Droughts: occurrence, evolution and impacts over India

Abstract

Novel approaches to assess the occurrence, distribution, trend, concurrence, and evolution of major drought types are required to understand their implications on terrestrial ecosystems especially agricultural productivity. The complexity of the drought phenomenon, intricate ecosystem-drought interactions, and interdependence of the drought characteristics make the drought assessment a challenging task. In addition to traditional droughts, flash drought is a newly discovered extreme event that has rapid intensification without sufficient early warning. Such flash drought poses a great threat to terrestrial ecosystems. The ecosystem resistance and vegetation adaptation to flash droughts are significantly dependent upon the accurate estimation of flash drought events and their interaction with ecosystem metrics such as GPP, NPP, and LAI. Therefore, in context of climate change, a better understanding of the droughts in terms of their occurrence, trend, concurrence, evolution as well as joint dependence of drought characteristics is important to investigate the implications on terrestrial ecosystem. This thesis presents the study carried out to deliver a comprehensive assessment of drought conditions over India and their implications on the terrestrial ecosystem. The initial part of the thesis is devoted to explain the drought from multi perspectives such as severity, distribution, trends, concurrence, and evolution. The investigation is carried out using the most widely used drought indices (SPI, SRI, SSI, and VCI) to monitor different drought types over 24 major river basins of India. The results show that hydrological and soil moisture droughts were observed to be more influential as compared to the meteorological and vegetation droughts in most of the river basins of India. Further, approximately 82% of concurrent droughts include soil moisture drought. This suggests that the soil moisture is more influencing rather than precipitation in the study area. The assessment of drought characteristics is approached from a joint dependence perspective in the second part. A copula based bivariate probabilistic analysis of drought characteristics across Indian river basins is carried out. It was observed that Southern Indian river basins have a higher exceedance probability and smaller joint return period compared to the Western river basins of India. This suggests that drought events in Western and Central India are more severe and longer whereas the ones in the south Indian river basins are more frequent but less severe. In the third part, flash drought identification and its impact on the regional terrestrial ecosystem was investigated. To account for terrestrial ecosystem, gross primary productivity (GPP) from MODIS was used to quantify the response of ecosystem to flash droughts in India. It was found that GPP responds to more than 95% of the flash droughts across India, with the highest response frequency occurring over Ganga basin and southern India while the lowest response across northeastern India. The discrepancies in the response frequency are majorly attributed to different vegetation resilience conditions across different parts of the country. The final part of the study is aimed to understand the impact of climate change on crop water requirement and productivity of major crops in Sikkim. The investigation is carried out using two crop models i.e., AquaCrop and CROPWAT in order to estimate crop yield and crop water requirement, respectively. From the investigation, an increase in the mean percentage change in the crop yield was observed over Sikkim during 2021 2099. This can be attributed to the suitable temperature profile, increase in the CO2 concentration, high elevation of the study area. The CWR and CIR investigation also suggests an increase in the CWR towards the end of the twenty-first century for rice and wheat over West and South Sikkim with respect to the baseline period.