If this rate of loss is likely present in other wetland sites means that in 15-20 years, the world has lost wetland estuary to 364,000 square kilometers.
If the average rate is the trend of the loss of wetlands in other sources means that in 15-20 years ago. The world lost wetland estuaries 364000 square kilometers.
If this is an average rate of loss in current trends in water-logged areas, etc. This means that in the first 15 - 20 years, the world has lost its water moistened mouth of the river to go up to 364,000 square kilometers.