Isotopic signatures of moisture recycling and evaporation processes along the Western Ghats orography

Abstract

The water isotopic composition (δ18O and δ2H) of humid tropical mountain belts is influenced by the limited isotopic elevation effect due to the narrow temperature gradient, high relative humidity, and the moisture feedback processes. The Western Ghats mountain forest ecosystem, recognized as a biological hotspot, contributes significantly to characterizing the hydrological and weather patterns over its windward and leeward sides. The Indian Summer Monsoon southwesterly winds advancing towards the Indian subcontinent through the Arabian Sea between June to September are obstructed by this mountain belt and get orographically uplifted, producing heavy rainfall. The present study intends to improve understanding of the variation of precipitation isotopic composition along multiple stations located at different elevations of the heavily regulated Periyar river basin (PRB), southern Western Ghats, and a coastal station (TRV). The δ18O variation with elevation across PRB reveals an abrupt depletion at the highland station and a pseudo-elevation effect due to the supply of evaporated local moisture from reservoirs and lakes. The contribution of recycled precipitation estimated using back trajectories suggests a maximum of 8.5% of locally recycled rainfall over midland station towards the September month, the daily contribution reaching a maximum of 50.7%. Though this approach did not show any signatures of moisture recycling in the highland station, the surface water (reservoirs and lakes) lc-excess (line-conditioned excess), and the d-excess (deuterium excess) values of precipitation suggest the higher rates of evaporation from large water bodies and their successive role in generating local rainfall. Statistically significant amount effects visible only along coastal stations with lower rates of precipitation and higher temperature indicate that precipitation along the Ghats region (midland and highland) is significantly controlled by orographic uplift of air moisture and the contribution from locally recycled moisture. This is further supported by the significant correlation of δ18O with regional convective processes along the Arabian Sea till midland station and further decrease towards the highland. The present study provides important information on the moisture feedback mechanism in the Western Ghats and the factors controlling the isotopic signatures over the region.