Flood Dynamics in the Sacramento Valley over the Last 30 Years: When and Where Were the Rest Stops for Shorebirds during Spring Migration?

Abstract:

Wetland dependent migratory shorebirds continue to suffer population declines, largely due to loss of habitat. A better understanding of the variability of flooded resting and feeding sites is needed over the large areas traveled by these birds, particularly in inland areas subject to cyclical droughts. We analyzed habitat availability patterns in the Sacramento Valley, an internationally important shorebird stopover site, using low cloud cover scenes during spring migration (February – June) from the 30-year Landsat Climate Data Record. We found that a simple band 5 threshold efficiently delineated open water over the past thirty years of surface reflectance data (Landsat 4-8). A random forest model for flooded and non-flooded regions (AUC = 0.9997444) and ROC optimization were used to determine the best threshold for the analysis. Using the time series of classifications, we assessed historical flooded habitat availability within each two week period during spring over the past 30 years. Flood irrigated agriculture on private lands represents the largest potential habitat area and consistently provides habitat at the early and late stages of migration. Rice alone represents five times the area of herbaceous wetlands and has a comparable probability of inundation as this wetland type in early spring (0.3). Flood extent is most restricted during the peak of spring migration when birds most need the habitat, which is limited primarily to federal and state managed lands (average 0.78 probability of inundation). The timing of drying and reflooding in agricultural fields could strongly influence habitat availability in the valley during the spring.