MODELLING EFFECTS OF CLIMATE CHANGE AND URBANISATION ON SUPPLY RELIABILITY OF STORMWATER HARVESTING AND MANAGED AQUIFER RECHARGE

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1 MODELLING EFFECTS OF CLIMATE CHANGE AND URBANISATION ON SUPPLY RELIABILITY OF STORMWATER HARVESTING AND MANAGED AQUIFER RECHARGE Gonzalez, D. 1, Clark, R. 2, Dillon, P. 1, Charles, S. 3, Cresswell, D. 2, Naumann, B. 4 1 CSIRO Land and Water, PMB 2 Glen Osmond, SA 5064, Australia 2 Clark and Associates, Adelaide 3 CSIRO Land and Water, PMB 5 Wembley, WA 6014, Australia 4 City of Salisbury, PO Box 8, Salisbury SA, 5108, Australia ABSTRACT Stormwater harvesting and Managed Aquifer Recharge (MAR) with corresponding treatment can economically deliver safe water supplies. Uptake will depend on accurately defining the reliability of supplies, particularly when banking water in a brackish aquifer. To address supply reliability iterative hydrological modelling was performed based on the Parafield stormwater harvesting MAR system in Salisbury, South Australia. The effects of reduced runoff from lower rainfall under a drying climate and enhanced runoff from increased impervious area were compared with a base case representing historical rainfall patterns and current catchment conditions. The climate change scenario with 7% less rainfall reduced demand met by 3% with an annual reliability of 99.5%. Increasing the impervious area within the catchment by 20% resulted in 8% more demand met with the same reliability, thus more than compensating for the reduction under climate change. INTRODUCTION Previous research on stormwater harvesting and Managed Aquifer Recharge (MAR) has focussed on water quality, risk management, economics, public acceptance and impacts to infrastructure (Dillon et al., 2014). Increasing the uptake of stormwater harvesting and MAR will depend on accurately defining supply reliability and water quality, particularly when banking water in a brackish aquifer. Climate observations and predictions for southern Australia indicate drier than average conditions could be expected in the future (Charles et al., 2003). Urban development and consolidation in growing cities is expected to result in increased runoff due to an increase in impervious areas. While urban stormwater is a climate-dependent water source, impervious catchments may be more resilient to reduced runoff than pervious catchments and if combined with MAR, could provide a reliable alternative water supply throughout droughts or a drying climate. In this study the contrasting effects of a drying climate and urban consolidation on the volumetric supply reliability of a stormwater MAR system were compared using an iterative application of the WaterCress hydrologic model. Average annual reliability was calculated from 100 iterations of 51 years. The Parafield Stormwater Harvesting Scheme in Salisbury, South Australia, that supplies water for non-potable uses, was used as the case study site. A base case was set to represent the current climate and catchment conditions and scenarios representing a drying climate and increase in impervious area were run separately and compared. METHODOLOGY The Water Community Resource Evaluation and Simulation System or WaterCress model was used to perform all simulations on a daily time step. 1

2 Runoff was routed through the catchment via routing nodes and subcatchment nodes represented areas of impervious and pervious surfaces. The Parafield Stormwater Harvesting Facility consisted of 93 ML active daily surface storage with a minimum detention time of 3 days from time of capture to injection. Injection and recovery from 4 aquifer storage and recovery (ASR) wells had a combined daily injection and extraction rate of 7.8 ML/day. The rainfall-runoff calculation was based on a linear equation following the Australian Rainfall Runoff method (Engineers Australia, 2014) and a small proportion (<6%) of flow representing runoff from pervious surfaces was modelled using a non-linear version equation based on the Australian Water Balance Model (Boughton, 2004). Modelled runoff was calibrated against gauged flow data from a 6 year period resulting in a strong positive linear correlation (R 2 = 0.95) between modelled and observed flows aggregated at a monthly interval. The target aquifer in this study had a groundwater salinity of ~2000 mg/l total dissolved soilds (TDS) (Miotliński et al., 2013) while stormwater was typically fresh (TDS < 200 mg/l). Storage losses through mixing in a brackish aquifer were accounted for by a single parameter loss equation with a coefficient fitted to result in zero aquifer storage at times when salinity spikes were observed. The storage loss model was calibrated against 10 years of recovered water salinity data resulting in a significant negative linear correlation (R 2 = 0.8) between effective freshwater storage and salinity (>400 mg/l TDS). Daily demand was calculated based on historical patterns of use; 30% as a uniform component and 70% as a seasonally varying amount. Seasonal patterns of use were calculated based on monthly variations in rainfall and pan evaporation and closely resembled those given in guidelines for irrigation of public open space in South Australia (SA Water, 2010). Three scenarios were run; (i) a base case representing current catchment and historical rainfall conditions, (ii) with rainfall representative of a drying climate, and (iii) with increased catchment impervious area representative of urban development and consolidation. Two synthetic rainfall sequences were used; (1A) representative of the historical rainfall sequence inclusive, and (1B) representative of a drying climate in southern Australia ( ) downscaled from a global circulation model projection. These sequences were generated for the Parafield Airport weather station. The historical sequence (1A) was conditional on atmospheric predictors (e.g. mean sea level pressure, wind speed and humidity (Charles et al., 2003) and calibrated against NCEP/NCAR Reanalysis data (Kalnay et al., 1996). The climate change sequence (1B) represented a high emission pathway with a representative concentration pathway (RCP) of 8.5 W/m 2 by the year 2100 and was based on the Geophysical Fluid Dynamics Laboratory (GFDL) Earth Systems Models (ESM) 2M. One hundred iterations of each sequence (1A and 1B) were generated by a stochastic downscaling model (nonhomogeneous hidden Markov model). Total catchment imperviousness was calculated at 38% as a sum of the impervious area from all subcatchment nodes. Based on land planning advice, further subdivisions and urban consolidation within the catchment it was considered that the total impervious area could increase by 20% (to 46% of the total catchment area) within the 50 year time horizon used in this study. The model applied this as an instantaneous increase and while this was not realistic it was useful for examining the impact 2

