## 4 HydroSAR Product Portfolio and Product Algorithms
### Intermediary Data Products and Visual Aids  
#### 4.2.1 Height above Nearest Drainage  
HAND is a terrain descriptor calculated from a hydrologically coherent DEM [37]. The HAND model normalizes topography based on the local relative heights observed along a drainage network (Figure 4). In the HAND grid, each pixel value indicates the vertical distance between a specific location and its nearest drainage stream. HAND can be calculated for individual hydrological basins such as HydroSHEDS [35] under the assumption that each basin is self-contained. HAND can also be calculated independent of the hydrological basin information in a tiled approach when calculating over large regions. In this case, a buffer is used to eliminate any edge effects due to tiling.  
Regardless of the domain, HAND calculation is performed the same way. First, the provided input DEM is conditioned to be hydrologically correct. This is achieved by filling in depressions, where a cell region has a much lower value than its surrounding neighbors. We then resolve the flat areas to identify drainage basins. With the drainage basins identified, the flow directions can be calculated, allowing for the generation of routing patterns, which are used to calculate the total accumulations showing the number of pixels draining into one another. At the final step, these accumulation values are used to calculate the HAND using an accumulation threshold and eight-directional flow, also known as D8. The accumulation threshold determines the drainage mask based on the total accumulation for a given cell, directly impacting the output. Figure 4 illustrates the relationship between the source DEM and the HAND product. Upon comparing the heights in Figure 4c, it becomes evident that the slope and trend of the DEM were removed from the HAND product.  
To determine an optimal accumulation threshold, we generated HAND solutions over the same area with various accumulation thresholds between 60 and 3500 pixels, demonstrating that the smaller the accumulation threshold, the more detailed the drainage network became. As the accumulation threshold became smaller, the river networks became more dendritic at finer scales. In order to create a globally uniform HAND, we selected an accumulation threshold of 100 pixels (corresponding to an area of about 9 ha), as this threshold allowed maintaining a close correspondence between the HAND-derived and HydroSHEDS-mapped drainage network. Our accumulation threshold was also close to  
Figure 4: An example of topographic heights and the HAND model: (**a**) Copernicus GLO\(-\)30 DEM; (**b**) HAND model height generated from the DEM; and (**c**) height comparison between the source DEM and the HAND model along a transect, indicated by the dashed white line in the sub\(-\)figures (**a,b**).  
the 10 ha threshold used in [38]. The HAND was then calculated using this threshold and by leveraging the psysheds library [39].  
Due to the large spatial extent of the HKH, we used a tiling scheme to calculate the HAND across our area of interest [39]. We started by defining a tiling scheme composed of \(1\times 1\) degree latitude and longitude chunks. Then, our strategy consisted of the following three steps. (1) We first buffered each \(1\times 1\) degree tile by 0.5 degrees, resulting in a \(2\times 2\) degree buffered tile with 50% overlap between neighboring tiles. We found that this 50% overlap minimized the basin boundary clipping effects and resulted in an HAND product without measurable seams at the tile boundaries. (2) We retrieved the GLO-30 DEM information for each buffered tile and computed the HAND across the \(2\times 2\) degree area. (3) Finally, we cropped the 0.5 degree overlap areas from the calculated HAND product and merge the cropped \(1\times 1\) degree tiles, resulting in a final seamless HAND product.