PUBLICATIONS. Water Resources Research. A new method for verification of delineated channel networks RESEARCH ARTICLE 10.

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1 PUBLICATIONS RESEARCH ARTICLE Key Points: Verification of delineated channel network Incision indices for checking the accuracy of channel network Appropriate thresholds for delineating channels Correspondence to: A. Sharma, Citation: Liu, Z., U. Khan, and A. Sharma (2014), A new method for verification of delineated channel networks, Water Resour. Res., 50, , doi:. Received 18 JUNE 2013 Accepted 12 FEB 2014 Accepted article online 19 FEB 2014 Published online 10 MAR 2014 A new method for verification of delineated channel networks Zhe Liu 1, Urooj Khan 1, and Ashish Sharma 1 1 School of Civil and Environmental Engineering, University of New South Wales, Sydney, New South Wales, Australia Abstract Several methods are used to delineate channel networks. The most widely used are the contributing area method, area-slope method, and grid network ordering method. The number of delineated channels depends on the threshold adopted when using each method. However, the appropriate threshold value required to delineate channel networks, and their corresponding accuracies, are still uncertain. The consistency between the delineated channels and actual channels can be evaluated by carrying out extensive field surveys, but these require significant time and cost. Accurate knowledge of delineated channel networks is vital, and is achievable more efficiently and simply. A new method of calculating the accuracy of delineated channel networks is introduced in this study. cross-section profiles throughout the channel network were examined and three new incision indices were derived: an incised channel index, a partially incised channel index, and a nonincised channel index. The indices were found useful for setting appropriate threshold values for actual channel networks. Three small catchments in Wellington, New South Wales (NSW), Australia, were investigated in this study. 1. Introduction Digital elevation models (DEM) have become increasingly common in the past few decades for terrain analysis and spatial modeling, one important use being automatic delineation of channel networks. Several methods have been proposed for this delineation [Costa-Cabral and Burges, 1994; Montgomery and Dietrich, 1988; Montgomery and Foufoula-Georgiou, 1993; Tarboton et al., 1988, 1991; Wilson and Gallant, 2000] three of which are well established and widely used: the contributing area method, the area-slope method, and the grid network ordering method. However, each of the said methods has certain requirements and limitations that make their universal applicability limited. The contributing area method requires a supporting area threshold. The channel networks are delineated when the flow accumulation area at a grid cell exceeds this threshold value [O Callaghan and Mark, 1984]. The area-slope method is slope-dependent; a threshold C is calculated from C 5 AS 2, where A is the specific catchment area and S is the local slope [Montgomery and Dietrich, 1988, 1989, 1992, 1994]. The grid network ordering method [Peckham, 1995] was developed from Strahler s stream order theory [Strahler, 1952, 1957], in which only those channels in which the grid order is greater than a certain threshold are extracted, and those below the threshold are deleted. networks delineated by any of these three methods, which are grid-based, are not totally consistent with real channel networks in the field. In particular, it has been noted that the origins of channel networks are not precisely located at the actual channel heads [Imaizumi et al., 2010; Martz, 1995]. The accuracy of delineated channel networks varies from method to method, and depends on the associated threshold values [Orlandini et al., 2011]. Accurate delineation of channel networks is important in many areas of investigation, including geomorphology, hydrology, eco-hydrology, and related disciplines [Band, 1986]. In hydrology, the accurate delineation of channel networks is useful in channel routing, distributed hydrological modeling, estimation of source areas, drainage density, and flow traveling time [Dietrich and Dunne, 1993]. In eco-hydrology and geomorphology, the accurate channel network delineation can be used in differentiation of diffusive and fluvial processes dominated regions, estimating sediment load, pollutants, and other ecological elements [Allan and Castillo, 2007; Clarke et al., 2008; Hynes, 1970; Nadeau and Rains, 2007]. The need for an accurate delineation becomes all the more important when the hydrologic model is not fitted to observed flows, but formulated based on measurements of catchment geomorphology and other characteristics. A mis-specification of the channel network in such cases is often a direct cause for bias in the hydrologic simulations at the catchment outlet. Some problems still remain in accurately delineating LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2164

