By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Following my recent Naturalist Note on water I was persuaded to write a follow-up note, and this is it!
Water is a key to life, for both plants and animals. When you think of water at Tryon Creek State Natural Area (TCSNA), you naturally think of the creek. But that’s a long way from the whole story. In a Naturalist Note1 published earlier this year, I included the following image of how much water was in the top 12” of soil at TCSNA in late April based on 22 soil samples.
Just as a reminder, I need to stress that this is only in the top 12” of soil. In much of the park, water available to the plants might be located as deep as 2.5 feet beneath the surface. At approximately 2.5 feet beneath the surface is a layer of clay which is relatively impermeable to plant roots (more on this in my next Naturalist Note.)
During the course of this summer, there was not much rainfall, as illustrated in the graph below, which is based upon rainfall at two local stations, one at the Lake Oswego City Hall, and the other at the Westlake Fire Station.
Rainfall April 23 to Aug 23, 2018
As documented in a recent Naturalist Note, light rainfalls such as the majority of those illustrated in this graph, may primarily end up in the crowns of trees and shrubs, and never even make it to the forest floor.
On August 22 and 23, 2018 I collected soil samples from approximately the same locations where I sampled the soil in April. I again took samples from only the top 12 inches. The results were stunningly different. Below is a diagram using the same number of pools which I described in the spring. This time, however, 42 of the pools are dry, and only 26 are full of water.
The number of Olympic-sized swimming pools that could be filled with water from the top 12 inches of soil found in late August 2018 at Tryon Creek State Natural Area.
Where did it go?
There are probably three main fates for the missing water in the top 12” of soil. First, any water at or close to the surface could simply have evaporated. I have no idea how much this would amount to, but in the top 1 to 2 inches of soil, I’m guessing this could be an important factor.
Subsurface Water Flow
Secondly, the water particularly in sloped areas, could have flowed down hill underground. Lateral underground movement of water is quite common. To illustrate this I poured several gallons of water onto the soil surface at a flat spot on the side of the Old Main Trail not too far from the Nature Center. After saturating the soil in this tiny area, I used a soil corer to create two 6” deep holes about an inch in diameter. I waited for several minutes until there was no freestanding water in either hole. Then I carefully poured water into the right hand hole, as seen in the picture below. Not surprisingly, the water flowed laterally underground, and appeared the other hole.
This subsurface water flow might be particularly important on the steep hillsides near the creek. To illustrate this effect, I found information online about two different watersheds. The first is the Tryon Creek watershed, which is the watershed in which the TCSNA is located. An analysis of this watershed by the City of Portland has shown that about 25% of it is made up of impermeable surfaces, like rooftops, sidewalks, driveways, streets, tennis courts and even swimming pools. An aerial photo of one small part of the watershed illustrates the extent of these impermeable surfaces.
Water falling on these impervious surfaces rapidly makes its way into Tryon Creek, thanks in part to storm sewer drains that are common in the city.
In contrast, the Fir Creek watershed, located in the vicinity of the Bull Run Reservoir in the foothills of the Cascades east of Portland is almost completely forested, with the only impermeable surfaces being a few roads in the area. These two watersheds are roughly similar in size.
The differences in the surfaces of these two watersheds creates an enormous difference in the water flow in the major creeks of the watershed. These differences are illustrated using data from the same “rain event” in both watersheds.
In this graph you can see that immediately after each large rainfall event that there is a sharp peak in the water flow in Tryon Creek. This sharp peak is followed by a slow decline in the water flow of the creek. It seems reasonable that the brief sharp peak in the creek depth is water running off the impermeable surfaces found in the watershed, and being quickly dumped into the creek by the storm sewer systems. The slower decline following the sharp peak, is, I assume, water actually slowly flowing through the soil and into the creek.
In contrast to the water flow in Tryon Creek, in the Fir Creek system, there is a slow but significant increase in the stream flow following the rains. I suspect this is because the water in this watershed all has to gradually seep through the soil, and slowly make its way down to the creek.
Water Usage by Plants
Thirdly, the water could have been extracted from the soil by the roots of plants, and subsequently evaporated from plant leaves. This would be one way in which soil water at some depths could be lost to the atmosphere. In studies2 of water usage by Douglas-fir (Pseudotsuga menziesii), for example, it was reported that a 60-foot tall tree with an 8 inch diameter used 5 gallons of water per day. A 91 foot tall Douglas-fir with a 14 inch diameter used 16 gallons per day. Our forest has numerous trees this big and bigger, so their water usage in summer for the forest as a whole could be more than we might first imagine.
Living as we do in an area with relatively dry summers and wet winters we could see dramatic changes from climate change. If we have a winter with subnormal amounts of rain, and warmer than average summers, there could be a large die off of moisture-loving plants which have lived in this area for some time. They will of course, be replaced with other plants, but the transition could be difficult.
2Wullschletter, Stan D., F. C. Meinzer and R. A. Vertessy. 1998. A review of whole-plant water use studies in trees. Tree Physiology.