Coupled effect of orography and rainfall on canopy heights in the Western Ghats and Meghalaya Plateau of India

Abstract

The Western Ghats and Meghalaya Plateau in India act as orographic barriers to incoming moisture from the Arabian Sea and the Bay of Bengal during the Indian Summer Monsoon (ISM), respectively, causing some of the highest global rainfall rates on their windward sides. Here, we investigate links between topographic parameters (elevation, hillslope, and relief) and rainfall distribution and the impact of the coupled topography-rainfall system on the vegetation height along these two barriers, which has not been studied previously. We use satellite-retrieved (TRMM) and gauge-interpolated (IMD) rainfall datasets along with spaceborne-LiDAR derived canopy height measurements. In both regions, the occurrence of orography-induced peak rainfall does not co-locate with the highest elevation; rather, rainfall peaks are usually at ∼750 m (±250 m) altitude for both regions, being situated about 25–30 km windward of their elevation peaks. The forced orographic lifting at these elevations is sufficient to create rainfall maxima. In the Western Ghats, rainfall peaks correlate with high topographic relief, but this relation varies in magnitude along the strike from south to north. Local mountain passes (Thalghat and Palghat Gaps) in the Western Ghats allow moisture movement further inland. The elevated central Meghalaya Plateau shows a stronger orographic control that induces higher rainfall. Rainfall distribution is primarily determined by these orographic barriers and their orientation to the prevailing wind direction. Topography-induced rainfall gradients significantly influence the canopy height and cause higher canopy clusters around rainfall peaks. We also delineate distinct homogenous zones to indicate the different topoclimatic-vegetation responses within these two regions.