Satellite Remote Sensing of Functional Flows to Improve Ecological Performance Metrics for the Sacramento-San Joaquin River Delta, California

Abstract:

Functional flows are a framework for measuring environmental flows of highly engineered river systems that are unlikely to return to their historic unaltered flow regimes. The Sacramento – San Joaquin River Delta in California is one such highly engineered system through weirs, levees and dams. The Yolo Bypass, a wildlife area and flood control mechanism that also provides some services of the historical river floodplain, is an important part of this river system. The Yolo Bypass is flooded when the flow in the Sacramento River rises above 1,600 m³/s and provides migratory passage for fish and habitat for migrating waterfowl. Once flooding occurs, Yolo Bypass is reconnected to the river allowing juvenile fish to enter and feed on the floodplain, which allows them grow faster and increase their chance of survival before they reach the ocean compared to moving down the main river passage. Currently, flooding into the Yolo Bypass is deemed “sufficient” for functional flows via proxy measures when flow over the Fremont Weir reaches a set target of 170 m³/s from November to March. However, only monitoring flow misses ecologically important factors such as area flooded, total days flooded, floodplain heterogeneity and connectivity, and rate of rise and fall of floodwaters. Multi-sensor satellite remote sensing can provide regular, high frequency time-series measurements allowing direct calculation of these ecologically relevant metrics. This study uses previously published methods to combine Sentinel-1 (SAR, 12-day revisit time) and Sentinel-2 (multispectral, 5-10-day revisit time) data to map surface water extent through time in the Bypass. The time series of water maps were used to create a novel annual spatio-hydrograph over the 2017-2019 to examine functional flow measures in the Yolo Bypass. When used in tandem with established flow monitoring, LiDAR-derived high resolution digital elevation models and flood inundation models, new ecological performance metrics can be derived and assessed to improve ecological outcomes of functional flows.