Reports by Consulting Hydrologist:
Garth van der Kamp
Global Institute for Water Security
University of Saskatchewan
July 15th 2021
The up-to-date Prospect Lake water level graph. During the past 15 days the lake water level has dropped by 10 cm (100 mm) which is equivalent to 100/15 = 6.7 mm/day, by evaporation of water from the lake surface. That is a very high rate of evaporation; the average summer-time rate of evaporation is about 4 mm/day. Perhaps this rapid water loss is not surprising considering the heat wave that came through last week.
We can expect the lake level to drop by another 20 or 25 cm by the end of August, barring heavy rainfall. The lake level is 13 cm below the top of the weir today. If the drought continues to September the lake level may drop by another 30 cm or more.
Evaporation from the lake amounts to a loss of water of 3 to 5 acre-feet per day, about 600 gallons per minute. That’s a pretty hefty water loss. Seepage past the weir to Tod Creek is very small by comparison: less than 10 gallons per minute, less than 2 % of the evaporation loss.
I’m giving these numbers just to make the point that the decline of the lake level during the summer is due to evaporation of water from the lake.
Garth van der Kamp, January 1, 2021
The water flow in Tod Creek varies very widely, from as low as 0.1 liter/second at the end of dry summers, to 5 cubic meters per second or more after heavy rainfalls in winter. This dramatic variation of the flow can be understood in relation to the climate, the lakes and wetlands, and the geology of the watershed. This essay provides a brief introduction to the hydrology of the watershed: how water moves above, over and below the ground surface.
In relation to water, the climate of the Tod creek watershed is summarized in Figure 1 which shows the average monthly values of rainfall and of potential evaporation in mm of water. (Evaporation includes both direct evaporation of water and transpiration from vegetation). The rainfall data are based on 30-years (1980-2009) of daily measurements of rainfall at Victoria airport. The potential evaporation is a theoretical estimate of the amount of evaporation that could be expected from a moist vegetated surface such as an irrigated field of grass. Evaporation requires heat energy and the potential evaporation therefore largely reflects the amount of solar energy that is available at the ground surface. The actual evaporation from the ground surface or from vegetation and from forests may be considerably smaller when moisture is limited, as in dry summers. Evaporation from lakes and wetlands is likely to be close to the potential evaporation.
Clearly, as any gardener knows, there is an excess of water available during the season of heavy rains from about October to March, and that is when the lakes fill up and overflow and the creeks run high. But during the hot dry summers evaporation and transpiration by vegetation greatly exceed the rainfall and draw on water stored in the soil and in lakes and wetlands. Thus in summer lawns go brown unless they are irrigated, the lake levels decline, and flows in the creeks are at a minimum.
Fig 1 – Average monthly precipitation and potential evaporation for Victoria BC
Average annual precipitation is about 880 mm and the average potential evaporation is about 725 mm. In other words, there is an annual surplus of water of at least 880 minus 725 or 155 mm. The actual surplus is considerably larger because some areas of the watershed such as pavement and roofs have little evaporation. The actual surplus could be determined by careful measurement of the creek flow out of the watershed because that is how the water surplus must leave the watershed. The water management problem, for fish, for water supply, and for agriculture, is that there is not enough storage capacity in the watershed to store all the winter surplus of water. By the end of the summer the water stored in lakes and in the soil and the groundwater is much depleted.
The Tod creek watershed
The watershed of Tod Creek is the total area for which, if water flowed downhill over the ground surface it would end up in the creek. It is shown in Figure 2. The area of the entire watershed including lakes is about 23 km2, and the portion of
Figure 2 — The Tod Creek watershed
the watershed that feeds water to Prospect lake is about 10 km2. Most of the watershed is comprised of forested hillslopes with shallow bedrock, and these, together with the lakes, dominate the flow regime of Tod Creek. The watershed includes several significant lakes: Prospect, Durrance, Killarney, Maltby. (Heal Lake used to exist but is now obliterated by the Hartland Landfill). Tod Flats has now reverted back to a wetland, although its original peat cover is largely gone after a century of cultivation. During the winter Tod Flats acts a shallow lake, and it dries out in the summer. Most of the water supply to Prospect Lake comes from Killarney Creek. Tod Creek starts at the outflow from Prospect Lake and is also fed by several small tributaries which fall dry in late summer, the chief of which is Durrance Creek, flowing out of Durrance Lake.
Hydrogeology of the watershed
The high variability of the flows in the creek is largely due to the geology of the hillslopes. These are underlain by crystalline bedrock at shallow depths with numerous outcrops (https://apps.nrs.gov.bc.ca/gwells/aquifers/). The top of the bedrock tends to be fractured, but at depths of more than a few meters there are only sparse factures which transmit little groundwater. That is why many wells are drilled to 100 m depth and more, to intercept sufficient groundwater flow for a domestic water supply. The bedrock surface is covered in a thin layer, usually just one or two meters of broken rock and gravel. This thin cover can store a lot of groundwater, but it is also highly permeable.
