Variability of water
This variability in water supply is largely influenced by climate. Together with geographical characteristics such as topography, soils, and land use, hydrologic variability affects the development and character of surface water systems such as lakes and rivers. Flow variability is also becoming increasingly recognized as an important factor in the health of riverine aquatic ecosystems. Extreme floods are important because many of the processes that shape the river occur during the largest floods, also known as reset events. Hydrologic conditions during extreme low flow periods are also important and can impact species selection.
Measuring, estimating, and/or predicting stream flow is an important task in surface water hydrology. There exist a variety of methods for monitoring stream flow and each method remains specific to a particular type of stream. The methods to quantify and monitor the stream flow are grouped into four categories:
•Direct measurement methods.
•Velocity area methods.
•Constricted flow methods.
•Noncontact measurement methods.
All necessary theoretical concepts of each process, their physical modelling and their real life measuring arrangements are discussed elaborately. Necessary schematics diagrams and practical photographs are used along with illustrative examples to make the theoretical concepts clear. More real life applications are considered in the last module in which Hydrological design concepts are developed. The single point or location at which all surface drainage from a basin concentrates as outflow is the basin outlet. The time required for the rain falling at the most distant point in a drainage area (i.e., on the fringe of the catchment) to reach the basin outlet is called the time of concentration. This is a very significant variable since only such storms of duration greater than the time of concentration will be able to produce runoff from the entire catchment area and cause a peak flow.