2 channel network and simulating the position of channel heads using DEM in computational hydrology and geomorphology [Imaizumi et al., 2010; Orlandini et al., 2011]. Orlandini et al. [2011] demonstrated that the most appropriate delineation method for different catchments varies with the geology, topography, and morphology of catchments. Field work in the Tennessee Valley by Montgomery and Dietrich [1992] and Dietrich et al. [1993] revealed that the exact thresholds required to delineate different first-order streams or channel heads in a catchment are themselves significantly different, even when using the same channel delineation method for all streams. Istanbulluoglu [2002] noted this variation in thresholds in initiating channels and proposed a model to simulate the variation. Montgomery and Dietrich [1992] suggested that the variation may be due to the range of hydrological and erosional processes for channel initiation within the catchment. For these reasons, a method has been proposed in the present study for evaluating the accuracy of delineated channel networks, and for determining the threshold values that are appropriate for different catchments. One of the difficulties in assessing the accuracy of delineated channel networks is the lack of appropriate indices or measures that can be used to establish the utility of one method versus another. To address this, three incision indices were developed in this study, based on channel cross-sectional shape (referred to here as the channel cross-section analysis (CCSA) method) in which the cross sections of all delineated channels are analyzed to determine the incision index throughout their entire length. The networks are then delineated by the above three methods and a range of threshold values for each. The CCSA, along with the three incision indices, determine the accuracy and appropriate threshold values for channel network delineation. 2. Background The location of the point where nonconfined flows converge into a confined flow with a definite path contained by banks is defined as the first-order stream or the channel head; it is the limit of the upstream boundary of the channel network [Anderson et al., 1997; Dietrich and Dunne, 1993; Montgomery and Dietrich, 1988, 1989]. networks comprise the distribution of tributaries and main streams in a catchment [Leopold et al., 1964], theoretically consisting of minor rills and ephemeral/perennial channels that show a clear watercourse in the headwater areas [Orlandini et al., 2011]. Depending on their geomorphological formation process, channel heads are divided into three categories: 1. Overland flow (OF) [Kirkby and Chorley, 1967]: OF channel heads occur when the accumulated runoff exceeds the sediment erosion threshold [Julian et al., 2012]. 2. Seepage erosion (SE) [Dunne, 1980]: SE channel heads are formed in gently sloping and flat areas where subsurface water flows reach the land surface [Julian et al., 2012] due to overland flow, and causes erosional seepage [Montgomery and Dietrich, 1989]. Most SE channel heads are around 1 m wide m deep. 3. Shallow landsliding (SLS) [Dietrich et al., 1986]: SLS channel heads form when landslides remove some accumulated colluvium in hollows [Dietrich et al., 1986]. Most SLS channel heads are found in steep areas [Montgomery and Dietrich, 1989] and are usually >1 m wide. Table 1 presents the geomorphological properties of channel heads in different catchment types, and lists channel heads ranging from 0.01 to 0.5 m width 3 approximately 1 m depth in flat areas. cross sections near channel heads, and downstream of the channel heads, resemble a U or V shape. Chorowicz et al. [1992] delineated channel networks automatically by giving first priority to U or V channel shapes. This method achieved relatively reasonable results, but the accuracy continued to be limited by the coarseness of the DEM grid (50 m 3 50 m) that was available at that time. Khan et al. [2013] briefly examined a random selection of channel cross sections at the start of networks in large catchments to verify the threshold between regions dominated by either diffusive or fluvial processes (although it is noted that this was not the main aim of their research). The width of channel heads in plain areas is approximately 1 m (Table 1). High-resolution (1 m 3 1 m) DEM data are essential for identifying the exact locations of channel heads. The lack of high-resolution DEM data in the past 20 years has hindered research focusing on the precise location of channel heads [Dietrich et al., 1993; Ijjasz-Vasquez and Bras, 1995; Montgomery and Foufoula-Georgiou, 1993]. The resolution of DEM data LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2165

