Rainfall Trends in India and their Impact on Soil Erosion and Land Management

Indrani Pal, Cambridge University, Geotechnical Engineering Group


Under the threat of global warming it is vital to determine the impact that future changes in climate may have on the environment and to what extent any adverse effects can be mitigated. In this research an assessment was carried out on the impact that climate trends may have on soil erosion and contaminant transport in India and examined the potential for top soil management practices to improve or maintain soil quality. Historical rainfall data from 50-135 years and extreme temperature data for 103 years were analysed and long term trends were assessed for various aspects of Indian climates using suitable statistical techniques. Results indicated that intra-region variability for extreme monsoon seasonal rainfall is large and mostly exhibited a negative tendency leading to increasing frequency and magnitude of monsoon rainfall deficit and decreasing frequency and magnitude of monsoon rainfall excess everywhere in India except in the peninsular Indian region. This is further exacerbated by increased and more variable extreme temperatures. Intra-region rainfall variability in India is linked to the pacific Southern Oscillation, where the associations of monsoon drought and El-Niño Southern Oscillation (ENSO) in the regions near to coast are greatest. 50¬years high resolution daily gridded rainfall data was analysed to set up certain indices for the extreme daily rainfalls to assess their changes for the six gridded regions of Kerala, the extreme south western state of India where monsoon rainfall initiates every year. This was also done for two study sites, namely Bhoj wetland area of west central India and Sukinda chromite mining site of central north east India. Significant decrease was found in monsoon and spring rainfall extremes and increase in winter and autumn rainfall extremes in Kerala that would affect the tendency of change in seasonal total rainfall as well. Decrease in monsoon rainfall in Kerala also indicate that monsoon rainfall is decreasing in India as a whole, increased occurrence of floods is expected in winter and autumn seasons, together with water scarcity are expected to be felt both in spring and monsoon seasons with a delaying monsoon onset in Kerala. Soil erosion studies were conducted for two northern most gridded regions of Kerala as an extended work of the related MPhil study, and contaminant transport with eroded sediments was looked at for the Bhoj and Sukinda sites using RUSLE2 model software and other suitable numerical methods. It was found that soil erosion depended on a complex interaction of climate, soil properties, topography, and cover management. An assessment on extreme climate patterns for Bhoj and Sukinda showed an increasing tendency of seasonal and annual rainfall extremes and temperatures leading to an increasing pattern of soil erosion at both the sites. However, a certain consensus was difficult to reach because of the complex interaction of climate and soil carbon that is a very important deciding factor for soil erosion potential. Vegetative cover and plant residue was found providing essential soil nutrients, enhancing soil properties and retarding rainfall impact on bare top soil leading to reduction of soil erosion. Therefore, a soil erosion and contaminant transport prevention plan should take care of the top soil such that it is not kept bare especially when rainfall intensity is high in a given year. This work as a whole has highlighted the importance of regional climatological analysis with the large scale spatial averages especially at local decision making level, which is very useful for the broad scenarios such as climatological and ecological risk management.