The Indian summer monsoon rainfall has decreased since 1950, and several hypotheses have been proposed to explain why. Most of these hypotheses involving weakening temperature gradients over the continent. This study explores the potential link between a weakening monsoon and widespread land use land cover (LULC) change from woody savanna to cropland in recent decades. Citing earlier studies, the authors note that:
Precipitation resulting from local land surface Evapotranspiration (ET) is known as recycled precipitation. Pathak et al. found that evapotranspiration from land surface vegetation plays a major role during the end of a monsoon. They observed that, during the initial phase of a monsoon, oceanic sources play a major role, and the soil is recharged with moisture. However, during the latter half of a summer monsoon (August and September), land surface ET increases as recycled precipitation increases, a pattern that is more prominent in the Ganga Basin and Northeast India. This recycled precipitation accounts for approximately 20–25% of the rainfall in North India (Ganga Basin) and Northeast India during August and September. Hence, deforestation associated with changes in LULC may affect ET and may significantly affect monsoon rainfall [Paul 2016: 1-2].
Summarizing their own study, the authors conclude:
Here, we performed a sensitivity analysis to quantify the impacts of large-scale conversion from woody savannah to crop land in India on monsoon precipitation. We found such a change results in decreased ET and subsequently a decrease in recycled precipitation leading to a decline in monsoon precipitation. This decline is similar in extent to the observed recent decadal weakening of monsoon precipitation. However, other reasons may account for this observed weakening, such as the warming of Indian Ocean SST [sea surface temperature] [Paul 2016: 5-6].
To better clarify causality of the weakening monsoon, given other potential factors, the authors propose that:
The future scope of this present work is to perform detection and attribution studies for potential declines of Indian monsoons with model runs forced with SST warming only, aerosol forcing only, LULC changes only and all controlling factors together [Paul 2016: 6].
Paul, Supantha, et al., 2016, Weakening of Indian summer monsoon rainfall due to changes in land use land cover, Scientific Reports 6:32177, https://www.nature.com/articles/srep32177.