Water-resources engineering is an area of professional practice that includes the design of systems to control the quantity, quality, timing, and distribution of water to meet the needs of human habitation and the environment. Aside from the engineering and environmental aspects of water-resource systems, their feasibility from legal, economic, financial, political, and social viewpoints must generally be considered in the development process. The successful operation of an engineered system usually depends as much on nonengineering analyses (e.g., economic and social analyses) as on sound engineering design. Examples of water-resource systems include domestic, commercial, and industrial water supply, wastewater treatment, irrigation, drainage, flood control, salinity control, sediment control, pollution abatement, and hydropower-generation systems.

Reference: Chapter 1 in Chin, David A. Water-Resources Engineering. (4th Edition). Pearson Education (US), 2020.

The waters of the earth are found on land, in the oceans, and in the atmosphere, and the core science of water-resources engineering is hydrology, which deals with the occurrence, distribution, movement, and properties of water on Earth. Engineering hydrologists are primarily concerned with water on land and in the atmosphere, from its deposition as atmospheric precipitation to its inflow into the oceans and its evaporation into the atmosphere. Water-resources engineering is commonly regarded as a subdiscipline of civil engineering, and several other specialty areas are encompassed within the field of water-resources engineering. For example, the specialty area of groundwater hydrology is concerned with the occurrence and movement of water below the surface of the Earth; surface-water hydrology and climatology are concerned with the occurrence and movement of water above the surface of the Earth; hydrogeochemistry is concerned with the chemical changes in water that is in contact with earth materials; erosion, sedimentation, and geomorphology are concerned with the effects of sediment transport on landforms; and water policy, economics, and systems analyses are concerned with the political, economic, and environmental constraints in the design and operation of water-resource systems. The quantity and quality of water are inseparable issues in design, and the modern practice of water-resources engineering demands that practitioners be technically competent in understanding the physical processes that govern the movement of water, the chemical and biological processes that affect the quality of water, the economic and social considerations that must be taken into account, and the environmental impacts associated with the construction and operation of water-resource projects.

Reference: Chapter 1 in Chin, David A. Water-Resources Engineering. (4th Edition). Pearson Education (US), 2020.

The water cycle describes where water is on Earth and how it moves. Water is stored in the atmosphere, on the land surface, and below the ground. It can be a liquid, a solid, or a gas. Liquid water can be fresh or saline (salty). Water moves between the places it is stored. Water moves at large scales, through watersheds, the atmosphere, and below the Earth's surface. Water moves at very small scales too. It is in us, plants, and other organisms. Human activities impact the water cycle, affecting where water is stored, how it moves, and how clean it is.

The natural water cycle is the water cycle under natural conditions. It does not include human influences on the global water cycle. Check this updated water cycle diagram from United States Geologic Survey to understand the human impacts https://www.usgs.gov/special-topics/water-science-school/science/water-cycle-diagrams

Reference https://www.usgs.gov/special-topics/water-science-school/science/water-cycle