Hydrologic Regime

changes with time in the rates of flow of rivers and in the levels and volumes of water in rivers, lakes, reservoirs, and marshes. The hydrologic regime is closely related to seasonal changes in climate. In regions with a warm climate, the hydrologic regime is affected mainly by atmospheric precipitation and evaporation; in regions with a cold or temperate climate, the air temperature is a leading factor.

The hydrologic regime of rivers manifests itself by daily, ten-day, monthly, seasonal, and long-term fluctuations. It consists of a number of characteristic periods (phases) that vary with seasonal changes in the conditions under which rivers are fed. These phases are known as high water, freshet, and low water. Rivers are fed unevenly in the course of a year because of varying amounts of precipitation and uneven melting of snow and ice and entry of their water into the rivers. The fluctuations observed in the water level are caused mainly by changes in the flow rate and by the effects of wind, ice, and man’s economic activities.

The hydrologic regime of lakes is determined by the relationship between the amount of precipitation reaching the lake’s surface, evaporation, surface and underground flow into the lake, and surface and underground outflow of water from the lake, as well as by the size and shape of the lake, the pattern of change in the surface area with change in level, and wind activity, which determines the size of the waves and the extent to which the level rises and falls. Fluctuations in the lake level may be seasonal, annual, or short-term.

The hydrologic regime of marshes is dependent on climatic and hydrologic conditions, terrain, and the nature of the vegetation. Man’s economic activities are introducing ever greater changes in the hydrologic regime.