Freshes – variable success
The most common way to describe environmental water requirements is in terms of considering events described with the following parameters:
- Magnitude – flow volume(ML/d);
- Duration – how long should that volume occur for to achieved the desired outcome (days);
- Timing – what time of year should the event/s occur;
- Frequency – how many events are required to achieved the desired outcome
- Independence – how should those events be spaced (days)
This model covers any flow requirement which can be described using the above five parameters and considered within a single year. (multi-year freshes are captured in a separate model). This model captures the range of flow requirements described below:
- Freshes: short-duration flow events that submerge the lower parts of the river channel. They are important for plants that grow low on the banks and provide opportunities for fish and other animals to move more easily along the river.
- High flows: larger and last longer than freshes. They usually fill at least half the river channel, and therefore are important for plants that grow higher up the bank. High flows also encourage some species of fish to move and breed.
- Bankfull flows: large enough to fill the river channel with little spill onto the floodplain. Bankfull flows have high energy; they move large amounts of sediment and help to shape the river channel. They can also act as a trigger for breeding in some fish species.
- Overbank flows: exceed channel capacity and spill onto the floodplain. These flows improve floodplain productivity, fill wetlands that provide feeding and breeding opportunities for waterbirds, fish, frogs and turtles, allow animals to move between the river channel and floodplain and help disperse plant seeds and fragments, nutrients and carbon.
The basic approach of defining water requirements in terms of magnitude, duration, timing, frequency and independence is well established. This model adds two key additional functionality elements:
Prevailing climatic conditions
The prescribed environmental water requirements are different in periods of drought where the water provision will be focused on biological functions that support species survival than in times of high rainfall where the environmental watering requirements are focused on the provision of conditions suitable for recruitment.
The prescription of water requirements as magnitude, duration, timing, frequency and independence allow for a binary assessment. That is, either all of the conditions were met or they were not. The Variable success reporting allows each of the five parameters to be represented as continuous functions to reflect the level of importance of each parameter. For example, the preferred duration of an event may be 10 days, however, a 9 day event may still provide some environmental value.
The purpose of the model is to represent the most common way that environmental water requirements are prescribed. The model extends this traditional prescription to also allow the consideration of the prevailing climate as well as the representation of partial success.
This model was developed initially in the eFlow Projector application (Marsh et. al. 2021) to support performance reporting by the Victorian Environmental Water Holder.
This model can be applied anywhere that flow requirements can be described in terms of magnitude, duration, timing, frequency and independence.
The flow time series is analysed to identify all event opportunities within the prescribed sub-period of the year (typically, Autumn-winter, summer-spring). These events are all considered to select the best set of event opportunities (based on the specified count, highest scoring when considering magnitude, duration and independence). The best events are summarised and an overall performance (0 to 100%) is produced for each water year of the record.
A continuous yearly value is determined by combining all scores (magnitude, duration, count and independence) into a summary score. These results are then aggregated to a binary yearly result, then to a temporal result based on the defined assessment parameters.
The temporal results are then analysed across locations to report an overall landscape risk by considering the simultaneous occurrence of failures across the system.
- Daily flow data
- Data – parameters to describe the variable season. This includes the start month (to determine the year) and the success % of each month. A binary season can be achieved by using only 0 and 100% season scores.
- Rules – the rules for each possible climate type. This includes a magnitude (flow threshold), duration (days), count and independence (days) target.
- Partial tables – the tables to define how partially achieving the rules can still carry some level of success. This includes a table for magnitude, duration, count and independence.
- Yearly time series of performance, including the magnitude, duration, count, independence, compliance and summary scores
- Events file containing all events identified, including start date, end date and duration.
- Yearly time series of assessment results
- Temporal time series of assessment results
- Spatial time series of assessment results
This plugin is written in Python and its underlying code is publicly available from the Eco Risk Projector computation repository.
Marsh, N., Marsh, Z., Stacey, M., and Toomey, M. (2021) Eflow Projector: Assessing the Performance of environmental watering, Proceedings of the 10th Australian Stream Management Conference 2020, Kingscliff, NSW