The potential of GRACE in assessing the flood potential of Peninsular Indian River basins

The existence of fewer gauging sites along with sparse network of rain gauges make the monitoring of floods difficult, especially over regions characterized with hydrologic heterogeneity. The availability of observations from satellite sensors makes it possible to study hydrological behaviour of such under-observed river basins. In this study, we explore the utility of GRACE derived terrestrial water storage estimates to monitor the flood events of varying magnitudes over the river basins on Indian Peninsula, that is known to be a region of high heterogeneity. We use terrestrial water storage estimates from Gravity Recovery and Climate Experiment (GRACE) satellite mission, rainfall and temperature data from India Meteorological Department (IMD), discharge data from Central Water Commission (CWC) and groundwater level data from Central Ground Water Board (CGWB) to evaluate and compare the variability of flood potential of six major river basins, which show unique hydrological characteristics. The flood potential is evaluated for these major basins in Peninsular India using Reager's Flood Potential Index (FPI) and the skill of FPI is assessed over these basins, for the period from 2003 to 2016. Significant variations are observed in the terrestrial water storage and FPI estimates of these basins, which reflects the diverse hydrological behaviour of the basins. Mahanadi basin shows extremely high positive FPI values, while Cauvery basin shows extremely high negative values, thereby confirming their respective flood and drought prone characteristics. The ability of FPI is established especially for basins with discharge values higher than 2000 m(3)s(-1). Moreover, good accuracy (> 90%) is observed over Mahanadi, Godavari and Narmada basins that are exposed with frequent mild, moderate and severe floods, using the threshold values of FPI. Among all the basins, Mahanadi basin shows high threshold FPI values of 0.40, 0.32 and 0.27 for severe, moderate and mild floods respectively. Godavari and Narmada basins have relatively lesser thresholds. The FPI values corresponding to severe, moderate and mild floods for Godavari basin are 0.25, 0.14 and 0.09, while those for Narmada basin are 0.25, 0.11 and 0.04. We hence, conclude that the index has the ability to monitor different levels of floods, over basins exhibiting diverse hydrological characteristics. These observations establish that GRACE derived terrestrial water storage anomaly (TWSA) can be used for flood monitoring applications, when discharge data is rarely available and the incorporation of terrestrial water storage estimates with the rainfall data in hydrological modelling may greatly help in assessing hydrological extremes.