A neural network classification framework for monthly and high spatial resolution surface water mapping in the Qinghai-Tibet plateau from Landsat observations

The Qinghai-Tibet plateau, known as the Asian Water Tower, has a significant area of water bodies that provide a wide range of valuable ecosystem services. In the context of climate change, the formation condition of surface water and water extent is changing fast. Thus, there is a critical need for monthly detection algorithms at high spatial resolution (~30 m) with good accuracy. Multiple sensors observations are available but producing reliable long time series surface water mapping at a sub-annual temporal frequency still remains a challenge, mainly due to data limitations. In this study, we proposed a neural network-based monthly surface water classification framework relying on Landsat 5/7/8 images in 2000-2020 and topographic indices, and retrieved monthly water mask for the year 2020. The surface water was mainly distributed in the central and western parts of the plateau and the maximum area of permanent surface water (water frequency > 60%) was 26.66*103 km2 in 2020. The overall, producer and user accuracies of our surface water map were 0.96, 0.94 and 0.98, respectively; and the kappa coefficient reached 0.90, demonstrating a better performance than existing products (i.e. JRC Monthly Water History with overall accuracy 0.94, producer accuracy 0.89, user accuracy 0.99, and kappa coefficient 0.89). Our framework efficiently solved the problem of missing data in Landsat images referring to the JRC and priori information and performed well in dealing with ice/snow cover issues. We showed that higher uncertainties exist on wetlands and recommended exploring relationships between water and wetlands in the future.