3 Table 1. Geomorphological Properties of Heads in Various Catchments Catchments Head Width (m) Depth (m) Catchment Area (m 2 ) Terrain Types Reference Trapper Creek, United States (specific Fairly steep OF and SE Istanbulluoglu [2002] catchment area) Black Hills and Willapa Hills, 81 < From moderate to steep OF, SE, and SLS Jaeger et al. [2007] United States Higashi-gouchi, Japan 26 >5 N/A Steep Mountain SLS Imaizumi et al. [2010] 50 <5 < ,000 Steep Mountain OF 72 <5 < ,000 Steep Mountain SE In the mid-atlantic, ,000 Ridge and valley SE Julian et al. [2012] United States ,000 Blue ridge N/A ,900 Piedmont N/A ,400 Coastal plain OF Marin County, United States N/A N/A 1 2 N/A Moderately steep SE Prosser and N/A 1 <0.5 N/A Gentle OF Dietrich [1995] is continuously improving, especially with the development of light detection and ranging (Lidar) technology, and this makes the use of detailed terrain analysis methods a meaningful task [Orlandini et al., 2003; Seibert and McGlynn, 2007; Tarboton, 1997; Tarboton and Baker, 2008]. In the present work, a 1 m 3 1m resolution DEM data generated from Lidar were used to accurately delineate first-order streams and precisely locate channel heads. 3. Study Area 3.1. Catchment Location The study areas were located in Wellington, which is an inland area in New South Wales (NSW), Australia. Three small catchments that contained no farm dams (Wellington catchments A, B, and C) were selected for this study (Figure 1). The catchments were located in flat terrain; and their areas were km 2 (A), km 2 (B), and km 2 (C), respectively. The latitude and longitude of centroid of Wellington catchments A, B, and C are S, E, S, E, and S, E, respectively Data NSW Land and Property Information (LPI), a division of the NSW Department of Finance and Services, provided high-resolution DEM (1 m 3 1 m) data in ARC/INFO ASCII Esri grid format and red/green/blue (RGB) aerial imagery of the Wellington area, created in March The images are in enhanced compression wavelet (ECW)-format orthorectified photographic mosaics, which were available at 50 cm resolution throughout the Wellington area, and at 20 cm resolution in the north-eastern region where catchments A and C are located. While the high resolution of the images played an important role in clearly showing the first-order streams and channel heads, sometimes the observations of channels and channel heads are limited due to the presence of vegetation because the size of channels near the channel heads can be much smaller than trees in some of these locations. 4. Methodology 4.1. Cross-Section Analysis (CCSA) Method Wellington catchment A was selected for the development of the CCSA method. First, to define the incision criterion for all channels in the Wellington catchments, 15 channel heads were identified in catchment A from the RGB images (Figures 2a and 2b). Then, using the ArcGIS system, a cross section of each channel head was drawn perpendicular to the flow direction. From this, cross-section profiles were prepared near the channel heads. The shapes and geometrical properties of the profiles were then analyzed for each channel (Table 2). It was found that: (1) all cross sections were either V or U-shaped and (2) for a width of 5 m (see below), the depth of the cross sections was 0.25 m. The similarities in geometry of cross sections, i.e., the V or U shape, are applicable for all channels and catchments, whereas the threshold of a certain depth in a particular width may vary in different topography regions. Cross-section profiles for two sample channel heads are shown in Figures 2c and 2d. LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2166

4 Figure 1. (a) Wellington catchment A, (b) Wellington catchment B, and (c) Wellington catchment C. The resolution of RGB images for Wellington catchments A and C is 20 cm. Resolution for catchment B is 50 cm. The nominal 5 m width mentioned above was dictated by the DEM resolution: since individual pixels represented 1 m 3 1 m on the ground, 5 m was equivalent to five pixels two on the left of the channel centre, two on the right, and one in the center. This allowed the cross-sectional shape of the channel to be determined. Although a 3 m width with one pixel on the left, one right, and one center would also give the shape in theory, the results may be misleading if local depressions are present. After performing