3 of urbanisation on the supply reliability of stormwater harvesting and MAR. Water utilities need to satisfy customer demand at a prescribed reliability level. In Perth, Western Australia the water utility has a target of 99.5% reliability for potable supplies (Gao et al., 2014). In the current study, 99.5% reliability was used as a hypothetical benchmark for potable supply while 95% reliability was assumed to be sufficient for non-potable supply e.g. municipal irrigation. mean annual recharge (Figure 1). At a supply reliability of 99.5%, demands met ranged from 901 ML for the climate change scenario to 1013 ML for the increased urbanisation scenario with the base case at 937 ML. At 95% reliability, demands met were 1006, 1065 and 1166 ML for the climage change, base case and increased impervious area scenarios respectively (Figure 1). For each scenario, 100 iterations of the 51 year sequences were performed at different demand levels to generate supply-reliability relationships within the range of % reliability. Mean annual supply reliability was determined from the mean annual volume of water supplied (from 51 years and 100 iterations) proportional to annual demand. RESULTS AND DISCUSSION The historical rainfall series (1A) had an total annual mean (from 51 years and 100 simulations) of 446 mm. The climate change series (1B) mean annual total was 7% less (415 mm) and mean number of rain days (>1mm) was 67 days, 12% less cf. the historical sequence. Seasonal differences between the series were evident in mean spring and autumn rainfall where the climate change series was 20% and 9% less than the historical series respectively. With fewer and larger storm events less rainfall is lost through initial losses and the proportion of flows exceeding harvesting capacity would also be reduced as on average there is more time between storm events. Trends between supply reliability and annual demand were similar across scenarios; high reliabilities were seen at lower demand rates and these dropped off sharply as demand approached Figure 1 Supply reliability curves for modelled scenarios. Compared with the climate change scenario with 7% less rainfall and 6% less runoff, demand met was only 3% less than the base case. When impervious area was increased this resulted in 15% more runoff, 9% more recharge and 8% more demand met compared to the base case. A larger increase in volume supplied was observed from urbanisation than the decrease expected under a drying climate. At 99.5% supply reliability, the base case resulted in 18% of mean annual rainfall being generated as runoff, 15% recharged and 13% supplied. The percentage decline in harvested volume recharged (5%) and volume supplied (3%) due to a drying climate was similar to the decline in rainfall (7%). In comparison, runoff from pervious rural catchments in southern Australia is expected to drop by 2-3 3

4 times the proportional rainfall decline under climate change (Dillon, 2011; Cai and Cowan, 2008). CONCLUSION This was the first known quantitative assessment of the volumetric reliability of a stormwater harvesting and managed aquifer recharge (MAR) scheme under climate change. A generic modelling approach was presented that is useful for the planning and design of stormwater havesting and MAR schemes and for increasing investor confidence. The model to account for depreciation of freshwater stored in a brackish aquifer was simple however was calibrated on 10 years of operational data. An increase in catchment impervious area of 20% was shown to more than compensate for the reduction in harvestable volumes due to a drying climate. ACKNOWLEDGEMENTS The authors acknowledge support of the partners to the Managed Aquifer Recharge and Stormwater Use Options research project. These are the National Water Commission through the Raising National Water Standards Program, the South Australian Government through the Goyder Institute for Water Research, CSIRO Water for a Healthy Country Program, City of Salisbury, the Adelaide and Mt Lofty Ranges Natural Resources Management Board, University of Adelaide, University of South Australia, South Australian Water Corporation and the former United Water International. REFERENCES Boughton, W. C The Australian water balance model. Environmental Modelling and Software, 19, Cai, W. & Cowan, T. (2008). Evidence of impacts from rising temperature on inflows to the Murray Darling Basin. Geophysical Research Letters, 35, L07701, doi: /2008gl Charles S. P., Bates, B. C. & Viney, N.R. (2003). Linking atmospheric circulation to daily rainfall patterns across the Murrumbidgee River Basin. Water Science and Technology 48 (7), Dillon, P.J. (2011). Water security for Adelaide, South Australia. Chapter 24, p in Water Resources Planning and Management, eds. R. Quentin Grafton and Karen Hussey. Cambridge University Press. Dillon P., Page, D., Dandy, G., Leonard, R., Tjandraatmadja, G., Vanderzalm, J., Barry, K., Gonzalez, D. and Myers, B., (2014). Managed Aquifer Recharge Stormwater Use Options: Summary of Research Findings, Goyder Institute for Water Research, Technical Report No. 14/X. Engineers Australia (2014). Australian Rainfall and Runoff. Stage 2 Revision Project 6. Loss Models for Catchment Simulation in Urban Catchments. Engineers Australia Feb Gao, L., Connor, J.D. & Dillon, P. (2014). The economics of groundwater replenishment for reliable urban water supply. Water, 6 (6), Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J., Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K. C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne, R. & Joseph, D. (1996). The 4

5 NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society, 77, Miotliński, K., Dillon, P. J., Pavelic, P., Barry, K., & Kremer, S. (2014). Recovery of injected freshwater from a brackish aquifer with a multiwell system. Groundwater, 52 (4), SA Water (2010). Code of Practice, Irrigated Public Open Space. D-AC F56- A7CB606F78DD/0/CoP_IPOS.pdf 5