To support whole of government priorities, at the time Topographic Data Management was tasked to develop a process for a stream ordered digital dataset at a consistent 1:100 000 (100k) scale. As part of this process, automatic modelling, methods, and scripting were tried and tested but did not meet standards or align with current feature datasets. By utilising the Strahler method and existing features within the digital Queensland watercourse line dataset, the favourable outcome was achieved. The result is called the consistent Statewide 100k drainage network.
The digital Queensland watercourse line dataset was stream ordered in 2018 using the Strahler (1957) stream ordering method with the addition of rules to accommodate different governmental mapping programs and the nature of the Queensland landscape.
The purpose of the consistent Statewide 100k drainage network is for use in Vegetation Management, Wetlands mapping, Land Administration, Emergency Management, monitoring of Climate Change, hydrological modelling, Topographic mapping and in the production of navigational and web-based mapping applications. The network displays the way or course through which surface water flows from time-to-time and comprises both natural and artificial features covering the State of Queensland. The network is segmented, concatenated and flow directed.
After the consistent Statewide 100k drainage network was selected, the Strahler classification of stream ordering was applied to it. The Strahler method is a top to bottom approach where the headwater stream is assigned with an order ‘n’. Every time two stream segments with order ‘n’ merge, the downstream order increases to ‘n+1’. Where an ‘n’ and ‘n+1’ merge the value remains ‘n+1’. And so on. However, because this classification was to be assigned to a subset of the drainage features of Queensland additional rules were devised and applied.
Those watercourse lines deemed to be a part of the consistent Statewide 100k drainage network were assigned with a stream order using the Strahler method (levels 1 – 9). Those deemed not to be part of the consistent Statewide 100k drainage network were assigned with a stream order of “Null”.
To allocate stream ordering, an algorithm was tested and refined which was accepted and adopted as a standard. The algorithm rule required that all Strahler classification streams with a value of ‘1’ had to be a minimum of 750 metres in length. Watercourse lines less than 750 metre (<750m) were manually reviewed.
Exception rules were applied in preference to the algorithm rule. They were:
Named streams were retained. After the algorithm was run, a search for any named streams not included (including those under 750m in length), was done manually. These features were then added.
When required to visually maintain straighter or more cartographically represented streams, varying angle distances from watercourse line segments were trialled, rejected, or accepted.
In braided stream networks, and data in areas of bifurcation, a manual review and upgrade was required. These anomalies were later referred to as areas of loops or circles.
Feature Manipulation Engine (FME) was used to carry out the automatic processing for the assignment of the stream order working via drainage basins. A digital elevation model (DEM) of the drainage basin was used to determine headwater and downstream. Once the automation was complete, various manual corrections and updates were made, and ongoing maintenance of the dataset continues to improve and enhance the dataset.
Additional rules/exceptions devised for use in assigning the Strahler stream order method on the consistent Statewide 100k drainage network are as follows -
Stream order of one is not allowed to be reduced in its number no matter what the situation.
Stream order can be reduced in its value in the case where a watercourse splits into multiple courses, and a major and minor channel was identified.
Stream order can be reduced in its value (e.g. 3 to 2) where a watercourse splits into multiple courses. But if the split course with the reduced value (e.g., 2) joins a watercourse from other distinct regions (e.g. stream order of 2), the reduced stream order value (e.g. 2) must not be considered in the stream order calculation of Strahler method for the segment from that join point. (e.g., stream order value is kept 2 from the join point, so as not to introduce falsely higher stream orders).
Other regulations for stream order process
Use standardised naming for each copied working 100K network basin being stream ordered.
e.g. Gilbert_100K_SO
Reference
Strahler, A. N. (1957), "Quantitative analysis of watershed geomorphology". Transactions of the American Geophysical Union, 38, No. 6, 913–920.