Monitoring spatio-temporal changes in wetlands with harmonized image series in Google Earth Engine

Study of rapidly changing lakes and wetlands with remote sensing methods is critical for understanding the climatic and anthropogenic effects. However, most of the studies search for the change of water body in specific time periods. Although this approach reduces the workload related to downloading and processing a large number of satellite images in computer environment, it actually causes ignoring some critical changes that occurred out of specified time periods. On the other hand, this situation reduces the data volume and the limited data causes problems for the management of water resources.  The Google Earth Engine (GEE) platform allows the opportunity to rapidly and practically process large-scale temporal data without downloading. In this study, areal changes in Lake Akşehir in Türkiye, from 1985 to 2020, were calculated and mapped by the GEE as a case. In order to calculate the changes, the Landsat 5 TM, 7 ETM + and 8 OLI&TIRS images were harmonized and created annual mosaics. The Normalized difference water index (NDWI) and the automated water extraction index (AWEI) were applied to these annual mosaics. By this approach, the change in the water area representing a shrank by 87% on average (according to the calculations 91% for the NDWI and 83% for the AWEI) from 1985 to 2020 was assessed practically and rapidly on annual mosaics created from all images between the studied period, instead of assessment based on images taken on only one date in the chosen years as in previous studies. Such an approach will provide time and labour savings and provide more meaningful and uninterrupted data for studies about changes in other wetland areas.