Capturing Spatial Gradients of Water Color and Clarity in Subtropical Reservoirs During Drought

Abstract Reservoirs have spatial variation in water depth, suspended matter, and biogeochemistry that can influence patterns of water clarity and color. Spatial surveys with sensor‐equipped boats matched in time with satellite image acquisition provide data‐intensive avenues for understanding spatial patterns of optical properties within reservoirs. We combined continuous field data from high‐speed spatial surveys with Sentinel‐2 imagery to map water clarity and color in six subtropical Texas reservoirs during summer. Reservoir arms had lower clarity and longer dominant wavelengths than the main bodies, indicating higher concentrations of optical constituents. Because surveys took place during a period of low inflows associated with drought, color and clarity patterns may have been associated mainly with phytoplankton communities and resuspension of shallow sediments, rather than direct river inputs. Whole system analysis of dominant wavelength showed that five reservoirs reflected green (550–569 nm) over most of their surface area, suggesting high concentrations of phytoplankton biomass, and one reservoir in a clay‐dominated watershed was primarily yellow to brown (570–583 nm). Water clarity spanned a wide turbidity range (3.18–95.2 NTU) and was low over most of the surface area of these systems (