High and Low Spell Analysis

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:

1. right-click on the time series name in the Results Manager tree menu
2. 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:

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