this analysis and referring Table 1, it was determined that, in the Wellington catchments, the CCSA criterion for an incised channel was, for a width of 5 m, the depth of cross section 0.25 m, and either V or U cross-sectional shape. A channel network was delineated for Wellington catchment A using the contributing area method at a threshold of 5000 m 2 (Figure 2b). Equally spaced (20 m distance) cross sections were drawn for all delineated channels and inspected to check whether they met the CCSA criterion. Initially, larger widths of cross section (around m, or more if needed) are considered to check the shape of channel and valley, after that the CCSA criterion is applied in central 5 m width. If some cross sections, especially in the upper reaches which were not clear or doubtful, more frequent cross sections at 3 m spacing were analyzed. The delineated channels were then divided into three categories: LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2167

5 Figure 2. (a) Wellington catchment A with 20 cm resolution RGB images. (b) Wellington catchment A with delineated channel network using contribution area method at the threshold of (c and d) Two examples of channel head cross sections which are incised in the sense of the index definition. (e) Cross section at upstream end of nonincised channel. Note that this cross section does not have U or V shape. 1. Incised channels, defined above; they are shown by blue lines in Figure 2b. 2. Partially incised channels, in which the cross sections met the CCSA criterion over only part of the channel length (mostly downstream). These channels extended further upstream toward their origin points/channels heads, as shown by green lines in Figure 2b. 3. Nonincised channels, which did not meet the CCSA criterion anywhere along their entire length (red lines, Figure 2b). Their cross sections were generally either straight lines or irregular shapes (i.e., neither U nor V shape), and therefore did not meet the criterion for incised or partially incised channels (Figure 2e). Nonincised channels were not visible in the RGB images. Contributing area method and area-slope method adopt D-infinity flow direction algorithm [Tarboton, 1997], while grid network ordering method uses D8 algorithm [O Callaghan and Mark, 1984]. LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2168

6 Table 2. Geometrical Properties of Cross Sections Near Heads Number Width (5 m)/depth (m) Incision Index It was noted that some of the delineated channels did not exist in the field. The number of false channels affects the accuracy of the delineated channel networks. Since no index had previously been put forward for analyzing network accuracy, three incision indices were introduced for this purpose in the present work: 1. Incised channel index (%): incised channels Incised channel index (1) Total number of delineated channels 2. Partially incised channel index (%): partially incised channels Partially incised channel index (2) Total number of delineated channels 3. Nonincised channel index (%): non-incised channels Non-incised channel index (3) Total number of delineated channels The sum of the three indices should always be equal to 100% Application of CCSA Method The incision indices were calculated for Wellington catchments B and C by the CCSA method. To calculate incision indices, a variety of channel networks were delineated using: 1. The contributing area method at thresholds of 15,000, 10,000, 5000, 2000, and 1000 m The area-slope method at thresholds of 2000, 1000, 500, 200, 100, and 75 m The grid network ordering method at the thresholds of order 8, 7, 6, and 5. First, the channel networks are delineated corresponding to low thresholds in each delineation method. Then, the cross sections are drawn perpendicular to these delineated channels near the upstream end. If the cross-section profiles met the CCSA criterion, they were categorized as incised channels, and the number of channels in this category was counted. If the upstream cross-section profiles did not meet the CCSA criterion, then a number of cross sections were drawn further downstream to determine whether the channels were incised there; if the CCSA criterion was met in the downstream cross sections, these channels were categorized as partially incised channels, and the number of these channels was counted. If the channel cross-section profiles did not meet the CCSA criterion at any point along the length of the channel, they were categorized as nonincised channels, and the number of these channels was counted. From these totals, the three incision indices were calculated from equations (1 3). The same method is repeated corresponding to higher thresholds and the incision indices are calculated again. The values of incision indices for catchments B and C, using the above three network delineation methods for all thresholds, are given in Figures 3a 3f and Table 3. (Note that the RGB images were not used in calculating the incision indices by the CCSA method.) While in this study no discontinuous channels were identified, if discontinuous channels are present then special attention needs to be given to categorize these channels. More cross sections need to be drawn LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2169

