DischargeFromSpace background

Technical Background

Introduction

Passive microwave satellite sensors provide near-daily, global coverage of the Earth’s land surface. At certain microwave frequencies, these instruments can penetrate cloud cover, making them particularly well suited for river monitoring. Extending earlier efforts based on wide-area optical observations (e.g., Brakenridge et al., 2005; Van Dijk et al., 2016), data from passive microwave missions such as AMSR-E, AMSR-2, TRMM, and GPM can be used to detect and track variations in river discharge. As river discharge increases, the water-covered surface area within a river reach expands. Since open water emits less microwave radiation than surrounding land, the brightness temperature of a pixel containing both land and water decreases (Brakenridge et al., 2007, 2012). The DFO Flood Observatory exploits this relationship by examining individual pixels of approximately 10 km × 10 km and tracking their microwave emissions over time. An increasing fraction of water within a pixel leads to a reduction in observed microwave radiation, producing a signal that responds directly to changes in inundated area and indirectly to variations in discharge. These microwave-derived signals are converted to discharge values through calibration against long-term (~10-year) records from in situ gauging stations or outputs from the Water Balance Model (WBM).