Water area timeseries

Timeseries of surface water area at river basin- and reach-scale

Using our annual dry season water, wet season water and permanent water time series rasters at 30m/pixel, we calculated water areas at river basin- and reach-scale for every year from 1991 to 2020. The time series of surface water areas that this yielded us enabled us to estimate their annual rates of changes at every one of our chosen basins and reachs.

River basins

We used the HydroSHEDS Basins dataset (Lehner et al., 2018), and chose its Level 7 basins for analysis.

An example of a basin from HydroSHEDS Basins Level 7 dataset. This is the basin of river Hemavathi in Karnataka, India.

River reaches

A reach of a river is a relatively short localized section of the river that has homogeneous characteristics, such as type of substrate, river flow, habitat, channel width, etc. Free and public datasets marking individual reaches of every major river in India are not available. In its place and as a reasonable approximation, we used transects spanning across river channels. We made our set of transects large and systematic, that finely and extensively samples the entire network of India's large rivers. We chose transect lengths and widths meaningfully, to avoid overlap and to account for generally increasing river widths as they flow from their source to delta.

An example of two neighbouring transects, at a bend of a river. These are comprised of 9 blocks of 3x3 JRC water dataset pixels. These are along the river Kaveri in Karnataka, India.

For locating the transects, we analytically created 68,368 roughly equally spaced points (~1km apart) along the length of every major river in India, using the HydroSHEDS flow accumulation data at 15-arc-sec resolution. Pixels with flow accumulation cell count value less than 5000 were ignored, because those typically represent river stretches too small to be faithfully captured given our choice of resolution for analysis. Once these transect locations were found, we combined them with HydroSHEDS flow direction data at 15-arc-sec resolution, to analytically create polygons at every transect location. We used the flow direction information to make transect polygons long, linear and in the direction perpendicular to the direction of river flow at that location.

The transects were constructed by stringing together blocks of 3x3 pixels of the JRC water dataset grid (30m x 30m), to achieve good coverage of the river reach's channel width and local surface characteristics. Since a river channel can be narrow or wide depending on how far downstream it is from the river's source, we made transects longer where the channel is likely to be wider. Transects of length 9, 11 or 13 blocks of 3x3 pixels, based on the flow accumulation at a given location, were a good fit of our analysis purposes.

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