Constraining river streamflow determination using bathymetry and slope from ICESat-2 satellite altimetry

In order to determine river streamflow over poorly gauged basins, spaceborne techniques are widely used to obtain hydraulic parameters like river height variation (H), slope (S), river width (W), velocity (V) and river cross section (or bathymetry). Conventional radar altimetry can only provide water height. It is also difficult to measure the slope of a reach even from multi-mission altimetry, due to the problems of the simultaneity and intersatellite biases.

Laser altimetry with ICESat-2 enhances the opportunity to constrain river streamflow. The Advanced Topographic Laser Altimeter System (ATLAS) on the mission carries 3 pairs of laser transmitters (one strong and one weak beam in each pair) with photon-counting detectors. ATLAS emits 532-nm laser pulses (green light) at a 10 kHz repetition rate. It detects individual photons at 70 cm along-track separation for each shot on the earth’s surface with ~17 m diameter footprint. The very dense measurements can provide the height profile of the cross section. Since the laser penetrates water, it can potentially measure the river bathymetry, at least the nearshore part, depending on the turbidity of the water. With its off-nadir beams, the system is able to provide the slope of a reach.

In our study, we process the point cloud of the river cross section and extract the nearshore river bathymetry. The slope is determined by three strong beams of one track. We also analyze the maximum depth of bathymetry in different seasons and different turbidity conditions. The water height of each beam is obtained from measurement of the center of the river (centerline from SWOT River Database), and validated with measurements of the weak beams.