Attribution of changes in annual runoff to vegetation change and climate change in a tropical catchment using a data-based approach

The ‘forest sponge effect’ hypothesis states that deforestation results in lower runoff during the dry season and higher runoff during the wet season, and vice versa. However, there are still contradicting results regarding this hypothesis. The objective of this research is to contribute to the validation of this hypothesis. The study area is the Merawu catchment (218.6 km2) located in central Java, Indonesia. We used a time series of the normalized difference vegetation index, mean annual temperature, mean annual potential and actual evapotranspiration, and the corrected mean annual rainfall and mean annual discharge of the study area from 1992 to 2014. Subsequently, a Mann-Kendall trend test and Sen's slope estimator have been used to determine the changes in all variables. The results show that climate change, in terms of an increase in precipitation, has not led to an increase in discharge due to higher water loss from the increase in mean annual actual evapotranspiration caused by higher vegetation density. Our findings show different directions of climate change and vegetation change that have decelerated the impacts on discharge, requiring further investigations.