Waitaki WQ Modelling Draft Report
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Waitaki Water Quality Catchment Modelling Prepared for Environment Canterbury May 2015 Prepared by : Christopher Palliser Sandy Elliott Sharleen Yalden Ude Shankar For any information regarding this report please contact: Christopher Palliser Scientist Catchment Processes +64-7-856 1748 [email protected] National Institute of Water & Atmospheric Research Ltd PO Box 11115 Hamilton 3251 Phone +64 7 856 7026 NIWA CLIENT REPORT No: HAM2015-002 Report date: May 2015 NIWA Project: ENC14202 Quality Assurance Statement Dr Annette Semadeni-Davies Reviewed by: Urban aquatic scientist NIWA Auckland Formatting checked by: A. Bartley Approved for release by: D. Roper © All rights reserved. This publication may not be reproduced or copied in any form without the permission of the copyright owner(s). Such permission is only to be given in accordance with the terms of the client’s contract with NIWA. This copyright extends to all forms of copying and any storage of material in any kind of information retrieval system. Whilst NIWA has used all reasonable endeavours to ensure that the information contained in this document is accurate, NIWA does not give any express or implied warranty as to the completeness of the information contained herein, or that it will be suitable for any purpose(s) other than those specifically contemplated during the Project or agreed by NIWA and the Client. Contents Executive summary ............................................................................................................. 5 1 Introduction .............................................................................................................. 6 2 Method ..................................................................................................................... 9 2.1 The CLUES model ...................................................................................................... 9 2.2 Measured water quality and flow data .................................................................. 25 2.3 Determination of suitable calibration sites ............................................................ 36 2.4 Calibration ............................................................................................................... 39 3 Results .................................................................................................................... 43 3.1 Model fit and uncertainty ....................................................................................... 43 4 Summary and conclusions ........................................................................................ 52 5 References ............................................................................................................... 53 Appendix A Waitaki sub-catchments ................................................................... 55 Appendix B Percentage of land use by area for each of the watersheds of the calibrated sites ................................................................................. 58 Appendix C TN and TP leaching rates .................................................................. 64 Appendix D Flow diversions ................................................................................ 87 Appendix E Detrended concentrations ................................................................ 91 Tables Table 2-1: Map key and annual nutrient loads for point sources. 12 Table 2-2: Combinations of land use, rainfall and soil types in the Upper and Lower Waitaki River catchments used in the customised version of CLUES (ECan, pers. comm .). 15 Table 2-3: Soils in the Upper Waitaki (ECan, pers. comm.). 17 Table 2-4: Soils in the Lower Waitaki (ECan, pers. comm .). 18 Table 2-5: Measured, default CLUES and customised CLUES flows for the water quality calibration sites in the Upper Waitaki. 20 Table 2-6: Measured, default CLUES and customised CLUES flows for the water quality calibration sites in the Lower Waitaki. 23 Table 2-7: Estimated flow diversions in the Waitaki River catchment. 25 Table 2-8: Measured median concentrations for TN and TP in the Upper Waitaki catchment. 28 Waitaki Water Quality Catchment Modelling Table 2-9: Measured median concentrations for TN and TP in the Lower Waitaki catchment. 32 Table 2-10: Map key for calibration sites. 37 Table 2-11: Adjusted TN and TP leaching rates for the Upper Waitaki. 40 Table 2-12: Adjusted TN and TP leaching rates for the Lower Waitaki. 42 Table 3-1: TN measured and modelled loads and yields at the calibration sites in the Upper Waitaki catchment. 44 Table 3-2: TP measured and modelled loads and yields at the calibration sites in the Upper Waitaki catchment. 