The Forest and the Water Cycle: A Walk in the Forest

By Paul Masterson

Although 7/10ths of the earth's surface is covered by water, the sobering fact is that 97% is saline and unusable. Only 3% is left to nourish and sustain all terrestrial life and fresh water aquatic life. The predicament is that while the supply remains constant, the demand increases.

It was last Fall at Thanksgiving that Brigid and her Grandpa walked a trail in Awenda Park enjoying the spectacle of autumn's brilliant colours. They recalled how the pleasant afternoon stroll had turned into a science lesson on how the changes in leaf colour happen. Today was almost an exact copy of that day a year ago. Brilliant, bracing, colourful but this time with a bank of storm clouds to the east.

As we find them, they have reached a small opening in the forested shoreline overlooking a small lake. A picnic table in the grassy area stands vacant as if waiting for the next visitor to lay sandwiches, pickles and cookies as they pause in their leisure walk.

"Grandpa! there's the Maple tree we saw last year. Its so beautiful. You told me that it drops over 200,000 leaves in a year."

Drawing close to the tree, their eyes are drawn to the intense crimson and yellow tones of nature's creation. The young girl runs over to the tree and runs her hand over the trunk. Meanwhile the old gentleman stands back scuffing at the leafy carpet as if looking for something.

"Brigid, do you remember what you asked when we were here last year?"

"Sure Grandpa, I asked you how many leaves would a forest of trees drop in a year."

Brushing some dirt and twigs from the picnic bench, Grandpa seats himself, stretches his legs and continues, "Do you recall my answer?"

"You said that on an acre of land over 10 million leaves weighing almost 1400 kgs. would fall."

Smiling to himself, the old man was pleased how well Brigid recalled their conversation. "Well done, Brigid, I thought by now you would have forgotten."

Joining her grandfather on the picnic bench Brigid explains, "After our walk last year, I did an essay for my English course on how we spent Thanksgiving. I wrote about our hike and all that. I found it really interesting."

From their vantage point overlooking the weed-rimmed lake they could see reflected sparkles of light caused by a gust of wind rippling the watery surface. "Your science teacher, Mr. Kingston ... Has he discussed the Water Cycle with your class?"

"Do you mean where it starts and goes and on and on?" volunteered Brigid.

"Yes, but because we are in a forest I have in mind the role forests play in the water cycle."

"I remember you saying, Grandpa, that the ground is more moist this time of year because growth has stopped so the roots and leaves don't need the water like in the spring and summer, so the water stays in the ground like a great reservoir."

At this moment, Brigid's attention is distracted as she sees a large bird rise suddenly from a weedy patch of the lake. She jumps up pointing. "Look Grandpa, that big bird! What is it?" With wings fanning the air like giant wands the bird quickly settles down in a swampy corner of the lake.

"It's a great blue heron. Did you see its long legs and how its neck was folded in while flying? It feeds on fish, frogs and crawfish. I saw it when I stood here a month ago."

Composing her excitement, Brigid resumes her seat and asks this question, "Grandpa ... Is transpiration part of the water cycle?"

"Oh yes; in fact it is one of the important benefits of the forest to the hydrological cycle. Your beautiful maple tree over there could, on a hot summer day, draw over 3,500 litres of water from the ground and release it into the air. That is about the same amount of water a family of four in Ontario would use in two and a half days."

"Golly that's really something. But what does hydra ... hydrologic ...?"

"Hydrological? It refers to the science of hydrology, which is the study of water, particularly how it occurs naturally, its properties, and how we get control and conserve it."

Not wanting to get too scholarly in their chat, Grandpa turns the discussion to another level. "Has Mr. Kingston reviewed the Water Cycle process with your class? You know ... rain falls from the clouds, but only some reaches the ground. Some evaporates as it falls, some is intercepted by the leaves and branches of trees and shrubs. Some reaches the ground and soaks in and some on the surface runs off into streams joining spring waters that come out of the ground. Some small streams join larger streams that flow to lakes and then the sea. Then water evaporates from the surface of all the water bodies, forms clouds and falls to earth again as rain."

Finishing his recitation, the old gentleman pauses to zip his jacket a little higher. The clouds seen earlier have shut out the sun and sent a chill into the air. "I think the rain is not too far off, Brigid. He doesn't wait for Brigid to comment but continues. "Well, as I said, transpiration is important but so is the soil and how well it allows the rain to soak in. We call this infiltration. The more the rain soaks in, the less runoff there is." Warming up to the subject, Grandpa is off and running. Surprisingly Brigid maintains her interest. "Other parts of the water cycle are affected by trees and the forest, starting with the interception of rain by leaves and branches."

