This book presents select proceedings of the International Virtual Conference on Trends in Hydrological and Environmental Systems (ITHES 2021). Various topics covered in this book include urban hydrology, hydrological extremes, statistical analysis of hydro-meteorological data, impacts of climate change, hydrological modelling, groundwater studies, water resource management and applications of RS & GIS in hydrology. The book also discusses various topics on applications of CFD in water resources and environmental engineering, water and wastewater treatment, solid waste management and air quality. The book will be a valuable reference for beginners, researchers, and professionals interested in environmental civil engineering, especially hydrological and environmental systems.