The Chapman watershed provides of 90% of the water supply within the Sunshine Coast regional district's water service area. The SCRD projects up to 2% population growth rate to 2036. The SCRD advocates for "intensive demand management", i.e. curtailing water use. Even with IDM, by 2036, the SCRD theorizes that it's goal of having adequate water storage capacity for a 1 in 25 year drought return scenario will not be met. It projects only having sufficient water capacity for 14 out of every 15 years. Without curtailment of demand, the regional district foresees only having adequate supply every 10 out of 11 years. What it means is that water demand if forecast to exceed supply during drought conditions every 25, 15 or 11 years under the indicated scenarios.
The SCRD is proposing to drain Chapman Lake during droughts by drawing it down as much as 8 meters. The depth of the lake at its deepest is about 32 meters so the draw down would represent well over 25% of the lake's volume due to it V-shaped basin.
Chapman Lake is inside Tetrahedron Provincial Park so the draw down would represent a significant change to the park's ecosystem and integrity.
One storage option is to engineer a lake at a gravel quarry well outside Tetrahedron Provincial Park. However, the gravel pit is still in operation and expected to continue until 2050.
Two options directly affect Chapman Lake.
- Draw down the lake using a floating pump. The diesel-powered pumps would operate 16 weeks and drawdown the lake 6.5 meters during a 1 in 20 year drought.
- Raise the lake by raising the existing dam from the existing 1.5 meters to 4 meters in height. It would flood an additional 5.6 hectares of land.
A consulting company rated the ecological disturbance of raising the lake as Very High and of the floating pump as Moderate-High. The deleterious effect of increased flooding is rated as Very High. The effect on the park and recreation is rated as Very High and High in order of the two options numbered above.