ENGINEERING HYDROLOGY: CHAPTER 064 - LOW-FLOW FREQUENCY ANALYSIS

1. DROUGHTS

1.01
While high flows lead to floods, sustained low flows can lead to droughts.

1.02
A drought is defined as a lack of rainfall so great and continuing so long as to adversely affect the plant and animal life of a region, and to deplete domestic and industrial water supplies, especially in those regions where rainfall is normally sufficient for those purposes.

1.03
In practice, a drought refers to a period of unusually low water supplies, regardless of the water demand.

1.04
The regions most subject to droughts are those with the greatest variability of annual rainfall.

1.05
Regions where the coefficient of variation of annual rainfall exceeds 0.35 are more likely to have frequent droughts.

1.06
Low annual rainfall and high annual rainfall variability are typical of arid and semiarid regions.

1.07
Therefore, these regions are more likely to be prone to droughts.

1.08

San Luis reservoir, in Central California, at drought level on September 20, 2009.

1.09
Studies of tree rings, which document long-term trends of rainfall, show clear patterns of periods of wet and dry weather.

1.08

Science Education Resource Center, Carleton College

1.10
The probability of periods of dry years, or droughts, must be considered in water resources planning.

1.11
Long-term records show a tendency for dry years to group together.

1.12
This indicates that the sequence of dry years is not random, with dry years tending to follow other dry years.

1.13
It is therefore necessary to consider both the severity and duration of a drought period.

1.14
The severity of droughts can be established by measuring
(1) the deficiency in rainfall and runoff,
(2) the decline of soil moisture, and
(3) the decrease in groundwater levels.

1.15
Alternatively, low-flow frequency analysis can be used in the assessment of the probability of droughts of different severities and durations.

2. LOW-FLOW FREQUENCY

2.01
Low-flow frequency analysis begins with the identification of the low-flow series.

2.02
Either the annual minima or the annual exceedence series are applicable.

2.03
In a monthly analysis, the annual minima series is formed by the lowest monthly flow volumes in each year of record.

2.04
In the annual exceedence method, the lowest monthly flow volumes in the record are selected, regardless of when they occurred.

2.05
In the latter method, the number of values in the series need not be equal to the years of record.

2.06
A flow-duration curve is a plot of ranked values of daily discharge, from high to low, for a given period, with percent time in the abscissas, as shown here.

2.07

2.08
The severity of low flows can be depicted with a flow-duration curve.

2.09
However, such a curve does not contain information on the sequence of low flows or the duration of possible droughts.

2.10
The analysis is made more meaningful by abstracting the minimum flows over a period of several consecutive days.

2.11
For instance, for each year, the 7-day period with minimum flow is abstracted, and the minimum flow is the average for that period.

2.12
A frequency analysis of the low-flow series results in a function describing the probability of occurrence of low flows of a certain duration.

2.13
The same analysis repeated for other durations leads to a family of curves depicting low-flow frequency, as shown in this figure.

2.14

2.15
This figure shows the average minimum discharge to be expected for return period, in years, in the abscissas, and indicated durations, in consecutive days, in the curve parameter.

2.16
For example, an average minimum discharge of 4 m³/s and duration of 7 days is expected every 2 years.

2.17
Likewise, an average minimum discharge of 3 m³/s and duration of 30 days is expected every 25 years.

3. STREAMFLOW REGULATION

3.01
Streamflow regulation with dams may be necessary to provide a minimum downstream flow for specific purposes.

3.02
The reservoirs serve as the mechanism to diffuse the natural flow variability into downstream flows which have lesser variability.

3.03
Regulation is often necessary for downstream low-flow maintenance, usually for the purpose of meeting agricultural, municipal, and industrial water demands, minimum instream flows for fisheries and wildlife conservation, navigation draft, and water pollution control.

Narrator: Victor M. Ponce
Music: Fernando Oñate
Editor: Flor Pérez

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