45 Table 3-3: TN measured and modelled loads and yields at the calibration sites in the Lower Waitaki catchment. 47 Table 3-4: TP measured loads and yields at the calibration sites in the Lower Waitaki catchment (see Figure 2-4). 49 Table A-1: Figure 1-1 key for sub-catchments. 55 Table B-1: Percentage of land use by area for each of the watersheds of the calibrated water quality monitoring sites. 58 Table C-1: TN and TP leaching rates for the Upper Waitaki (ECan, pers. comm.). 64 Table C-2: TN leaching rates for the Lower Waitaki ( ECan, pers. comm .). 71 Figures Figure 1-1: Waitaki River sub-catchments showing streams of order ≥ 3 and lakes. 8 Figure 2-1: Schematic of SPARROW sources and transport. 10 Figure 2-2: Waitaki River catchment showing streams of order ≥ 3, lakes and point sources. 11 Figure 2-3: The land use in the customised CLUES (ECan, pers. comm .). 14 Figure 2-4: Waitaki River sub-catchments showing streams of order ≥ 3, lakes and calibration sites. 38 Figure 3-1: Measured vs modelled TN and TP loads for the calibration sites in the Upper Waitaki. 50 Figure 3-2: Measured vs modelled TN loads for the calibration sites in the Lower Waitaki. 51 Waitaki Water Quality Catchment Modelling Executive summary Environment Canterbury requires a whole-of-catchment model for the Waitaki River catchment to provide a catchment-wide view over the likely impacts of land use changes on nutrient (total nitrogen and total phosphorus) loads. While the CLUES (Catchment Land Use for Environmental Sustainability) model was identified as being suitable for this purpose, the model required customisation to increase the spatial representation of current land use and soil drainage characteristics because of the diverse nature of the land uses, soils and rainfall. The development of the customised version of CLUES and its calibration against measured nutrient loads are described in this report. Additionally, the methods used to calculate loads for calibration are described, the sites suitable for calibration are identified, and the modelled or predicted loads are compared with the measured ones. The model was applied and calibrated separately for the upper and lower sections of the Waitaki Catchment. The calibration results for the model varied with the Root Mean Square Error (RMSE) being 3 m 3/s for flow over the entire catchment, 34.35 t/y for total nitrogen in the Upper Waitaki catchment, 10.38 t/y for total nitrogen in the Lower Waitaki catchment, and 2.00 t/y for total phosphorus in the Upper Waitaki catchment. Environment Canterbury instructed that total phosphorus calibration was not to be done in the Lower Waitaki. Uncertainty in the modelled loads arises from a number of sources, most notably limited data availability for nitrogen and phosphorus concentrations, both in terms of the number of sites for which water quality is also recorded and the percentage of water quality readings from these sites that were censored. With respect to the latter, techniques were used to enable the use of these concentration data. There is also uncertainty around the flows connected to hydro schemes within the catchment that were used to calculate loads. It was therefore considered wisest to focus on modelled percentage changes when comparing the results from various scenarios. The CLUES model does not simulate groundwater and assumes that groundwater lags are zero (i.e., that stream concentrations reflect current land use). This means that the model calibration discussed here adjusted key coefficients (e.g., TN and TP yields and stream attenuation coefficients) to match current nutrient load observations. If there are significant groundwater lags in the region, then the CLUES results are likely to under-predict stream loads – that is, the effects of the recent growth in dairying will not yet be fully shown in the measured loads. Waitaki Water Quality Catchment Modelling 5 1 Introduction Environment Canterbury (ECan) requires a whole-of-catchment model for the Waitaki River catchment to provide a catchment-wide view of the likely impacts of land use changes on nutrient (total nitrogen or TN, and total phosphorus or TP) annual loads in the river drainage network. The Catchment Land Use for Environmental Sustainability model (CLUES; Elliott et al. 2008, Semadeni- Davies et al. 2011) was identified as a suitable model. This model has been set up on a regional basis and has been calibrated nationally. However, to update and improve the spatial representation of the catchment, the default CLUES model was customised and recalibrated specifically for this application. The model was customised (called customised CLUES in this report) in order to provide ECan with a model that is better able to provide robust, defensible estimates of nutrient loads under current and future scenarios at a limited number of nodes in the Waitaki River