basins of interest to produce monthly estimates of basal runoff.<br> Please read the README_TotalRunOff.txt

格陵兰境内5个流域（NEGIS、Northern Lakes、Southern Lakes、Store及Watson）2017至2021年的月均总径流量（涵盖地表径流与基底径流）。该径流量通过将地表融水（Surface Melt）、冰芯层滞留量（Firn Retention）、冰面径流变化（Supraglacial Change）与冰下径流（Subglacial Runoff）求和，得到流域尺度的月度估算值。<br>1. 地表水量基于选定的区域气候模型（Regional Climate Model，RCM），通过累加地表融水与液态降水（降雨）进行月度计算。<br>2. 冰芯层滞留量通过在流域积雪/冰芯覆盖区域，用区域气候模型的地表融水量减去其估算的径流量计算得到。<br>3. 冰面径流变化体积估算由兰卡斯特大学采用下述方法完成：冰面湖范围通过RT算法（RT algorithm）对哨兵2号（Sentinel 2）影像中的水体分类勾勒得到；湖深通过辐射传输模型（radiative transfer model）计算（详见章节2.2.6.1 分析与算法理论基础文档（Analysis/Algorithm Theoretical Basis Document，ATBD）），其中湖深由哨兵2号红波段影像（Band 4，第4波段）反演获取；针对每个独立湖体多边形，以湖深乘以湖面积得到单湖体积，再对全流域范围求和得到总湖体积。体积不确定性通过与ICESat-2卫星（ICESat-2）断面数据对比确定：以ICESat-2断面积分得到的湖深与哨兵2号反演的对应湖深断面之间的最大百分比差值表征不确定性。研究发现，红波段反演得到的湖体积最高被低估33.5%。部分冰面湖中的融水在融季结束后不会离开冰盖，而是在冬季冻结留存，目前难以准确估算其占比，本实验数据集假设该部分水量为9月观测水体体积的25%，并在冬季以冻结形式留存。<br>4. 冰下融水通量图以全格陵兰为范围计算，再针对目标流域进行积分，得到基底径流的月度估算值。<br>请参阅README_TotalRunOff.txt 文件