7 Figure 3. Incision indices for: (a) Wellington catchment B, (b) Wellington catchment C, using contributing area method for channel network delineation, (c) Wellington catchment B, (d) Wellington catchment C, using area-slope method for channel network delineation, (e) Wellington catchment B, and (f) Wellington catchment C, using grid network ordering method for channel network delineation. upstream and downstream of the discontinuous section. If both upstream and downstream cross sections meet the CCSA criterion, the channel can then be categorized as an incised channel Validation of CCSA Method To verify the above incision indices, an alternative set of incision indices was calculated using the RGB images (referred to as the incision index from RGB ). To calculate these indices, the channel heads were located on RGB images in catchments B and C and the channel networks were delineated using the same methods and thresholds as above. After that the channels delineated by the above three methods at all thresholds were compared with the actual channels visible in the RGB images. LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2170

8 Table 3. Incision Indices of Networks Delineated From Three Network Delineation Methods at Different Thresholds for Wellington Catchments B and C a Delineation Methods Thresholds Catchments Incised Index (%) CCSA Partially Incised Index (%) Nonincised Index (%) Incised Index (%) RGB Partially Incised Index (%) Contribution Area 15,000 B C ,000 B C B C B C B C Area Slope 2000 B C B C B C B C B C B C Grid Network Ordering Order 8 B C Order 7 B C Order 6 B C Order 5 B C a Incision indices calculated using: (1) CCSA method and (2) RGB images. Nonincised Index (%) If the entire length of a specific channel was exactly superposed on the actual channel in the RGB image they were categorized as incised channels, that is, incised for their whole length, following which the number of channels in this category was counted. If the delineated channels appeared to extend upstream beyond the channel heads on the RGB images they were categorized as partially incised channels, and the number of such channels was counted. If the delineated channels were not visible in the RGB images they were categorized as nonincised channels, and the number of such channels was counted. From these totals, the three incision indices were calculated from equations (1) (3) and are presented in Figures 3a 3f and Table 3. The total numbers of channels in each category for both the CCSA method and the RGB image method, using the three methods of channel network delineations, are given in Figures 4a 4f and Table Discussion 5.1. Variation of Incision Indices The incision indices calculated by the CCSA method and the RGB images agreed very closely in catchments B and C, as determined by all three channel network delineation methods (Figures 3a 3f and Table 3). For all three methods, only the major channels were delineated when the thresholds were set at a very high value; since these were very well-incised channels, only the incised channel index was calculated and the other two indices took a zero value (Figures 3a 3f and Table 3). Thus no minor channels small tributaries or first-order streams were detected in this delineation. LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2171

9 Figure 4. incised, partially incised and nonincised channels for: (a) Wellington catchment B, (b) Wellington catchment C, using contributing area method for channel network delineation, (c) Wellington catchment B, (d) Wellington catchment C, using area-slope method for channel network delineation, (e) Wellington catchment B, and (f) Wellington catchment C, using grid network ordering method for channel network delineation. Conversely, at very low threshold values, almost all major and minor channels were detected, but several false (nonexistent) channels were also indicated; therefore the nonincised channel index was significantly higher (Figures 3a 3f and Table 3). Further, several channels appeared to extend beyond their origin points/channel heads; thus the partially incised channel index was high (Figures 3a 3f and Table 3). The delineated channel networks from intermediate threshold values generally showed relatively reasonable results that were consistent with actual channels, where the nonincised and partially incised channel indices took reasonable values (Figures 3a 3f and Table 3). By the contributing area method, the 5000 m 2 threshold produced reasonable values of incision indices for both catchments B and C (Figures 3a and 3b and Table 3), with the number of nonincised and partially incised channels being reasonably low. A threshold of 200 m 2 was appropriate for the area-slope method, and a threshold of six was appropriate for the LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2172

