Using ECOSTRESS to Observe Diurnal Variability in Water Temperature Conditions in the San Francisco Estuary

Abstract—The San Francisco Estuary and Sacramento-SanJoaquin River Delta (Bay Delta) is a highly sensitive and criticalhabitat for the Delta Smelt, an endangered endemic fish, withwater temperature being a key determinant of habitat suitability.This study investigates the relationship between open watersurface and subsurface conditions from spaceborne thermal measurements(ECOSTRESS and Landsat-8) and in situ sensor datafrom the California Data Exchange Center (CDEC), respectively,to produce estimates of spatially continuous bulk temperaturein the Bay Delta. We found that ECOSTRESS and Landsat-8 surface temperature measurements are well-correlated withbulk water temperatures (N = 236, r = 0.907 and N = 226, r= 0.976, respectively). For the ECOSTRESS-in situ comparison,accounting for time of day improved the correlation betweensurface and subsurface conditions (r = 0.946, 0.881, and 0.944for morning, midday and evening, respectively). We found thatECOSTRESS surface temperatures were warmer than bulktemperatures in the midday period (2 C peak, at 2PM) andcooler in the morning and evening periods (-1 C peak, at6AM). We also found that a simple harmonic regression modelcan capture the diurnal variability of the skin effect to predictbulk water temperature (RMSE = 0.809 C). With ECOSTRESS,we found that across the Bay Delta, including open waters andpelagic bays, temperature conditions causing stress and mortalityfor the Delta Smelt were persistent throughout the day duringsummer months. ECOSTRESS is a unique dataset capable ofinforming conservation efforts in the Bay Delta.