WHAT IS HYDROLOGY?
Hydrology means the science of water. It is the science that deals with the occurrence, circulation and distribution of water from the earth and the 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 drawing support from allied sciences, such as meteorology, geology, statistics, chemistry, physics and fluid mechanics. Hydrology is basically an applied science. To further emphasise the degree of applicability, the subject is sometimes classified as
1. Scientific Hydrology – It is the study which is concerned chiefly with academic aspects
2. Engineering or Applied Hydrology – Is it the study concerned with engineering applications
In general, Engineering Hydrology deals with
1. Estimation of Water Resources
2. Study of processes such as Precipitation, Run-off, Evapo-Transpiration and their interactions
3. Study of problems such as floods, droughts, and the strategies to combat them
WHAT IS HYDROLOGIC CYCLE?
Water occurs on the earth in all its three states that is 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, stream flow and ground water 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.
A convenient starting point to describe the cycle is in the oceans. Water in the oceans evaporates due to the heat energy provided by solar radiation. The water vapour moves upwards and forms clouds. While much of the clouds condense and fall back to the oceans as rain, a part of the clouds is driven to the land areas by winds. There they condense and precipitate onto the landmass as rain, snow, hail, sleet, etc. A part of the precipitation may evaporate back to the atmosphere even while falling. Another part may be intercepted by vegetation, structures and other such surface modifications from which it may be either evaporated back to the atmosphere or move down to the ground surface.
0 – Evaporation from Ocean
1 – Raindrop evaporation
2 – Interception
3 – Transpiration
4 – Evaporation from land
5 – Evaporation from water bodies
6 – Surface runoff
7 – Infiltration
8 – Groundwater
9 – Deep percolation
A portion of the water that reaches the ground enters the earth’s surface through infiltration, enhances the moisture content of the soil and reaches the groundwater body. Vegetation sends a portion of the water from under the ground surface back to the atmosphere through the process of transpiration. The precipitation reaching the ground surface after meeting the needs of infiltration and evaporation moves down the natural slope over the surface and through a network of gullies, streams and rivers to reach the ocean. The groundwater may come to the surface through springs and other outlets after spending a considerably longer time than the surface flow. The portion of the precipitation which by a variety of paths above and below the surface of the earth reaches the stream channel is called runoff. Once it enters a stream channel, runoff becomes stream flow.
The sequence of events as above is a simplistic picture of a very complex cycle that has been taking place since the formation of the earth. It is seen that the hydrologic cycle is a very vast and complicated cycle in which there are a large number of paths of varying time scales. Further, it is a continuous recirculating cycle in the sense that there is neither a beginning nor an end or a pause. Each path of the hydrologic cycle involves one or more of the following aspects:
(i) transportation of water
(ii) temporary storage
(iii) change of state for example, (a) the process of rainfall has the change of state and transportation, and (b) the groundwater path has storage and transportation aspects.
| Transportation Components | Storage Components |
|---|---|
|
Precipitation |
Storage on the land surface (Ex. Depression storage, Ponds, Lakes, Reservoirs, etc) |
|
Evaporation |
Soil moisture storage |
|
Transpiration |
Groundwater Storage |
|
Infiltration |
|
|
Run-off |
|
HORTON'S HYDROLOGIC CYCLE
A qualitative Hydrologic Cycle was introduced by Horton, which is generally known as Horton’s representation of Hydrological Cycle. It illustrates very clearly the storage and transportation components and their relative positions in the cycle. The below figure is read in the counter clockwise manner and is self explanatory.
Schematically, the inter-dependency of the transportation components can be represented as in the below figure (Transportation components of Hydrologic Cycle). The quantities of water going through various paths of the hydrological cycle in a given system can be described by the continuity principle known as water-budget equation or hydrologic equation.It is important to note that the total water resources of the earth is constant and the sun is the source of energy for the hydrologic cycle. A recognition of the various processes such as evaporation, precipitation and groundwater flow helps one to study the science of hydrology in a systematic way. Also, one realises that humans can interfere with virtually any part of the hydrologic cycle, e.g. through artificial rain, evaporation suppression, change of vegetal cover and land use, extraction of groundwater, etc. Interference at one stage can cause serious repercussions at some other stage of the cycle.
The hydrological cycle has important influences in a variety of fields including agriculture, forestry, geography, economics, sociology and politics. Engineering applications of the knowledge of the hydrologic cycle, and hence of the subjects of hydrology, are found in the design and operation of projects dealing with water supply, irrigation and drainage, water power, flood control, navigation, coastal works, salinity control and recreational uses of water.