10 Table 4. Incised s, Partially Incised s and Nonincised s for Three Network Delineation Methods at Various Thresholds for Both CCSA Method and RGB Imagery in Wellington Catchments B and C Delineation Methods Thresholds Catchments Contribution Area Incised s CCSA Partially Incised s Nonincised s Total Incised s RGB Partially Incised s Nonincised s 15,000 B C ,000 B C B C B C B C Area Slope 2000 B C B C B C B C B C B C Grid Network Ordering Order 8 B C Order 7 B C Order 6 B C Order 5 B C Total grid network ordering method for both catchments (Figures 3c 3f and Table 3). Comparison of the channel delineation methods showed that the area-slope method was the most accurate of the three. This is consistent with previous reports that concluded that the channel head locations are strongly controlled by the area-slope threshold, especially in regions where the channel heads were not formed by landslides [Dietrich et al., 1993, 1992; Hattanji et al., 2006; Montgomery and Dietrich, 1994]. The delineated channels corresponding to a 5000 m 2 threshold for contributing area method, a 200 m 2 threshold for the area-slope method, and a threshold of six for the grid network ordering, are verified by overlaying the raw area-slope point of the extracted channel network on a binned area-slope curve for the entire catchment (figure not shown here but available from the authors on request). The raw area-slope points corresponding to the extracted channels are found to be located in the fluvial process dominated region, i.e., after turnover of the areaslope curve, which confirms that the selected thresholds are appropriate. The differences between the CCSA and RGB image incision indices were negligible (Figures 3a 3f and Table 3). Therefore, should RGB imagery not be available for a particular region, the CCSA method can also be used to calculate the incision indices if proper incision criterion threshold is set on the basis of Table 1 and analyzing some incised cross sections in that region. After that, an appropriate threshold for each delineation method can be estimated from the incision indices for river network delineation. Such incision indices are highly useful in hydrological and geomorphological investigations for delineating appropriate channel networks that have few nonincised and partially incised channels. High-resolution DEM data (1 m 3 1mor finer) are essential for applying the CCSA method and calculating incision indices because the width of most channel heads in flat terrain is around 1 m (Table 1). Coarse DEM (e.g., 25 m 3 25 m or larger) cannot detect first-order streams adequately because the grid size is much larger than the width of the channel heads/first-order channels. As indicated in section 4.1, 1 m 3 1 m resolution DEM was adequate to capture the shape of channel heads/first-order channels in the flat Wellington catchments. LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2173

11 The CCSA method is sensitive to the incision criterion for the channel. The incision criterion of 5 m wide 3 25 cm deep is carefully decided after analyzing Wellington catchment A and channel heads information from previous researches, presented in Table 1. The more accurate assessment of incision criterion can be made if high resolution DEM data, i.e., 25 cm and 50 cm, is available in the future. Other catchments require that an appropriate incision criterion ought to be established before applying the CCSA method. One of the limitations of the CCSA method is that it is unable to differentiate between channels and valleys. The channel may not exist at the valley bottom but if the valley is too narrow and fulfils the CCSA criterion, it will be categorized as an incised channel. This limitation can be overcome if higher resolution DEM/Lidar data, i.e., 25 cm 3 25 cm or 50 cm 3 50 cm, becomes available in future, which will help draw more accurate and realistic channel cross-section profiles and mark the channel banks Variation of Incised, Partially Incised, and Nonincised s The numbers of