This lecture initiates the discussion on the hydrological cycle, which involves the circulation of water through the hydrosphere, atmosphere, and lithosphere. Water moves through processes such as precipitation, runoff, groundwater flow, evaporation, and transpiration, forming a continuous cycle. Precipitation, the discharge of water in solid or liquid form, plays a key role in distributing water across land and subsurface areas. Evaporation and transpiration return water to the atmosphere, where it cools, condenses, and falls again as precipitation. This cycle ensures water's continuous transformation between its liquid, vapour, and solid states, maintaining Earth's water balance.