Isotopic study on the effect of reservoirs and drought on water cycle dynamics in the tropical Periyar basin draining the slopes of Western Ghats

Apart from extreme hydrological events, such as, droughts and floods; large water bodies and land use patterns can significantly influence the water cycle dynamics in river basins. Assessment of cumulative effects of extreme events, water bodies and land use pattern on the hydrological cycle is not available in most parts of the world. Periyar river basin, draining the southern Western Ghats is recently been subjected to severe drought (2016) and devastating flood (2018); each leading to huge economic and ecological loss. This study uses stable isotopes (delta H-2 and delta O-18) to understand the water cycle dynamics in Periyar river basin in relation to drought and reservoirs. The variable d-excess and lc-excess have been analyzed to understand the evaporative controls in determining the water cycle mechanism during drier seasons. The seasonality of isotopic imprints in different hydrologic compartments revealed higher variability in the rainwater during northeast monsoon (NEM) and pre-monsoon (PREM) owing to the presence of cyclonic circulations and evaporative fractionation. The higher rates of depletion in precipitation did not bring any subsequent depletion in the accumulated surface and groundwater rather it produces enrichments due to the evaporative loss of heavier isotopes owing to the drier weather conditions. The physiography of the basin has significantly influenced the distribution of delta H-2 and delta O-18 in the surface and groundwater indicating a higher rate of mixing in the middle stretch (midland) suggesting an altitudinal control in the basin. Shallow dug wells situated close to the reservoirs inherited the evaporation and isotopic signatures specific to the recharge source. The present study highlights the impact of drought on the water cycle dynamics of southwest India through increased evaporation signals (more negative lc-excess), reduced effects of elevation (altitude effect) and distance (continental effect) along the basin. The evaporation signals are more conspicuous over reservoirs and exhibit a significant role in recharging the shallow dug wells in their proximity. The lc-excess reveals that northeast side of the basin receives significant amount of recirculated moisture from the windward side of Western Ghats, as observed in an earlier study over the drier Tamil Nadu state. The present study thus underlines the effective use of stable isotopes in studying the impact of weather fluctuations (as extreme events) in the hydrological circulation which is not yet been utilized in many parts of the world.