Hydrologic data generally consist of a sequence of observations of some phase of the hydrologic cycle made at a particular site. The data may be a record of the discharge of a stream at a particular place, or it may be a record of the amount of rainfall caught in a particular rain gage.
Although for most hydrologic purposes a long record is preferred to a short one, the user should recognize that the longer the record the greater the chance that there has been a change in the physical conditions of the basin or in the methods of data collection. If these are appreciable, the composite record would represent only a nonexistent condition and not one that existed either before or after the change. Such a record is inconsistent.
3. Introduction
• Hydrologic data generally consist of a sequence of observations of some
phase of the hydrologic cycle made at a particular site. The data may be
a record of the discharge of a stream at a particular place, or it may be a
record of the amount of rainfall caught in a particular rain gage.
• Although for most hydrologic purposes a long record is preferred to a
short one, the user should recognize that the longer the record the
greater the chance that there has been a change in the physical
conditions of the basin or in the methods of data collection.
• If these are appreciable, the composite record would represent only a
nonexistent condition and not one that existed either before or after
the change. Such a record is inconsistent.
4. • Flow duration curve is a plot of discharge versus percentage of time for which the
discharge is available. It is obtained from hydrograph data. The flow or discharge can be
expressed as cubic meters per second, per week or other unit of time. If the head at
which the flow is available is known, the discharge can be calculated in terms of the
kilowatts power (P) using following equation,
P=(0.736/75)*Qphη kW
• The flow duration curve becomes the load duration curve for hydroelectric plant and
thus it is possible to know the total power available at the site. The maximum and
minimum conditions of flow can also be obtained by the flow duration curve where
minimum flow condition decides the maximum capacity of plant that can be improved by
increasing the storage capacity. Figure shows that flow duration curves are of no use
where the time sequence of the flow is of importance such as in the study of floods.
Flow duration curves
5. • The flow-duration curve is a cumulative frequency curve
that show the percent of time specified discharges were
equaled or exceeded during a given period.
• It combines in one curve the flow characteristics of a stream
throughout the range of discharge, without regard to the
sequence of occurrence.
• If the period upon which the curve is based represents the
long-term flow of a stream, the curve may be used to
predict the distribution of future flows for waterpower,
water-supply, and pollution studies.
Cond….
8. Mass Curve
• It is a plot of cumulative volume of water that can be stored from a stream
flow versus time in days, weeks or months. It shows a mass curve,
Maximum intercept between line AB and mass curve is known as reservoir
capacity
• The unit used for storage is either cubic metre or day-second-metre. A day-
second-metre is the flow at the rate of 1m3/sec for one day and equal to
60*60*24=86400 m3.
• The capacity of plant is based on the storage capacity, which can be
modified by storage for the same mass curve, The water stored in dams is
called pondage and water stored in upstream reservoir is called storage.
9. • A small storage is used to meet the fluctuating demand for small
period (one day). When powerhouse is away from the main
storage, a small pondage is provided near the powerhouse.
• Sometimes surge tank is used. The capacity of reservoir, made
for a period of deficiency to make available the flow of water at a
required rate, is studied by mass curve.
CONTD….,
12. Software for Duration curves calculations
• A tool from the older generation that allows calculating and rendering
of duration curves. For example, these curves are often used in the
assessment of hydrological drought.
• Duration curves can be created from the entire imported time series
or from defined parts, such as annual or monthly segments.
• The FDC 2.1 tool was created for the analysis of flow duration curves.
Into the module can be imported daily or weekly types of time series.
In addition to discharge time series can be for the calculation and
analysis used variable time series of a continuous measurements,
which are used in hydrology and hydrogeology.
13. Stream flow Analysis and Assessment Software (SAAS)
• The aim of that software is to increase our understanding of flow regimes,
linkages to other physical processes, the combined influence on ecological
function, and the result of altering these processes on the integrity of river
systems.
• We are interested in methods to identify and quantify ecologically-important
components of flow regimes, assess their alteration through time with respect
to a reference condition, and relate hydrological alteration to changes in
ecological condition.
• We are also interested in using this information to characterize and classify
both natural and altered flow regimes. Thus the analyses and related methods
in SAAS reflect a focus on environmental flow assessments.
14. Conclusion
• The present study introduces a variety of nonparametric
quantile-estimation procedures useful for estimating
and interpreting FDCs & MCs. The traditional period-of-
record FDC & MCs can be interpreted as representing
the magnitude and frequency of daily stream flow
during the period of record.
