Forest Canopy Greatly Reduces Flooding

By Don Gasper

Deforestation for any reason in the heavily forested Mountain State will, in fact, change streamflow and have an impact on flooding in downstream areas.

We are fortunate to have US Forest Service (USFS) research carried out right here in our area. In the late ‘50's and early ‘60's, a study at the USFS Fernow Experimental Forest near Parsons in north-central West Virginia was conducted to examine the influence of timber cutting on flooding. On an undisturbed control watershed through a nine-year study period, average monthly precipitation (rain and snow), total runoff, and the percentage of runoff as related to precipitation was determined and reported by Ken Reinhart and others at the experimental station in 1963 (see Table 1).

During the winter dormant season (November-April) runoff was 60% of the total precipitation. However, during the growing season (May-October), runoff carried by streams was on average only 23% of the total precip- itation. This means during summer season over 75% of the rainfall does not even reach the stream channels of fully forested watersheds!

Evapotranspiration – rain and snow intercepted and evaporated plus that which enters the soil and is withdrawn by vegetation roots and transpired out their leaves – causes such differences in runoff. It is fairly obvious from the data that a typical, forested watershed greatly reduces runoff flow during the growing season. Furthermore, data from more than 40 years of records at Parsons, WV, indicate that the highest streamflows from a studied watershed occurred in the dormant season when there were no leaves and no evapotranspiration (see Table 2).

Storm Events, Stream Flows and Leaves

Table 2 also shows that 14 of the 20 greatest rainfall events took place during the growing season. Yet Table 3 shows that of the top 20 stream flows only 7 occurred during the growing season. Note, the correlation is greater with the season than with the magnitude of the storm. In fact, the second and third largest storm events over these 40 years were recorded to be only the 19th largest flow and the other failed to even register in the top-20. These two storms took place in June and August.

It is clear that many (70%) of the greatest storms took place during the growing season when the forests soils are drier. Flooding was significantly reduced by the drying of forest soils. The water holding capacity of forest soils is so increased by tree root up-take of soil moisture, that the first two inches or more of rain does not run-off. Some soils sometimes can take up twice this much.

This point was made by the authors in their 1963 report, but the effectiveness of the forest canopy in reducing summer flows remains greatly underappreciated. They end with this statement.
 

In the region where The Fernow is located, flood occurrence is greater in the dormant season than in the growing season. At the gaging station on the Cheat River near Parsons, WV, 4 miles from The Fernow experimental watersheds, there have been 135 occurrences of discharge above the base of about 10,000 c.f.s. since 1913. Of these, 102 occurred in the dormant season and only 33 in the growing season..

Clearcutting is an extreme example of deforestation and canopy reduction. Canopy reduction for any reason (timbering, settlement, surface mining, etc.) in the heavily forested Mountain State will in fact increase flooding in downstream areas. These same investigators note that more recent studies show a canopy reduction of 30% results in noticeably higher peak flows.

Peak flows we know are the destructive out-of-bank flows that flood across the flood plain of the valley floor. While great floods are produced by great storms, the resultant flooding from even these are reduced by a forest soil storage capacity greater by the leaf canopy demand for soil moisture. Flows of lesser magnitude are controlled even more.

In Reinhart’s study in 1963, a storm hydrograph was developed for the control watershed and for a nearby watershed that was clearcut (See Figure 1). A storm hydrograph is simply the amount of water that flows through a stream over the course of a storm. These hydrographs show streamflow for the control and the clearcut watershed in response to a summer storm. The flow units are in cubic feet per second per square mile in order to compare the two slightly different-sized watersheds. The author notes the flow from this summer storm was nine times greater from the clearcut watershed, and "Instantaneous peaks on the clearcut in the growing season were increased on the average by 21%."

Data obtained locally, and broadly applicable, by these superb investigators supports the conclusion that the forest canopy greatly reduces flooding in summer when most storm events occur. Table 1 can instruct us further in that the average annual rain and snowmelt totals are 58.45" and that measured run-off is 24.33", about 7" drops to deep seepage. The rest, 27" just over half, is evapotranspired by the leafed canopy of this fully forested watershed. A clear-cut removed would mean the stream channel immediately below would have to carry "over twice as much flow" (24" + 27" for 51") as it has in the last 80 years. For about 5 years this will be the situation until enough regrowth and canopy appears and the tree roots begin to be effective in again picking up soil moisture and drying the soil.