incised, partially incised, and nonincised channels obtained by the CCSA method and from RGB imagery were very similar, but there were slight differences in all methods of river network delineation (Figures 4a 4f and Table 4). Such differences were negligible for higher thresholds but somewhat greater for low thresholds. At higher thresholds, fewer channels are delineated, but only incised channels are represented. This result was obtained by the CCSA method and confirmed on RGB images, with little or no distinction between the two. At lower thresholds, however, significantly more channels were delineated, but some of these channels did not satisfy the CCSA criterion due to their complex cross-sectional shapes, but were clearly visible in the images (Figures 4a 4f and Table 4). Another reason for the differences is that some channels that satisfied the CCSA criterion were not clearly visible in the images due to the presence of large overhanging trees, dense shrubs, etc. Another cause was the higher resolution of the RGB images (50 cm for catchment B and 20 cm for catchment C). Due to the coarser resolution of the catchment B images, some channels were not clearly visible despite their U or V crosssectional shape. The discrepancy between the number of channels calculated by CCSA and visible in RGB images was therefore slightly greater in catchment B (Figures 4a, 4c, and 4e) than in catchment C (Figures 4b, 4d, and 4f and Table 4) but, overall, the numbers of incised, partially incised, and nonincised channels were consistent. 6. Conclusion A new CCSA method along with three incision indices incised channel index, partially incised channel index, and nonincised channel index is introduced here to test the accuracy of delineated channel networks. The method was developed for Wellington catchment A and applied in Wellington catchments B and C. The incision indices calculated using the CCSA method were verified against RGB images of the areas. Agreement was found to be very close, with minor discrepancies due to the complex cross-sectional channel shapes in some regions, different image resolutions in the two catchments, and some natural obstructions to visibility in the images in places (large trees, dense shrubs, etc.). High-resolution DEM data, 1 m 3 1 m or finer, are essential for the CCSA method and for calculation of incision indices. Coarser resolution would affect the accuracy of the results and would be unable to distinguish narrow, shallow first-order channels. The CCSA method along with incision indices proved useful for testing the accuracy of delineated channel networks at various thresholds using the three commonly applied channel delineation methods: contributing area, area-slope, and grid network ordering. The CCSA method and incision indices were also helpful in assigning the appropriate threshold for channel network delineation in order to avoid the production of large numbers of nonincised and partially incised channels. Like other methods, the CCSA method also has some limitations. The CCSA method is sensitive to the choice of the incision criterion threshold and a proper threshold needs to be set before applying this method. The CCSA method is unable to differentiate between valleys and channels especially if the valleys are too narrow. This limitation will be overcome if high resolution, i.e., 25 or 50 cm DEM/Lidar data becomes available. An appropriate measure of accuracy in channel network delineation, and the appropriate thresholds, is essential requirements in hydrological and geomorphological studies. Prior to this study, no such methods LIU ET AL. VC American Geophysical Union. All Rights Reserved. 2174

12 or indices existed. The use of RGB imagery in this analysis proved an efficient and economical alternative to cumbersome and costly field surveys. Acknowledgments All the authors are most grateful to NSW Land and Property Information (LPI), who provided the highresolution DEM data and RGB imagery. This research could not have been conducted without this data. We also acknowledge useful discussions with Narendra Kumar Tuteja, who has been associated in similar types of research with the second and third authors. References Allan, J. D., and M. M. Castillo (2007), Stream Ecology: Structure and Function of Running Waters, Springer, Dordrecht, The Netherlands. Anderson, S. P., W. E. Dietrich, D. R. Montgomery, R. Torres, M. E. Conrad, and K. Loague (1997), Subsurface flow paths in a steep, unchanneled catchment, Water Resour. Res., 33(12), Band, L. E. (1986), Topographic partition of watersheds with digital elevation models, Water Resour. Res., 22(1), Chorowicz, J., C. Ichoku, S. 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