Reaching down and picking a maple leaf from the ground, Grandpa lays it on his palm. "As the rain falls, it strikes branches and the flat surface of leaves. In a very light rain about 40% of the rain reaches the ground. In a heavier rainfall of 2 inches, 92% reaches the earth. Obviously during the dormant season when there are no leaves, the amount of rain reaching the ground is higher."

Grandpa stoops and scoops some earth into his hand. Then taking Brigid's hand, he cups it and drops in the dirt. "A few minutes ago I talked about the rain soaking the soil or infiltrating it. Rub the soil between your fingers and tell me how moist it is."

Rubbing the dirt with her fingers Brigid pauses. "It's kind of dry, Grandpa."

"Do you think it could take more water?"

"Yes."

Motioning the youngster to dump the soil from her hand he carries on. "Soil is able to absorb a lot of moisture. In one study they found that the soil not only soaked up the rain that fell but also absorbed the water running off a nearby field. Can you think of a soil with a texture that absorbs very quickly?"

A brief moment passes as Brigid thought about it. "Sandy soil?"

"Right on. Rain percolates easily through sand. However sand doesn't hold moisture for long and dries quickly. On the other hand, a heavy clay soil does not absorb readily but when soaked it can hold moisture for a long time.

Finding a stout stick and using it as a trowel, Grandpa digs into a small nearby depression in the ground, collects some soil from beneath the humus layer, rolls it in his hand, fingers it and turns to Brigid.

Somewhat puzzled, Brigid watches the performance.

"I'm examining the soil for its texture. Here take some and rub it between your fingers."

Following his directions, Brigid waits for Grandpa's explanation.

"This soil has a loose texture. Grittiness indicates the presence of sand. It also has a soft and pliable feel which means there's clay present. I also see some organic matter. With those clues the soil would be classed as loam."

Continuing their walk Grandpa pursues the subject of runoff. "The soil's texture and depth and the depth of the forest litter and humus layers affect the ability to prevent or reduce runoff. Forests like we're in, and those throughout the temperate zone of Canada with well developed organic layers, are excellent terrain for infiltration and runoff prevention. Forest fires, especially very intense ones, consume the litter and the humus layers. This will have a noticeable influence on infiltration."

Checking his watch, Grandpa, realizes that he has over extended this science lesson even though Brigid appears ready to hear more. However, he feels that they should get on their way as the grey overcast is not promising. "Let's hurry along, Brigid, we are going to get some rain."

Glad to oblige Brigid asks, "Why do they call the forest a giant reservoir, Grandpa?"

"I guess because it is, Brigid. Although the capacity for storing moisture or water, if you will, varies greatly with soil depth and texture. Shallow forest soils, generally less than 20 inches deep, can, after a long dry spell, retain several inches of water. Water in the soil is used by trees and other plants, it also moves into porous layers called aquifers and flows to springs and wells. We find lots of examples of aquifers in Tiny Township."

At this point along the trail they cross a small draw with a flow of water. This reminds Grandpa of his next point.

"The stream is part of a runoff. Actually Brigid, except where logging, grazing or a heavy fire have exposed the mineral soil, runoff in the forest is virtually absent."

For the next short while the path led them away from the lake through a grove of conifers towards the parking lot and their car. Here Grandpa concludes his lesson. "We all accept the stabilizing effect on soil of undisturbed forests. We know that erosion and sedimentation can happen along a forest stream through the action of water on the stream's bank and bottom. When forest soil is changed by land clearing, logging, fire or other disturbances, the possibility of erosion becomes high. Logging certainly has some effect on the water cycle, however clearing for agriculture has a greater impact."

"By the way Brigid, can you guess what condition has the greatest impact on the water cycle?"

"No ... what?"

"Urbanization! Studies have shown that runoff from urbanized forest land is several times higher than what occurred before the development. Oh yes, lawns have very low infiltration rates, one test showed that it was one-sixth the amount that a forest would handle. The forest is the best of all natural covers to build ground water storage capacity, lower overland flow, runoff and erosion."

Reaching their car, they felt drops of rain which caused Brigid to quip: "The water cycle has started again, Grandpa."

Reference: Canadian Institute of Forests, Southern Ontario Section.