Most of the rain water that soaks into the forest soil during the winter rains seeps rapidly down the slopes to where it emerges in ephemeral small rivulets which fall dry quickly after the rain ceases. These rivulets feed the larger creeks, leading to peak flows at the end of heavy rain events. The deeper intact bedrock has very low porosity and a few fractures so that it can store and transmit only small amounts of groundwater. This bedrock groundwater, together with water remaining in the shallow soil, is used by the trees during the summer so that very little of it reaches the streams in the valley bottom. In the fall, when the rains come back, the depleted groundwater storage must first fill by infiltration of the rain water before small rivulets begin to flow again.
Hillside groundwater observation wells operated by the provincial water agency, typically show the water table rising rapidly to near the ground surface after heavy winter rains and then dropping to far below the ground surface in the summer, mostly due to water uptake by the trees. A typical observation well record is shown in Fig 3. This observation well is located on the south-facing hillslope of Lauwelnew (Mount Newton) in the Hagan Creek watershed. This well is 150 m (500 ft) deep, drilled in granite bedrock.
Fig 3. Groundwater level record for BC Observation well No. 343
The bedrock hillslopes are the dominant feature of the Tod Creek watershed. Small persistent seeps and springs of groundwater do occur in a few places, notably from the remnants of the limestone that was quarried above the downstream reach of the Creek below the Butchart dam. The valley bottom between Prospect Lake and the Butchart Dam is filled with deposits of clay and sand and peat. These become saturated during the winter and contribute many seeps of groundwater, but in summer the valley bottom vegetation draws on this stored groundwater so that the water table declines, and the seeps dry out.
Water balance of the lakes in the watershed
Each of the larger lakes in the Tod Creek watershed fills and overflows during the winter. The water level of lakes works like a bank balance. The water level rises when the input (flow into the lake plus rain on the lake) exceeds the withdrawals (flow out of the lake plus evaporation from the lake). Also, the higher the lake water level rises above the bottom sill of the outflow point, the stronger the outflow becomes.
The figures below show the water outflow and level changes of Prospect lake in relation to the top of the weir which controls the outflow (occasionally beavers raise this elevation, using the weir or the constriction of the creek below the Goward Road bridge as a handy start for a beaver dam). These data are based on occasional measurements of the lake level at the Goward bridge and estimates of the flow under the bridge using floating chips and a stopwatch to estimate the speed of the flow.
Figure 4 shows the outflow from the lake in relation to the level of the lake above the top of the weir. (The weir has a horizontal top about 4 m wide). Such a graph of flow in relation to water level is referred to as a “rating curve” or “stage-discharge curve” in hydrology and allows estimation of the flow for any water level within the range of the curve. The maximum observed outflow exceeds 3 cubic meters per second (cms) when the water level rises above about 0.60 m. When the water level declines to just above the top of the weir the flow estimates are uncertain, and some are influenced by beaver-built obstructions and by accumulation of branches and other floating debris at the weir.
Figure 4 Estimated flow rate of Tod Creek at Goward Rd bridge in relation to the lake level above the top of the weir
Figure 5 shows how the water level of Prospect Lake changed from early September 2018 until December 2020. Once can see how the lake fills to a level above the weir in November and occasionally rises to a peak after heavy rains. In the springtime the water level declines because the rain and flow into the lake slow down to a rate less than the outflow. At that time of the year evaporation (see Figure 1) is still quite low. At some point in May or June the increasing rate of evaporation from the lake and the lower rainfall and inflow cause the lake level to decline below the level of the weir. From June to September the lake level depends mostly on the balance between rain on the lake versus evaporation from the lake. In the summer of 2019, the water level dropped to 25 cm below the top of the weir. Allowing for rain on the lake, this shows that lake evaporation during those months was about 4 mm per day. In the exceptionally dry summer and early fall of 2018 the water level dropped to 35 cm below the weir.
Fig 5 Water level of Prospect Lake relative to the top of the weir
The summer-time loss of water from the lake by evaporation, at 4 mm per day, equals about 50 L/sec. This rate of evaporation, estimated from the observed water level change is slightly lower than the estimated potential rate of evaporation (Figure 1), and compares well with the 5 mm/day evaporation rate from the Sooke Lake reservoir estimated by Werner (MSc thesis, 2001). During the summer of 2019 the observed leakage past the weir was less than 2 L/sec.
Data for the water-level regime of the other lakes in the Tod Creek watershed are not available, but the water level of all these lakes declines to the point that flow through the outflow channels is zero and there is at most some seepage through dams, if any. This water level regime of the lakes is an important factor in the flow of Tod Creek because the summertime drop represents a large decline in the amount of water stored in the lakes. For example, a 30 cm drop of the water level of Prospect Lake below the top of the weir is equivalent to a water loss of 183 acre-feet. This loss must be made up by rain and inflow in the fall before the lakes again contribute to flow in the creek.