This article provides the introduction to the fundamental concepts behind the hydrologic cycle, brief history of the evolution of scientific hydrology and its connection with fundamental physical laws.
History & Evolution
Hydrology is defined as the study of the occurrence, distribution, movement & properties of water on earth”. A brief description of the origin & history of hydrology is summarized below;
- About 4000 B.C., a dam was constructed across the Nile River to permit reclamation of previously barren lands for agriculture production.
- An overview of Greek and Roman history showed that they built aqueducts that cover the vast lands.
- The concept of relationship of precipitation & stream flow was first conceived by early Greek Philosophers. The hydrological cycle theories were first conceived by Greek Philosophers. The quantification of rainfall data and development of mathematical models were started at the start of 18th century.
- Rapid development in hydrologic sciences showed that the the application of computer models provided a unique opportunity to hydrologists to analyze vast and complex hydrologic data.
Importance of Hydrology
Hydrology involves intense exploration of water resources. The dramatic increase in the population of the world has increased the water demand that put a great load on natural water systems. Hydrology is an integral part of various discipline especially Environmental Engineering & Environmental Sciences. Hydrological studies provides necessary information in engineering design & analysis such as design of water reservoir, groundwater reservoir, water distribution systems, storm water sewers and impact assessment studies of all water resource & public health projects.
Application of Physical Laws
The relationship of various physical laws in connection to hydrologic cycle are;
- Newton’s Law of Motion
- Laws of Thermodynamics and
- Flick’s Law of diffusion
Various biological & chemical laws are also applicable but application of physical laws are most important and dominant.
The Hydrologic Cycle
The hydrologic cycle is basically the movement of water above and below the surface of earth. The cycle consists of various hydrological processes some of them are listed below;
- Depression storage
- Flood routing
- Sediment Transport etc.
The Sun, which drives the water cycle, heats water in oceans and seas. Water evaporates as water vapour into the air. Ice and snow can sublimate directly into water vapor. Evapotranspiration is water transpired from plants and evaporated from the soil. Rising air currents take the vapour up into the atmosphere where cooler temperatures cause it to condense into clouds. Air currents move water vapour around the globe, cloud particles collide, grow, and fall out of the upper atmospheric layers as precipitation. Some precipitation falls as snow or hail, sleet, and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Most water falls back into the oceans or onto land as rain, where the water flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff and groundwater are stored as freshwater in lakes. Not all runoff flows into rivers, much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers, which store freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as groundwater discharge. Some groundwater finds openings in the land surface and comes out as freshwater springs. Over time, the water returns to the ocean, where our water cycle started.
Watershed is the area that topographically appears to contribute all the water that passes through a given cross-section of the stream. It consists of one or more catchment area and it has a specific boundary.
The Water Balance
It is the application of the law of mass conservation to a watershed, a state or a country. Mathematically,
Change in storage of water = Precipitation+ groundwater inflow – groundwater outflow – Evapotranspiration – Surface runoff
A water balance can be used to help manage water supply and predict where there may be water shortages. It is also used in irrigation, runoff assessment, flood control and pollution control. Further it is used in the design of subsurface drainage systems.