Abstract
Abstract. Streamflow permanence is an aspect of great legal importance in Brazil, because streams, depending on their flow regime, are protected or not by law. Various methods, from field methods to computational methods, are used to determine the flow regime of streams, however some are too time spending and computational methods usually need gaging information as input. Furthermore, computational methods used to extract drainage networks do not identify the flow regimes of streams, and modelled drainage networks always need to be refined manually, as some authors indicate that up to 55% of modelled drainage length is ephemeral in some cases. This work proposes a semi-automatic computational method to determine the flow regime of first order streams, which uses 11 morphometric attributes of the mini drainage basins of these streams to develop a classification model using decision tree algorithms. WEKA package was used to perform the data mining process, which resulted on the development of a compact 8 node decision tree. Ten-fold stratified cross-validation was used to validate the model, which obtained an accuracy rate of 70%. The drainage network of the study area extracted with the classical approach was refined after the result of the classification was obtained. Quantitative analysis of channel length by Strahler order shows an overall reduction of 25% in channel length after refinement was undertaken, and for 1st order streams, as much as 31% were classified as ephemeral. Modelling the drainage areas of headwater streams represents a new approach to determining stream flow permanence, and inclusion of new attributes in the model may yield better results in future research.
Highlights
The flow regime of rivers is an aspect of great legal importance in Brazil, as the perennial and intermittent streams are protected by the Brazilian Forest Code (BRASIL, 2012), while ephemeral ones are not
Junior & Andreoli (2015) used hydrological parameters and water balance to determine the best timing of the year for field investigations of stream; Panero et al (2006) performed a principal component analysis on hydrochemical data to classify perennial and intermittent rivers; the NC Division of Water Quality (2005) has drawn up a manual and a form to determine the flow regime of stream in the field; Porras & Scoggins (2013) determined the probability of stream being perennial, through statistical analysis of river monitoring data; Jaeger et al (2019) used the PROSPER model, developed by the USGS to determine the probability of stream being perennial; Williamson et al (2015) used the TOPMODEL model to classify the flow regime of stream
To run the model and classify the mini-basins as perennial or ephemeral, the decision tree built in the Waikato Environment for Knowledge Analysis (WEKA) was transposed to the IDL/ENVI environment, which allows the raster classification to be performed
Summary
The flow regime of rivers is an aspect of great legal importance in Brazil, as the perennial and intermittent streams are protected by the Brazilian Forest Code (BRASIL, 2012), while ephemeral ones are not. It is known that the lower the Strahler order of a stream, the greater the chance of it being ephemeral (NC Division of Water Quality, 2005) This is true for hydrological models used in the extraction of drainage networks, in which much of the hydrography generated is ephemeral, because the models either perform the extraction of features from the terrain (data mining models), or are obtained by the classical methodology, based on a contribution area threshold above which the pixels are considered as part of the drainage network (O'CALLAGHAN & MARK, 1984). In this work perennial river are those which maintain streamflow over at least 90% of the year; intermittent rivers maintain streamflow only during the rainy season and are associated to arid climates; and ephemeral streams only exist during or immediately after a rainfall event
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