A number of water use scenarios for the Chapman water system were developed based on the results from water consumption and temperature and precipitation data between 2009 and 2013 (daily data from the Sechelt AUT climate station) and 2003 and 2013 (monthly from the GGP climate station). The scenarios included a 1, 2, and 3 °C increase in average July and August temperature under normal precipitation conditions and under drought conditions. The baseline data used to develop the scenarios included the estimated increase in water consumption per increase in temperature of 34 LCD based on the correlation between average daily LCD and mean daily temperature, an increase of 1.75 LCD per decrease in mm of precipitation, the year that experienced the highest average daily consumption in July and August of 1022 LCD (2010) and MDD from 2013 (25,980 m3).
The scenarios are based on a 2% rate of population growth (SCRD, 2013) and include a business as usual water demand based on no reduction in average water use, a moderate decrease in water consumption of 12% based on implementing stage 2 watering restrictions during July and August, and an intensive demand management (IDM) scenario based on a 20% reduction as outlined in the SCRD’s Comprehensive Regional Water Plan (2013).
Table 6: Variables and values used to develop scenarios for the sensitivity analysis
| Variable | Value |
| Average July and August LCD (2010) | 1022 LCD |
| Maximum daily demand (2013) | 25,980 m3 |
| Average July and August temperature | 17.4 °C |
| Total July and August precipitation (average 2010) | 32.4 mm |
| Estimated increase in consumption per increase 1 °C | 34 LCD |
| Estimated increase in consumption per decrease in 1 mm rain | 1.75 LCD |
| Estimated increase in consumption under drought conditions | 56.7 LCD |
| Estimated savings from business as usual demand | 0% |
| Estimated savings from Stage 2 watering restrictions (moderate) | 12% |
| Estimated savings from Intensive Demand Management | 20% |
| Water treatment plant daily capacity | 26,900 m3 |
| Equivalent of total water storage capacity | 1,800,000 m3 |
The results from the sensitivity analysis show that under a worst case scenario (3 °C increase in average daily July and August temperature with no precipitation under business as usual water demand), average daily water consumption would exceed the over design capacity of the water treatment plant by 2015, maximum daily demand would exceed the threshold this year (2014) and total July and August consumption would exceed 1.8 million m3 by 2019. The worst case scenario for the moderate demand scenario (12% decrease in consumption) shows that average daily water consumption would exceed capacity by 2020, maximum daily demand would exceed capacity by 2015 and total July and August consumption would exceed 1.8 million m3 by 2024. The worst scenario for the IDM scenario (20% decrease in consumption) shows that average daily water consumption would exceed daily capacity by 2024, maximum daily demand would exceed capacity by 2019, and total July and August consumption would exceed 1.8 million m3 by 2028. The tables below highlight the years that July and August demand is projected to exceed the current capacity of the water treatment plant and the equivalent to the lake supply for all scenarios.
Table 7: Scenarios indicating the year that July and August consumption will exceed defined threshold for current daily water treatment capacity and the equivalent volume of water stored in Chapman and Edwards Lake, BC (with no change in normal precipitation)
| Scenario | Average daily consumption > 26,900 m3 | Total July and August consumption > 1.8 M m3 | Maximum Daily Demand < 26,900 m3 | ||||||
| 1 °C | 2 °C | 3°C | 1 °C | 2 °C | 3°C | 1 °C | 2 °C | 3°C | |
| BAU | 2020 | 2019 | 2017 | 2024 | 2023 | 2021 | 2014 | 2014 | 2014 |
| 12% | 2027 | 2025 | 2023 | 2031 | 2029 | 2027 | 2018 | 2016 | 2015 |
| 20% | 2031 | 2029 | 2027 | 2035 | 2033 | 2031 | 2025 | 2023 | 2021 |
Table 8: Scenarios indicating the year that July and August consumption will exceed defined threshold for current daily water treatment capacity and the equivalent volume of water stored in Chapman and Edwards Lake, BC (under drought conditions)
| Scenario | Average daily consumption > 26,900 m3 | Total July and August consumption > 1.8 M m3 | Maximum Daily Demand < 26,900 m3 | ||||||
| 1 °C | 2 °C | 3°C | 1 °C | 2 °C | 3°C | 1 °C | 2 °C | 3°C | |
| BAU | 2018 | 2016 | 2015 | 2022 | 2020 | 2019 | 2014 | 2014 | 2014 |
| 12% | 2024 | 2022 | 2020 | 2028 | 2026 | 2024 | 2018 | 2016 | 2015 |
| 20% | 2028 | 2026 | 2024 | 2032 | 2030 | 2028 | 2022 | 2020 | 2019 |
The increase in temperatures over the next 35 years can reasonably be expected to occur at a consistent, yet steady rate, if indeed it does occur. In other words, it is unlikely that in the next 5 to 10 years we will experience a 3 degree increase in average summer temperatures. However, it can be reasonably assumed that a 1 or 2 degree increase in average summer temperatures will be experienced. Furthermore, in the next 5 to 10 years, the Sunshine Coast may experience another drought similar to the drought experienced in 2012. These conditions may result in the current capacity of the existing water treatment plant to be insufficient in meeting local water demands in the next 5 to 10 years. Based on the results from the sensitivity analysis, it is expected that the Chapman water system will exceed the daily capacity at the water treatment plant at least once a year indefinitely if temperatures and precipitation remain at normal conditions. If, during the next 5 to 10 years, average temperatures in July and August increase by 1 degree and the Sunshine Coast experiences a drought, it can be reasonably assumed that the current capacity of the water treatment plant will be unable to keep up to average daily demand as early as 2018 if water use habits remain business as usual.
In regards to supply, the capacity of Chapman and Edwards lake to augment flows in Chapman Creek during the months of July and August (and into September), appears to be fairly over the next 10 to 15 years. Due to the uncertainties in the amount of water that will be supplied to the catchment area below the lakes and above the intake to the water treatment plant, the exact implications of less snowpack in the winter, earlier spring freshet, and less precipitation during the summer season are unknown. However, if the watershed experiences a steady decrease in snow water equivalent in the coming years, then the amount of water required to augment flows from Chapman and Edwards Lakes in Chapman Creek to meet the demand of the water treatment plant can be expected to increase at a fairly rapid rate, especially during drought conditions.
The sensitivity analysis provided two key projections: (1) the current capacity of the existing water treatment plant could be exceeded regularly in the next 5 years and (2) total water consumption during the summer could regularly exceed the equivalent to the available water supply in the next 10 years if water demand remains business as usual, snow pack continues to decrease in the winter, the summer temperatures increase by an average 1 or 2 °C, and prolonged droughts become more common.