2.1.4.1.1 Streamflow data
Streamflow data from 37 gauging stations were used in the rainfall-runoff model Australian Water Resources Assessment (AWRA) landscape model (AWRA-L) and river routing model (AWRA-R) in the bioregional assessment (Table 6). Of these, streamflow data from five catchments for AWRA-L calibration were located outside of the Namoi river basin (Figure 28). The following criteria were used in catchment selection for AWRA-L calibration such that they (i) have at least 10 years of data since 1983, (ii) are not impacted by major coal mine or other developments, (iii) have no regulation (e.g. dams, weirs), (iv) are close to the Namoi subregion and (v) are not nested within a larger catchment. All gauging stations used in AWRA-R river routing modelling of the Namoi river basin in an earlier study (Lerat et al., 2013) were also selected.
Although there is some paucity of streamflow gauging stations in the unregulated tributaries, the spatial density of the streamflow gauging stations seems adequate along the Namoi River and its anabranches downstream of Narrabri. Up to December 2012, the available streamflow data length ranges from 14 to 121 years including gaps. The median data length is 47.5 years and the mean is 54.0 years. One-quarter and three-quarters of all the stations have at least 71 and 36 years of streamflow data, respectively, however only 30 years of flow data from 1983 onwards are used in the modelling. Six stations had less than 30 years of data available since 1983.
Table 6 Details of the 37 streamflow gauging stations used in rainfall-runoff model and river routing model for the Namoi subregion
AWRA-L = Australian Water Resources Assessment landscape model, NA = data not available, D/S = downstream, U/S = upstream
Data: NSW Office of Water (Dataset 1, Dataset 2, Dataset 3)
Figure 28 Streamflow gauging stations given in Table 6 for the Namoi subregion
2.1.4.1.2 River cross-sections data
River cross-section data are used in AWRA-R to compute instream evapotranspiration and rainfall, instream capacity and losses to groundwater (Lerat et al., 2013; Dutta et al., 2014). The cross-sections for 23 streamflow gauges used in AWRA-R calibration were obtained from NSW Department of Primary Industries (Bioregional Assessment Programme, Dataset 4). Any potential changes in cross-section due to scour and re-deposition of sediment after peak flows are not considered.
AWRA-R simulations are done at locations where cross-section data are unavailable. Obtaining channel cross-sections requires detailed surveys which are time-consuming and carried out under strict guidelines (Stewardson et al., 2005). Regional hydraulic geometry models can be obtained using proxies that can be readily obtained (e.g. catchment area and mean annual streamflow). Using data from about 400 stations in Queensland, Tennakoon and Marsh (2007) developed functional relationships of modest explanatory value (r2≈0.3) between top width and mean channel depth with catchment area and mean annual streamflow. The cross-section for the remaining 21 streams at the outlet is determined by assuming a trapezoidal shaped cross-section with bottom channel width (L) and height (H) (Figure 29) able to accommodate AWRA-L simulated maximum streamflow. The 21 stream nodes where the assumed trapezoidal sections were used are: 9, 12, 14, 15, 16, 17, 19, 20, 21, 22, 24, 25, 27, 30, 31, 33, 34, 36, 37, 38 and 39. Figure 4 in companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018) shows the location of these nodes.
Figure 29 Shape of trapezoidal section assumed for headwater catchments in AWRA-R simulation
AWRA-R = Australian Water Resources Assessment river model, H = height, L = width
The flow equation for a trapezoidal weir with side slopes vertical to horizontal ratio of 4 to 1 estimates height (H) for a given flow Q as:
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(4) |
where Cd is the coefficient of discharge (assumed as 0.62 for Cippoletti weir; Daugherty and Franzini, 1965) and g is gravity acceleration (9.81 ms-2). Using the simulated maximum flow value for the headwater catchment, the high coefficient of discharge for a trapezoidal weir was adjusted to match the maximum flow height for a nearby catchment assuming that the adjusted Cd takes care of roughness of the channel (e.g. Manning’s n) and other variables that govern flow and channel cross-sectional area relationship. It is also assumed that cross-sections at a nearby gauging station with a comparable catchment area or at a gauging station with comparable mean annual streamflow provides a reasonable estimate of bottom channel width (Bioregional Assessment Programme, Dataset 4). The Cippoletti weir cross-sections do not incorporate overbank geometry, thus the assumption is reasonable for the stream cross-sections for the headwater catchments which are unlikely to overtop the stream bank.
This process may be simplistic but there are no suitable data to evaluate the approach including data related to calculating the flow using the Manning’s equation. Any systematic errors may be compensated through calibration. Furthermore, as the Bioregional Assessment Programme is reporting on the relative difference of hydrological response variables between the baseline and coal resource development pathway (CRDP), any error introduced by the above assumption would cancel out.
2.1.4.1.3 River reach lengths
River reach lengths are used in AWRA-R to compute instream actual evapotranspiration and rainfall fluxes, instream capacity and groundwater recharge from irrigated areas (Lerat et al., 2013; Dutta et al., 2014).
Reach lengths are quantified for all rivers in the reach, including the main channel and tributary channels. River reach lengths are obtained from the River Styles spatial layer for NSW, obtained through digitisation of high resolution aerial or satellite imagery with field validation from different sources (NSW Office of Water, Dataset 5). Visual assessment showed that these data were more accurate than drainage networks derived from the DEM data, particularly in meandering sections of the river. The river reach was clipped using catchment boundaries defined in the AWRA-R modelling domain (see Section 2.6.1.3 in companion product 2.6.1 for the Namoi subregion (Aryal et al., 2018)); and each river reach length was manually computed using GIS software (Bioregional Assessment Programme, Dataset 4). These lengths are planar and can be different from on-ground lengths, particularly in steep areas.
2.1.4.1.4 Irrigation areas and crop types
The AWRA-R river model needs details of irrigated areas and crop types in each river reach in which irrigation is present in order to determine areal extent and crop factors of the most common crop types (Dutta et al., 2014) to calculate water usage by irrigated crop.
Areas and crop types for each reach are sourced from the Namoi Integrated Quantity-Quality Model (IQQM) (Bioregional Assessment Programme, Dataset 6). The information in the Namoi IQQM was summarised by reach in order to determine crop types and associated crop factors (Bioregional Assessment Programme, Dataset 6).
