Overview

Spell Analysis is a common way to consider the extremes of the hydrograph. Low Spell Analysis calculates statistics based on when the stream flow drops to or below a predefined threshold level. Similarly, High Spell Analysis calculates statistics based on when flow equals or exceeds a predefined upper limit.

Low Spell Analysis

The key to successfully using spell analysis is to select meaningful thresholds. The Spell Analysis tool allows you to input a discharge that has been determined to be important for the particular analysis. For example, for a low-flow spell analysis one may be interested in how frequently the depth of flow over riffles drops below that which is navigable by some important fish species. This low-flow threshold can be used to determine the frequency and duration of low-flow events, and even the average minimum flow of each event. In addition to the user-defined threshold, a threshold can be defined in terms of properties of the flow, such as the mean flow. By defining flow thresholds in terms of some intrinsic factor of the flow record the same spell analysis may be applied to several flow records to help interpret differences in the flow regime.

Low-flow spell analysis examples:

Imagine that the spawning of some native fish species occurs during periods of stable low-flows where water velocities and depths are low and temporally stable. This same period of low-flow happens to be the time of peak demand for irrigation water to be delivered down the stream. The frequency and duration of the low-flow periods could be compared between the natural low-flow regime and a synthetic flow regime proposed by the irrigation facility.
Consider the position of the irrigation facility in the example above:- Are we confident that it is the stable low-flows that facilitates species spawning and recruitment? This hypothesis may be tested by comparing a long data set of temporally collected recruitment data with detailed spell analysis of the flow record over the same period. Do periods of high recruitment follow periods of low-flow?

Contents

High Spell Analysis

The High Spell Analysis is conducted in exactly the same manner as the Low Spell Analysis, however it is the frequency, duration and magnitude of events above the threshold that is important in this case.

High-flow spell analysis example:

Fluvial Geomorphologists (those that describe the evolution of the landscape by water) have dedicated considerable attention to describing "channel forming flows". That is those flows with the capacity to transport sediment and reform the channel. One can consider not only the frequency of such flows (as described in flood frequency analysis) but also the duration that is critical to being able to do work on the stream.
Consider a small hydroelectric power plant that discharge near bankfull flow when operating at peak discharge. If this discharge corresponds to that required to transport sediment, then the management of the power station may be required to not increase the rate of sediment transport due to the operation of the power plant. To address this problem one may compare the high-flow spell analysis of the natural hydrograph with that of a modified hydrograph due to power generation.

Figure 1. Illustration of the High Spell Analysis tool.

Accessing the Tool

A Spell Analysis can only be created for flow time series (i.e. time series with units of volume/time). It can also only be created for one time series at a time. To create a high-spell or low-spell analysis:

right-click on the time series name in the Results Manager tree menu
select Statistics » High Spell Analysis or Statistics » Low Spell Analysis

Once created, the Spell Analysis will be visible in the Results Manager custom charts and statistics pane.

Configuration

The spell analysis is configured within the Options menu item (Figure 1). The configuration options are described below.

Year Start

The year start defines the day and month on which the water year starts.

Threshold

The threshold flow (above or equal to which a high-flow spell commences, or below which or equal to which a low spell commences) can be selected using several alternative criteria:

Mean daily flow: A number of mean daily flows (can be a decimal)
Standard deviation: The mean daily flow plus some number of standard deviations of the daily flow
Percentile: A percentile of the daily flow, for example the 10th percentile flow is that flow which is exceeded 10% of the time
User defined: A user defined flow value

Independence Criteria

Independence criteria determines if spells above or below a threshold (defined above) are separate events or part of the same event. Where consecutive spell events are not deemed to be independent, their duration is simply added. Two alternative independence criteria are available;

Number of days between spells: the number of days between the last day of an event and the start day of the next event.
Number of days between peaks: the number of days between event peaks.

