Basics of Hydrology

What is Hydrology?

Hydrology is the science that encompasses the occurrence, distribution, movement and properties of the waters of the earth and their relationship with the environment within each phase of the hydrologic cycle.

What is Hydrologic cycle ?

Hydrologic Cycle is a Water Cycle. It involves continuous movement and distribution of water betweeen Earth surface and Atmosphere. Some of the important processes involved in Hydrologic Cycle are: Evaporation of water from water bodies, Transpiration of water from plants, Condensation of water vapor in clouds, Precipitation in the form of rain and snow, Runoff and subsequent flow through streams, Infiltration of precipitation, and Recharge of aquifers.

Source: http://www.srh.noaa.gov/jetstream/atmos/hydro.htm

Hydrology and Hydrologists!

Water is one of our most important natural resources. Without it, there would be no life on earth. The supply of water available for our use is limited by nature. Although there is plenty of water on earth, it is not always in the right place, at the right time and of the right quality. Adding to the problem is the increasing evidence that chemical wastes improperly discarded yesterday are showing up in our water supplies today. Hydrology has evolved as a science in response to the need to understand the complex water systems of the Earth and help solve water problems. Hydrologists play a vital role in finding solutions to water problems, and interesting and challenging careers are available to those who choose to study hydrology.

(Source: United States Geological Survey)

Hydrographs

Hydrograph

Hydrograph is the graph of Flow vs. Time. The unit of flow could be cfs, m3/s, etc and the unit of time could be hours, days, months etc. The area under a hydrograph between two points (time interval) represents the volume of water passing through point of interest in that time interval. The total area under the hydrograph represents total runoff volume for that storm event. Hydrologists can use the measured or synthetic hydrographs to estimate the peak flows and total volume for the storm event of interest. The peak flows and volume are necessary to design the hydraulic structures such as dams, detention ponds, stormwater infiltration facilities, reservoirs, storage tanks, or water quality treatment devices. Hydrographs are also used to show the hydrologic effects of existing or proposed watershed projects and land use changes, for example, they can be used on a development site to compare pre-development hydrograph to post-development hydrograph to analyze the impact of the project on downstream communities.

Types of Hydrographs

Following is the partial list of hydrographs commonly used in Watershed studies.
• Natural hydrograph--obtained directly from the flow records of a gaged stream.
• Synthetic hydrograph--obtained by using watershed parameters and storm characteristics to simulate a natural hydrograph
• Unit hydrograph--a discharge hydrograph resulting from 1 inch of direct runoff distributed uniformly over the watershed resulting from a rainfall of a specified duration.
• Dimensionless unit hydrograph (DUH)--a hydrograph developed to represent several unit hydrographs; plotted using the ratio of the basic units time to peak and peak rate; also called an index hydrograph.
(Source: Part 630 National Engineering Handbook, Chapter 16 Hydrographs)

Hydrologic Routing

As the runoff travels through streams and lakes, the flow rate changes, due to factors such as channel attenuation, on-stream detention, etc. The method used to estimate such modification is called ‘routing’. Routing helps hydrologists predict the translation of hydrograph shape as it travels downstream.

Here's a great textbook on Hydrology that I like a lot.