Flood Susceptibility Modeling in a Subtropical Humid Low-Relief Alluvial Plain Environment: Application of Novel Ensemble Machine Learning Approach

This study has developed a new ensemble model and tested another ensemble model for flood susceptibility mapping in the Middle Ganga Plain (MGP). The results of these two models have been quantitatively compared for performance analysis in zoning flood susceptible areas of low altitudinal range, humid subtropical fluvial floodplain environment of the Middle Ganga Plain (MGP). This part of the MGP, which is in the central Ganga River Basin (GRB), is experiencing worse floods in the changing climatic scenario causing an increased level of loss of life and property. The MGP experiencing monsoonal subtropical humid climate, active tectonics induced ground subsidence, increasing population, and shifting landuse/landcover trends and pattern, is the best natural laboratory to test all the susceptibility prediction genre of models to achieve the choice of best performing model with the constant number of input parameters for this type of topoclimatic environmental setting. This will help in achieving the goal of model universality, i.e., finding out the best performing susceptibility prediction model for this type of topoclimatic setting with the similar number and type of input variables. Based on the highly accurate flood inventory and using 12 flood predictors (FPs) (selected using field experience of the study area and literature survey), two machine learning (ML) ensemble models developed by bagging frequency ratio (FR) and evidential belief function (EBF) with classification and regression tree (CART), CART-FR and CART-EBF, were applied for flood susceptibility zonation mapping. Flood and non-flood points randomly generated using flood inventory have been apportioned in 70:30 ratio for training and validation of the ensembles. Based on the evaluation performance using threshold-independent evaluation statistic, area under receiver operating characteristic (AUROC) curve, 14 threshold-dependent evaluation metrices, and seed cell area index (SCAI) meant for assessing different aspects of ensembles, the study suggests that CART-EBF (AUCSR = 0.843; AUCPR = 0.819) was a better performant than CART-FR (AUCSR = 0.828; AUCPR = 0.802). The variability in performances of these novel-advanced ensembles and their comparison with results of other published models espouse the need of testing these as well as other genres of susceptibility models in other topoclimatic environments also. Results of this study are important for natural hazard managers and can be used to compute the damages through risk analysis.

New evidence of mid- to late- Holocene vegetation and climate change from a Neolithic settlement in western fringe of Central Ganga Plain: Implications for Neolithic to Historic phases

Here we present a continuous palaeoclimatic record of 5980 years (7230 cal BP–1250 cal BP) from Hetapatti, a Neolithic site situated on the western fringe of the Central Ganga Plain. The region was a center of reurbanisation following the decline of the Harappan civilisation and is considered a hub of economic, political and religious evolution since the sixth millennium BC. Hetapatti contains an uninterrupted sequence from Neolithic to Historic (Gupta) period. The study integrates two different approaches (phytoliths and carbon isotope analysis) to infer vegetational and climatic changes and to understand their relationship to the cultural sequence. Our results show a gradual transformation from a warm and humid climate into a warm and dry climate from the Neolithic to Historic (Gupta) period. We find an abrupt weakening of the ISM at ~2080 cal BP driving a warm and dry climate. The comparison of our data with other high resolution regional and global records led us to hypothesise a synchronicity in this warm and dry trend, coeval with the Roman warm period (RWP). The observed variation in vegetation and climate might have driven by the fluctuations in the Indian Summer Monsoon (ISM). Additionally, the phytolith analysis provides evidence of cereal crops including rice, wheat, barley, millets, etc. suggesting a crop affinity with the earlier Indus crop package. It is interesting to note that the site displays an uninterrupted cultural sequence from Neolithic to Historic times, with artefacts of each phase exhibiting technological and stylistic developments from the preceding culture. From this we infer that the well adapted socio-ecological strategies and availability of perennial rivers may have helped the ancient civilisation to absorb the stress relating to a varying climate.