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. Hydrometric monitoring of smallwater bodies (1–10  ha ) remains rare, due to their limited size andlarge numbers, preventing accurate assessments of their agriculturalpotential or their cumulative influence in watershed hydrology. Landsatimagery has shown its potential to support mapping of small water bodies, butthe influence of their limited surface areas, vegetation growth, and rapidflood dynamics on long-term surface water monitoring remains unquantified. Asemi-automated method is developed here to assess andoptimize the potential ofmulti-sensor Landsat time series to monitor surface water extent and meanwater availability in these small water bodies. Extensive hydrometric fielddata (1999–2014) for seven small reservoirs within the Merguellil catchmentin central Tunisia and SPOT imagery are used to calibrate the method andexplore its limits. The Modified Normalised Difference Water Index (MNDWI) is shown out of sixcommonly used water detection indices to provide high overall accuracy andthreshold stability during high and low floods, leading to a mean surfacearea error below 15 %. Applied to 546 Landsat 5, 7, and 8 images over1999–2014, the method reproduces surface water extent variations acrosssmall lakes with high skill ( R2=0.9 ) and a mean root mean square error(RMSE) of 9300  m2 . Comparison with published global water datasetsreveals a mean RMSE of 21 800  m2 ( +134  %) on the same lakesand highlights the value of a tailored MNDWI approach to improve hydrologicalmonitoring in small lakes and reduce omission errors of flooded vegetation.The rise in relative errors due to the larger proportion and influence ofmixed pixels restricts surface water monitoring below 3  ha withLandsat (Normalised RMSE  =  27 %). Interferences fromclouds and scan line corrector failure on ETM + after 2003 also decrease thenumber of operational images by 51 %, reducing performance on lakes withrapid flood declines. Combining Landsat observations with 10  m pansharpened Sentinel-2 imagery further reduces RMSE to 5200  m2 ,displaying the increased opportunities for surface water monitoring in smallwater bodies after 2015.

Surface water monitoring in small water bodies: potential and limits of multi-sensor Landsat time series