GRACE-based Assessment of Climatic Controls on Hydrological Variables

Abstract

Anthropogenic climate change is now well-established and is expected to have serious implications on Terrestrial Hydrologic cycle. This inspired scientific investigation to assess the impact of various climatic processes on hydrologic system to disentangle climate change impacts from natural climate variability. The present thesis utilizes satellite remote sensing-based observations, specifically the unprecedented data derived from the Gravity Recovery and Climate Experiment (GRACE) satellite mission to study climatic controls of various hydrologic variables for global river basins in the initial part of the thesis followed by regional scale analysis for India. Since 1901, global temperatures have risen by 0.89 oC seriously impacting moderate to extreme rainfalls. However, few assessments of changes in global water balance have been conducted. The effect of rising temperatures on water recharge and availability for 31 major river basins across the world was investigated using GRACE derived Terrestrial Water Storage (TWS). Relative recharge, a new parameter defined to assess the direct effect of temperature, was found to reduce with increasing temperature in 23 river basins with 11 of them showing significant reduction. Precipitation is another important climatic factor that greatly influences hydrologic processes. Although several studies have examined the effects of precipitation on GRACE based TWS, groundwater and river discharge in individual regions with specific importance to years with precipitation deficit or excess, very few studies have investigated the comparative hydrologic sensitivity of global river basins to Precipitation. Moreover, limited studies have assessed the hydrologic impact of Potential Evapotranspiration (PET), another climatic variable which also includes humidity, wind speed and net radiation besides temperature. The present thesis investigates the sensitivity of interannual variability in storage change, Evapotranspiration and river discharge with interannual variability in precipitation and PET for 31 major global river basins using a Budyko framework-based sensitivity analysis. The estimated sensitivity values were further compared across these river basins and checked for any relationship with Aridity Index of the river basins. Water demand in India is growing due to its increasing population, economic growth and urbanization. The next part of the thesis examines climatic controls of regional hydrologic cycle in India. The knowledge of the interdependencies of large-scale hydrometeorological processes is crucial for efficient water resources management. A correlation and Independent Component Analysis (ICA) based spatiotemporal analysis of Precipitation, Evapotranspiration, Surface Soil Moisture (SSM), Root Zone Soil Moisture (RZSM), TWS and groundwater is proposed to study such interdependencies at intra- and interannual time scales. Recent studies based on GRACE suggested that groundwater has increased in central and southern parts of India. However, surface water, which is an equally important source of water in these semi-arid areas has not been studied yet. Surface water trends were examined using Surface Water Extent (SWE) and SSM were selected as proxies of surface water storage. Increasing trends in SWE and SSM were observed for all study regions and rainfall was found as the causal factor of TWS, SWE and SSM for 2 study regions in the south-eastern and west central part of India