How the Spell Analysis tool handles overlapping spells between time periods

When analysing annual time periods, it is necessary to know if we are in an event at the end of the time-period. The Spell Analysis Tool records this information and considers it for the start of the next period. The occurrence of the event is recorded for the period in which it began, however the length is recorded in whichever period it occurs. If a spell starts in one period and ends in another (e.g. a flood that spans more than one water year) the occurrence of the spell is recorded for the first period, and the duration of above spell periods is attributed to the year in which it occurs. It is therefore possible to have a year when the only spell is one that spans from the end of the preceding year, in this case there will be no spell in the year, but there will be a spell duration recorded for the year. The Spell Analysis Tool also compares independence criteria between analysis periods. That is the number of days between peaks or the end and start of spells can span the end of an analysis period without affecting the output.

Length and Volume Criteria

In addition to the spell threshold, optional spell length and volume criteria can be defined. A high- or low-flow spell only occurs if the spell threshold, length and volume criteria are all met. The available length and volume criteria are:

Minimum spell length: the minimum duration of a spell
Maximum spell length: the maximum duration of a spell
Minimum spell volume: the minimum volume of a spell

If the spell length and volume criteria are set to 0, then these criteria will not be applied.

Outputs

Overview

The Overview menu item (Figure 1) summarises the high-flow, or low-flow, spells of the selected time series.

The following values are reported:

Reported Value	Description
General
Period	The reporting period, which can be whole record or annual.
Start and End	The start and end dates of the reporting period.
Number	The number of spells for the reporting period.
Duration
Duration – Longest	The single longest spell during the reporting period.
Duration – Mean	The average duration of spells for the reporting period.
Duration – Standard Deviation	The standard deviation of spell duration for the reporting period.
Duration – Total	The total duration of spells for the reporting period.
Period Between
Period Between – Longest	The single longest period of time between spells for the reporting period.
Period Between – Mean	The average period of time between spells.
Period Between – Standard Deviation	The standard deviation of the period of time between spells.
Period Between – Total	The total period of time between spells for the reporting period.
Magnitude
Peak – Mean	The mean peak for high spells or minima for low spells is reported.
Peak – Standard Deviation	The standard deviation of the peak for high spells or minima for low spells is reported.
Spell Volume – Mean	The average spell volume.
Spell Volume – Standard Deviation	The standard deviation of the spell volume.
Spell Volume – Total	The total volume of spells in the reporting period.

Spells

The Spells menu item (Figure 1) displays the time series of high-flow, or low-flow, spells. Flows that do not meet the specified threshold, length and volume criteria are not plotted. The time series can be viewed as a graph or a table.

Day of Year

The Day of Year menu item (Figure 1) displays the time series of high-flow, or low-flow, spells in terms of the day of the water year in which they occur. Flows that do not meet the specified threshold, length and volume criteria are not plotted. The time series can be viewed as a graph or a table.

Spell Start Date

The Spell Start Date menu item (Figure 1) displays cumulative exceedance percentiles for the annual spell start dates (expressed as day of water year). The exceedance percentiles can be viewed as a graph or a table.

Spell End Date

The Spell End Date menu item (Figure 1) displays cumulative exceedance percentiles for the annual spell end dates (expressed as day of water year). The exceedance percentiles can be viewed as a graph or a table.

Spell Peak Date

The Spell Peak Date menu item (Figure 1) displays cumulative exceedance percentiles for the date (expressed as day of water year) of the spell peaks (for high-flow spells) or minima (for low-flow spells). The exceedance percentiles can be viewed as a graph or a table.

Spell Peak Frequency

The Spell Peak Frequency menu item (Figure 1) summarises the number of spell peaks (for high-flow spells) or minima (for low-flow spells) occurring each month. The frequencies can be viewed as a bar chart or a table.

Spell Duration

The Spell Duration menu item (Figure 1) displays cumulative exceedance percentiles for the spell durations. The exceedance percentiles can be viewed as a graph or a table.

Acknowledgements

This material has been adapted from:

Marsh, N. (2004) RAP river analysis package: user guide, version 1.1. CRC for Catchment Hydrology, Australia, Jan 2004. www.toolkit.net.au/rap

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