Satellite and Airborne Altimetry for Surface Water Monitoring at Local to Regional Scales

Climate changes and human activities profoundly influence the storage and distribution of surface waters. Understanding surface water dynamics, characteristics, and impacts require comprehensive monitoring data sets. However, the Earth’s surface monitoring networks are sparse because of large distances, limited access, and low population densities in remote areas. Remote sensing techniques have led to large datasets at multi-temporal and spatial scales, becoming a critical supplement to Earth’s surface observations. Specifically, satellite and airborne altimetry provide a repetitive remote sensing approach for detecting water surface elevation (WSE) of inland water bodies at different scales.

Hydrological applications of satellite altimetry depend on its accuracy, sampling frequency, and availability. Several space-borne altimeters operate with advanced techniques, such as Synthetic Aperture Radar (SAR) and multiple laser beams, which have been validated to be useful in inland water monitoring. The quality and quantity of the measurements of surface waters have been considerably enhanced. Additionally, airborne altimetry provides flexible and detailed measurements of water bodies for local fluvial systems. Using satellite and airborne altimetry to monitor surface waters of different scales and to cope with complex hydrological problems is the primary motivation of this thesis.

This thesis intends to use data from currently operational satellites, such as Sentinel-3 A/B and Ice, Cloud and land Elevation Satellite-2 (ICESat-2), and airborne altimetry to monitor water surfaces at various scales ranging from small streams with a few meters in width to large rivers with kilometers in width and large reservoirs. Moreover, the altimetry datasets were combined with other satellite products, in-situ observations, and models to analyze complex hydrological problems covering different scales.

Unmanned Aircraft Systems (UAS) altimetry has high accuracy (approximately 3 cm) and spatial resolution (a few centimeters) over small rivers. Thus, it was used to measure WSE for a small stream, i.e., the Vejle River in Denmark. The longitudinal WSE profile of the Vejle River was clearly depicted by UAS altimetry. Subsequently, UAS altimetry and hydrodynamic models were combined to determine the Strickler coefficient (Ks), predominantly controlled by the presence and character of submerged vegetation. The calibrated Ks showed strong seasonal and longitudinal variations due to the vegetation growth cycle, variable vegetation density, and species composition. This case showed how UAS altimetry and hydrodynamic modeling could be combined to improve the understanding and parameterization of small rivers.

For global medium streams and large rivers, satellite altimetry provides measurements of WSE at Virtual Stations (VS), which is the intersection between satellite ground tracks and river sections. River discharge can be estimated with stage-discharge (SQ) rating curves at VS. A hydrodynamic model was used to simulate stage, WSF, and discharge at six backwater-affected reaches and generate SQ/SFQ rating curves everywhere along the reaches. The simulated rating curves indicated that SQ rating curves were inadequate for estimating discharge, and the uncertainties were high, which could be reduced by SFQ rating curves. The potential of using SFQ rating curves and ICESat-2 to estimate dis-charge was validated. This case shows the possibilities of ICESat-2 laser altimetry for EO-based discharge estimation.

At a regional scale, water storage dynamics on the North China Plain (NCP), which faces severe water scarcity, were investigated. Sentinel-3 A/B altimetry was used to estimate the volume of surface water storage (SWS). The accuracy of Sentinel-3 A/B is approximately 20 cm over large reservoirs in NCP. Combined with other earth observation datasets, total water storage (TWS) and its components were estimated. SWS shows an increasing trend after implementing large inter-basin water transfer projects. We further estimated groundwater storage variations by subtracting the SWS and soil water storage from the TWS and found that the inter-basin water transfer projects altered the decline trends of groundwater storage in part of the region. This case shows how satellite altimetry and other Earth Observations (EO) can be combined to monitor water resources at a regional scale.

The results of this study highlight the versatility of satellite and airborne altimetry at different scales. Using advanced remote sensing technologies, the operational satellite altimetry missions can provide WSE with high quality and quantity, which offer new opportunities for surface water monitoring and modelling. In addition, UAS altimetry supplements small-scale studies where satellite altimetry is inaccessible.