A prototype algorithm for daily water hyacinth monitoring at Hartbeespoortdam, South Africa, from Sentinel-3 OLCI data

Abstract. Water HYacinth (WHY) is one of the world’s most disturbing invasive aquatic plant species, characterised by a high spatial and temporal variability. Remote sensing is a valuable approach to monitor WHY, as the dense, floating mats of vegetation can be detected using various satellite instruments, for example, OLCI on Sentinel-3. The multi-spectral instrument features only moderate spatial resolution (300 m), however, it achieves global coverage in two days, and with two instruments currently in orbit, it provides an opportunity to monitor WHY at near-daily resolution. This is crucial, considering that WHY cover patterns are highly variable due to the plants’ rapid reproduction and the influences of wind and hydrodynamics. We present the development of an algorithm for the creation of daily WHY maps by: (1) deriving WHY cover patterns from both OLCI instruments using the Normalized Difference Vegetation Index, NDVI; (2) merging the data sets into one with near-daily resolution; and (3) filling the gaps (due to missing observations or cloud interference) using a spatial-temporal interpolation scheme. We show that the gap-filling strategy leads to a consistent daily time series of WHY cover for the study region and increases the number of days with observations by 55%. A leave-one-out analysis showed that the interpolation algorithm performs well even for longer periods without observations, unless WHY cover patterns change abruptly. The presented algorithm is computationally light-weight and, although developed for Hartbeespoortdam Reservoir (South Africa), is easily adaptable for use with other water bodies. The prototype WHYmapping algorithm was used to analyse eighteen months of data (July 2022 – December 2023). In the future, long time series of daily WHY maps may provide an evaluation tool for WHY management and benefit water management strategies directly by allowing continuous monitoring of WHY.