4.2. Hydrologic Cycle
The hydrologic cycle is the descriptive term applied to the general circulation of water from the oceans to the atmosphere, to the ground, and back to the oceans. Water is circulated on the planet earth mainly by solar and planetary forces. To understand the complicated relationship between precipitation and runoff, it is imperative to get a complete insight into the hydrologic cycle (Fig. 4.2).
The cycle is also defined as “the sequence of events which mark the movement of water from the water bodies as well as earth surface to the atmosphere and back to the earth and subsequently to water bodies via surface, subsurface routes.”
4.2.1. Components of Hydrologic Cycle and Important Terminology
Components of a hydrologic cycle can be explained through Fig. 4.2, which clearly indicates that the cycle has no fixed beginning and end, but the processes involved are following a strict path.
Atmosphere: It is a mixture of gases: gaseous envelop surrounding the earth is atmosphere. From a hydrologic cycle point of view, water vapor is the important component of the mixture.
Precipitation: Atmospheric moisture reaching the ground or earth in liquid or solid form is known as precipitation. Liquid: rainfall, drizzle; solid: snow, hail, etc.
Interception: Temporary storage of water by leaves or canopy of vegetation.
Surface Detention: The thin film of water above the land surface is called surface detention. Surface detention causes surface runoff due to the development of head at the upstream side of the surface. The intercepted water ultimately reaches the earth and only a limited quantity of intercepted water gets evaporated back to the atmosphere, depending upon the climatic characteristics of the watershed.
Surface Runoff: The movement of water above the surface under the action of gravity is surface runoff.
Depression Storage: The precipitating water and surface runoff stored in uneven surfaces, such as pits, ponds, and lakes, causes depression storage and is static.
Channel Runoff and Channel Storages (Dynamic Storage): As water accumulates in the depressions, and after the depressions are filled, the head further develops and then the water flows into channels. The water storage in channels is called dynamic storage.
River Runoff: The movement of water under gravity through permanent or semipermanent well-defined natural channels is called river runoff.
Evaporation: It is the transfer of water from the water bodies to the atmosphere due to heat energy.
Infiltration: The entry of water through the surface into the soil is called infiltration. The infiltrating water replaces the air present in the soil pores.
Percolation: The downward movement of water within the soil under the action of gravity is percolation.
Soil Moisture: The water stored in the unsaturated soil media is soil moisture (if both air and water are present in the soil pores, then it is soil moisture).
Field Capacity (FC): The maximum amount of water that the soil can retain under gravity is called field capacity.
Soil Moisture Deficiency (SMD): It is the departure of soil moisture from its field capacity. SMD = FC − existing moisture (infiltrating water first meets the SMD and brings the soil to its field capacity).
Saturated Zone: Soil pores are completely filled with water and there is no air present in the soil pores. This soil zone is known as aquifer and the water stored is called groundwater storage.
Groundwater: Water present in the saturated zone (or aquifer) is called groundwater. It may be either stationary or nonstationary.
Groundwater Flow: The slow movement of water within the saturated zone due to potential difference is groundwater flow.
Groundwater Table: It is the surface separating the saturated and unsaturated zones, where the pressure is atmospheric.
Interflow: It is the percolating water reappearing at the surface in the river flow—the contribution of water to river flow from the unsaturated zone.
Base Flow: The contribution of water to the river from the saturated zone or contribution of groundwater to the river flow is called base flow.
Water Divide: It is the ridge line passing through the highest points of an area. It separates one catchment from others. It is a function of outlet point.
Catchment Area: It is the area of the catchment enclosed within the water divide.
Closed Basin or Blind Basin: It is the area within the catchment which does not contribute to runoff toward the river drainage system.
Drainage Basin Area: It is the area in the catchment within which runoff originates and passes through a single outlet.
Effluent River: When groundwater contributes to river flow, it is an effluent river.
Influent River: When river flow contributes to the groundwater, the river is known as influent.
Alluvial River: Alluvial river is one which flows through stretches formed by sedimentation.
Perennial River: There is continuous flow that is present in the river all the time. Rivers originating from the Himalayan glacial region are mostly perennial rivers, such as Ganga.
Intermittent River: There is nearly continuous flow for most part of the year but may dry under severe dry conditions.
Ephemeral River: Such a river carries water only when storm rain occurs.
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