Evaporation
Introduction
Here is the ultimate “selfie”! What a beautiful planet and just look at all that water and cloud. But there is another perspective. Even though planet Earth is covered by more water than land, more than 99 percent of Earth's water is unusable by humans.
A mere 0.3 percent of our fresh water is found in the surface water of lakes, rivers, streams and wetlands. It is evaporation that generates and circulates the pure fresh water that we all consume.
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We will look at the principals involved with evaporation, take a closer look at how evaporation is working on Hotel Lake today touch briefly on water licences and their impact and finally widen our viewpoint and examine how evaporation will be affected by global warming and climate change.
Evaporation
As water moves through the hydrologic or water cycle, it evaporates and rises from Earth's surface as water vapour, which condenses in the atmosphere, forming clouds that release precipitation, which falls to the Earth's surface and flows through rivers and lakes and eventually into our oceans. It is from all these bodies of water that evaporation initiates this perpetual cycle of replenishment.
Evaporation is the process by which liquid water is transformed to water vapour and enters the atmosphere. Heat is the primary driver of evaporation. When the water in Hotel Lake is warmed, particularly during the summer, the water molecules move and vibrate more quickly allowing molecules at the surface to escape into the atmosphere as molecules of water vapour. Wind across the surface of the water will accelerate evaporation. As these water molecules rise into the atmosphere, they leave behind whatever salts, pollutants or other impurities that were dissolved or mixed in with the lake water.
This process of purified water vapour rising from water bodies such as Hotel Lake is called evaporation and it is an integral part of the water cycle, and indeed the cycle of almost all life on planet Earth.
Exactly how fast water evaporates depends on many variables and although air temperature and wind are the main drivers, there is much more that has been learned recently on this subject.
The International Atomic Energy Agency (IAEA) makes the general statement that overall, about 20% or 1/5th of water inflowing into lakes is lost to evaporation. In very dry and hot environments this percentage can rise to about 40%.
Evaporation at Hotel Lake
In a 2006 study by Kerr Wood Leidal Assoc Ltd., pre-existing data for sunshine and temperature, at locations remote from Hotel Lake, were incorporated into three separate methods of calculating evaporation. In addition, actual UBC evaporation records were also used in a separate calculation. It is interesting to note that an assumption was also made that Hotel Lake’s water temperature is: “equal to the mean monthly air temperature”. So far as can be seen, no real data from Hotel Lake was included. The three methods of calculation, combined with the UBC data, were published in the following table of the KWL study:
For Hotel Lake it is estimated that 680.8mm or 26.8 inches of lake water will evaporate yearly and about half of that leaves the lake in June, July and August.
Looking at the “Average” June, July and September figures, it is calculated that a total of 341.1mm, or 13.43 inches of water will evaporate during those three months. During these three hot and dry months, Hotel Lake will drop of approximately one inch per week.
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When we look at our actual water level readings in 2023 and 2024 we see the same general pattern with the lake dropping about one inch per week during June, July and August.
In July of 2024 the water level in Hotel Lake dropped 4 inches which is right on schedule. But on July 29, 3/4 of an inch of rain fell on Hotel Lake which caused the lake to rise accordingly.
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The three summer months are most interesting because generally, evaporation is at its highest level and there is no rainfall and, because the lake is below the outlet of Hotel Creek, little or no water is leaving the lake by that route. However, water is being pumped out of Hotel Lake by private property owners and water license holders and those licensed amounts are public knowledge and we comment on these further down on this page. Water is also removed by the SCRD but only in the event of emergency.
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There remains an additional long-standing-questions as to whether water is being added or removed via seepage and that is discussed in some detail on our Watershed page.
How Much is an “Inch”
If Hotel Lake level drops 1 inch, this amounts to approximately 233,217 cubic feet which we convert to 1,452,670 imp gallons; Imperial gallons is the unit of measurement for BC government water licences.
The Kerr Wood Leidal report summates the usage of all riparian-domestic licenced and unlicensed users, stating they use 32,300 cubic meters per year = 7,105,006.7 imp gallons. This amount represents 4.89 inches a year, which is about 0.4 inch per month or 0.1 inch per week. However the Kerr Wood Leidal report acknowledges that “a large fraction of water withdrawn by riparian licencees typically returns to the lake through groundwater discharge. This is known as “return flow”. So if all those assumtions are correct and the lake level drops 1 inch per week, largely due to evaporation, then far less than 1/10 of that is from riparian water user consumption.
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Moving on a little further with domestic consumption, we are pleased to include this excellent map that shows the SCRD North Pender Water System and if your wondering who is on that system and who is not...well here is the big picture.
The SCRD holds several very large licences to pump water from Hotel Lake but, rather than draw down fully on those licences, has opted to pursue infrastructure design improvements that rely on Garden Bay Lake for its primary supply. This arrangement permitted the SCRD to declare a self-imposed “In Emergency Only” limitation on water withdrawals from Hotel Lake, an arrangement that has reduced lake level fluctuations and draw downs that caused great concerns in the past.
​So far, so good! Its worth paying attention so that our very small and shallow Hotel Lake is always treated with respect and not drawn down beyond what is currently believed to be an environmentally healthy balance. One outstanding concern is that in 2023 the lake, for reasons not clear, dropped to a historically low level; it is a question that needs an answer.
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In other places all around the world, lakes and humans are suffering the consequences of bad decisions.​​
Evaporation, Environmental Change, Rising Global Temperatures
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As long as new demands are not placed on Hotel Lake, it seems that this lovely little lake may continue to florish. But there is another elephant stomping around heavily on the phenomena we all rely on, thats right, evaporation does have its limits.
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Global Warming, a term that seemed to have been replaced by the currently popular expression Environmental Change, is becoming hard to ignore. The earth’s temperature is rising to historically high levels and predicted to rise further. More than just interesting, this is about to become a really big deal and humans are compelled to start considering the limits of “evaporation” as a way for humans to keep cool. Will global warming transform hot-weather-events to alarming scenarios that increasingly threaten human life? In the decades ahead, will summers around the world be so hot that evaporation falls short of what humans need to cool down and to survive. The 10 minute video, below, presents a very engaging discussion of how evaporation works; importantly, it ends with a glimpse of how evaporation and global warming are intertwined in such a way that, in many regions of the planet evaporation, at a personal level, may not keep people cool enought to survive extreme heat events.
While looking at the realatively comfortable relationship that we have with evaporation here on the West Coast, it is important to broaden our view and include other regions of our planet where conditions are drastically different. The video below explores such a region and gives new understandings of the many places on our planet that face different and more challenging environmental consequences than we.
References
2006 Kerr Wood Leidal Consulting Engineers -DRAFT- Hotel Lake Hydrologic Analysis
This study was necessarily expanded to address directions of the BC Environmental Board Decision of August 2005. Available in our Library by clicking above:
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North Pender North-Pender-Harbour-Water-System-Background-Information.pdf
Available in our Library by clicking above:
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