The science and study of water, including its physical and chemical properties and its occurrence on earth. Most commonly, hydrology encompasses the study of the amount, distribution, circulation, timing, and quality of water. It includes the study of rainfall, snow accumulation and melt, water movement over and through the soil, the flow of water in saturated, underground geologic materials (groundwater), the flow of water in channels (called streamflow), evaporation and transpiration, and the physical, chemical and biological characteristics of water. Solving problems concerned with water excesses, flooding, water shortages, and water pollution are in the domain of hydrologists. With increasing concern about water pollution and its effects on humans and on aquatic ecosystems, the practice of hydrology has expanded into the study and management of chemical and biological characteristics of water.