Hydrology is the study of the movement, distribution, and quality of water on Earth's surface and below. It involves the study of the hydrological cycle, which includes the processes of evaporation, condensation, precipitation, runoff, and infiltration. Water resources engineering, on the other hand, deals with the planning, design, and management of water resources systems, including dams, reservoirs, canals, and irrigation systems.

K.C. Patra's book, "Hydrology and Water Resources Engineering," is a comprehensive guide that covers the fundamental principles of hydrology and water resources engineering. The book is widely used as a textbook in universities and colleges and is also a valuable resource for practicing engineers and researchers.

For those interested in downloading and reading the book, a PDF version of "Hydrology and Water Resources Engineering" by K.C. Patra is available online. The book can be downloaded from various online sources, including online libraries and bookstores.