Dynamics of open-surface water in Guanting Reservoir basin: a Google Earth Engine platform study

Open-surface water plays an important role in social-ecological system．Dynamics of open-surface water has significant effect on variations in water resource supply and ecological integrity．Timely long-term monitoring of open-surface water would provide guidance for protection and sustainable utilization of water resources．The Google Earth Engine platform was used to extract 5196 Landsat TM, ETM+ and OLI images from 1987 to 2018, which were then processed and composed to medians per year．The two-step urban water index and slope-mask method were then used to obtain long-term dynamics of open-surface water in Guanting Reservoir basin．Validation revealed an overall accuracy of up to 93.5% with a Kappa coefficient of 0.87．Piecewise linear regression was applied to analyze the dynamics of open-surface water．The open-surface water from 1987 to 2018 experienced a slow increase, followed by a rapid decrease, and a sharp increase．The average annual changes in open-surface water area were found to be 5.50 km² during the slow increase period from 1987 to 1996, −7.45 km² during the rapid decrease period from 1997 to 2010, and 15.19 km² during the sharp increase period from 2011 to 2018．Variations in annual precipitation had major influence on the area of open-surface water, with a regression determination coefficient of 0.29 (P < 0.01)．Anthropogenic impacts, i.e., increased water consumption from both open-surface water and groundwater, and land use change associated with open-surface water, became more notable after 1997．This study provides helpful tips for water resource protection and regional sustainable development in the context of rapid urbanization．