How Much Moisture Really Does Evaporate During Winter in Southcoastal BC?

In the last blog, I suggested that we need to design leachate collection to accomodate all of the precipitation from October through March because of low evaporation potential during these cooler, high rainfall months in Southcoastal BC. I used the example of a one hectare outdoor site in Abbotsford that would be required to collect and store 10,290,000 L or 2.3 million gallons of water, or all of the precipitation from October through March.

This recommendation is significantly different than the recommendations and calculations in our British Columbia Compost Facility Requirements Guideline (, which suggests that all of the winter precipitation can magically be evaporated depending on how we design our compost facility. This blog will challenge the assumptions used in the BC guideline, which in my opinion, is giving us a false sense of security about how much leachate actually needs to be captured. In the next blog, I will discuss the economic and business potential of the assumptions used in the BC guideline, assuming that they were correct. I will also discuss what is required to make the evaporation potential work for us.

Most of us have observed evaporation from composting material, so in that way, the theory of evaporation makes sense.

The theory of moisture evaporation makes sense because we can see it!

We also need to understand that we have to evaporate up to 400 L of water per tonne of composting material simply to reduce moisture from 65% to 50% during the composting process as outlined in an earlier blog (

For the purposes of demonstrating that we cannot account for significant evaporation of precipitation that enters the composting material during the cool high rainfall season, I will use one of the scenarios outlined in the Compost Facility Requirements Guideline. This example is an outdoor windrow facility operates with 3 m deep and 7.6 m wide piles with 1.5 m aisles. The guideline suggests that only 37,000 L of a potential 245,000 L rainfall in a 25 mm rainfall on a one hectare pad. This suggests that only 15% of the rainfall needs to be captured, and 85% of the rainfall is evaporated by the composting material in the windrows. In the case of the Abbotsford example, according to the guide, the leachate collection only needs to store 15% of what I suggested needs to be collected, 155,400 L instead of 1,029,000 L!

Which estimate is correct? This has a very signficant affect on designing our composting facilities, and estimating leachate storage and treatment costs. In order to answer this, we have to do the following:

1. evaluate the evaporation theory suggested in the Compost Facility Requirements Guideline to determine if it is correct.

2. We need to go back to the source data used to verify the evaporation model to see if the model accurately predicts the amount of actual leachate collected (

4. We also have to offer alternate explanations of where the precipitation went.

In summary:

1. The theory of evaporation is technially correct in estimating the potential evaporation energy in composting material, but it does not include the important principle of the water content of air relative to air temperature as provided by the psychometric curve. Air at 60 C holds up to 20 x more water than air at 10 C, so we need to think through what happens as warm air moves upward in a compost pile when the outdoor temperature is 10 C or less?

2.  When we look at the source documents used to verify the evaporation theory (an outdoor windrow facility in Washington State,, we see that 60% of the precipitation was captured as leachate in a similar pile configuration as the example above, where the theory based on this facility predicted only 15% capture!

3. The unaccounted for precipation from the facility in Washington State (4000 m3) could have been absorbed into the composting material in the windrows.

I will provide an additional blog post in a couple of weeks to provide the detailed calculations to support this. In the meantime, you are most welcome to test it out on your own!

The estimates of potential leachate from an outdoor composting pad provided in the BC Compost Facility Requirements Guideline vastly underestimates the actual leachate produced, giving a false sense of security that leachate from composting facilities is not a serious concern. From my observations of compost facilities in southcoastal British Columbia, the outdoor facilities that truly have been capturing the leachate have been surprised at the volume. Should it have been a surprise? No!

Yes, there is some evaporation during composting. We can see it. But in a cold, high rainfall climate such as our winters in British Columbia, if we can evaporate the 400 L per tonne that we need simply to reduce moisture content during composting from 65% to 50%, we are doing fantastic!

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