Hydrology means the science of water. It is the science that deals with the occurrence, circulation and distribution of water on the earth and earth’s atmosphere. As a branch of earth science, it is concerned with the water in streams and lakes, rainfall and snowfall, snow and ice on the land and water occurring below the earth’s surface in the pores of the soil and rocks. In a general sense, hydrology is a very broad subject of an inter-disciplinary nature depending upon allied sciences, such as meteorology, geology, statistics, chemistry, physics and fluid mechanics. Hydrology also deals with estimation of water resources, study of precipitation, runoff, evapotranspiration and their interaction. It also involves the study of problems such as floods and droughts and strategies to combat them. Hydrology is basically an applied science.
Engineering hydrology deals with all these aspects which are pertinent to planning, design and operation of hydraulic engineering projects for the control and use of the available water. Hydrology finds its application in the design and operation of water resources projects to estimate the magnitudes of flood flows at different times of a year to decide reservoir capacity, spillway discharge, dimensions of hydraulic structures etc.
Hydrologic Cycle
Water occurs on the earth in all its three states, viz. liquid, solid and gaseous and in various degrees of motion. Evaporation of water from water bodies such as oceans and lakes, formation and movement of clouds, rain and snowfall, streamflow and groundwater movement are some examples of the dynamic aspects of water. The various aspects of water related to the earth can be explained in terms of a cycle known as the hydrologic cycle. Hydrologic cycle is the water transfer cycle, which occurs continuously in nature. The three important phases of the hydrologic cycle are
- Evaporation and Evapotranspiration
- Precipitation
- Runoff
The total water of earth, excluding deep ground water, is in constant circulation from the earth (including oceans) to atmosphere and back to the earth and oceans. This cycle of water amongst earth, oceans and atmospheric systems is known as hydrologic cycle. The hydrologic cycle can be visualized to begin with the evaporation (due to solar heat) of water from the oceans, streams and lakes of the earth into the earth’s atmosphere. The water vapour, under suitable conditions, get condensed to form clouds moving with wind all over the earth’s surface and which, in turn, may result in precipitation (in the form of rain water, snow, hail, sleet etc.) over the oceans as well as the land surface of the earth. Part of the precipitation, even while falling, may evaporate back into the atmosphere. Another part of the precipitation may be intercepted by vegetation on the ground or other surfaces. The intercepted precipitation may either evaporate into the atmosphere or fall back on the earth’s surface.
The greater part of the precipitation falling on the earth’s surface is retained in the upper soil from where it may return to the atmosphere through evaporation and transpiration by plants find its way, over and through the soil surface as runoff, to stream channels and the runoff thus becoming stream flow. Yet another part of the precipitation may penetrate into the ground to become part of the ground water. The water of stream channels, under the influence of gravity, moves towards lower levels to ultimately meet the oceans. Water from ocean may also find its way into the adjoining aquifers. Part of the stream water also gets evaporated back into the atmosphere from the surface of the stream. The ground water also moves towards the lower levels to ultimately reach the oceans. The ground water, at times, is a source of stream flow. Further, it is a continuous recirculation cycle in the sense that there is neither a beginning nor an end or a pause.
The hydrologic cycle is a very complex phenomenon that has been taking place since the earth formed. It should also be noted that the hydrologic cycle is a continuous recirculation cycle with neither a beginning nor an end. Hydrologic system is defined as a structure or volume in space surrounded by a boundary that receives water and other inputs, operates on them internally and produces them as outputs. The global hydrologic cycle can be termed a hydrologic system containing three subsystems such as the atmospheric water system, the surface water system and the subsurface water system. Another example of the hydrologic system is storm-rainfall-runoff process on a watershed. Watershed (or drainage basin or catchment) is a topographic area that drains rain water falling on it into a surface stream and discharges surface stream flow through one particular location identified as watershed outlet or watershed mouth. Each path of the hydrologic cycle involves one or more of the following aspects.
- Transportation of water
- Temporary storage
- Change of state
Hydrologic Systems
The hydrologic system was defined as a structure or a volume in space, surrounded by a boundary, that accepts water and other inputs, operates on them internally, and produces them as outputs. The structure (for surface or subsurface flow) or volume in space (for atmospheric moisture flow) is the totality of the flow paths through which the water may pass from the point it enters the system to the point it leaves. The boundary is a continuous surface defined in three dimensions enclosing the volume or structure. A working medium enters the system as input, interacts with the structure and other media, and leaves as output. Physical, chemical and biological processes operate on the working media within the system; the most common working media involved in hydrologic analysis are water, air and heat energy.
Global Hydrologic Cycle
The global hydrologic cycle can be represented as a system containing three subsystems: the atmospheric water system, die surface water system and the subsurface water system. Another example is the storm rainfall- runoff process on a watershed, which can be represented as a hydrologic system. The input is rainfall distributed in time and space over the watershed and the output is stream flow at the watershed outlet. The boundary is defined by the watershed divide and extends vertically upward and downward to horizontal planes.
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