In this interval, 2 or 3 moderate storms can occur that would have the erosional effect of a much larger storm. "Over twice as much flow" will cause channel scour of the banks and bottom, and "head-cutting" wherein the tiny drainage channel extends itself upstream (both are measured in the Fernow Studies). Tiny open channels begin as underground drainage "pipes," and it is here these processes begin. Both head-cutting and scour produce sediment, and more sediment may be produced from the enlarging channel than from surface disturbance. These surface gullies, roads and other disturbance do produce noticeable erosion and sediment that all kinds of Best Management Practices (BMP’s) have been developed for, but there are no BMP’s for channel erosion other than canopy preservation.

As the underground piping system of macropores is enlarging it is very likely to plug-up causing the flow to build and perhaps produce a surface flow for a distance, or enter and greatly enlarge other macropores or produce slips as flow paths reorganize. We see this in open channels how in small channels a small amount of added sediment will begin to undermine the channel’s capacity to carry water.

The tiniest open channel already has sand and pebble bars and deposits of sediment that build up on riffles. In addition to channel enlargement due to the "over twice as much flow" that the channel must carry and its subsequent scour, bars and riffle deposits are actively causing further channel erosion and producing more sediment. Bars obviously deflect the flow into the bank, and that causes more bank erosion, perhaps even tree topple. When sediment builds up on a riffle it dams and effectively lowers the bank there, and out-of-bank flooding can occur. Once out of the channel, the flow can erode a new channel, producing an enormous amount of new sediment.

Both dams and bars are proportionally magnifying processes carrying far downstream with greater and greater impacts. These effects of clear-cuts, and any canopy reduction beyond 30% when peak flow increases are easily noticeable, are suffered far below – far off-site, generally on a downstream neighbor or community [bold by editor].

Clearly deforestation can produce added summer flooding that results in soil loss, structural loss, loss of life and channel destruction. Forest stands and their canopies are most important in reducing summer flooding. It is necessary to preserve the canopy of every stand. To reduce the canopy by one-half would be imprudent, with soil loss and flooding below, and sediment created to degrade the channel below. This may leave the community with no alternative but stream channelization that reduces wonderfully interesting streams to featureless drainage ditches. To reduce a canopy by three-fourths would be "deforestation" and destructive of community

values. Clearcutting would be only a very limited acceptable forest management practice – nearly a crime against society and the environment. State permits would be required to manage this vital stewardship of these fragile processes in today’s damaged watersheds.

There follows just two studies, one in the north, and one south, that are of such a nature that they demonstrate clearly the effect of canopy reduction on channel scour. The watersheds are undisturbed except the trees were cut – and increased sediment load was measured in the stream just below.

At a New Hampshire site in 1965 only tree-felling took place. Trees were not removed. There was no other disturbance. They produce the following table (Table 4) of the increased sediment in their weir (dam). They note that much of the sediment was delivered after the increased flow had begun to remove sediment stored in the streambed after the first two years. "Headcutting" and "bank scour" had taken place. They do, of course, report increased flow occurred as well as this increased sediment.
 

The C-1 watershed at the USFS Hydrological Lab in North Carolina reports "All trees in this catchment were cut, first in 1939, and again in 1962. The logs were not removed either time – there were no skid trails, no roads, and almost no soil disturbance. Yet annual sediment transport was still well above reference levels" over twenty years later. This very surely had to be due to channel enlargement as it had to "carry over twice as much flow" annually.

Table 2.  Twenty Largest Storm Events Recorded at Nursery Bottom in Descending Order
 

Rank	Amount(mm)	Duration (hours)	Dates	Type
1	154	42	11/2-6/85	Hurricane Juan-Rain
2	121	49	8/15-17/75	Rain
3	119	87	6/20-24/72	Hurricane Hazel-Rain
4	114	14	10/15/54	Rain, then snow
5	114	49	3/5-7/67	Rain
6	97	49	12/8-10/72	Rain
7	93	67	9/27-30/64	Rain
8	92	42	5/23-24/68	Rain
9	92	25	6/5-6/81	Rain
10	91	33	10/19-22/85	Rain
11	90	53	5/31-6/2/74	Rain
12	89	22	9/13/88	Rain
13	89	16	7/8-9/85	Rain
14	88	88	3/19-22/63	Rain
15	86	19	9/29-30/73	Rain 1/2, snow 1/2
16	84	63	9/9-11/60	Hurricane Donna-Rain
17	84	68	4/2-7/87	Rain
18	83	38	7/2-4/78	Rain
19	83	26	2/9-10/57	Rain
20	81	58	10/7-9/76	Rain

 

Table 3.  Watershed 4 Streamflow Peaks, in mm
 

Rank	Date	Peak flow
1	11/4/85	161 Juan
2	10/15/54	114 Hazel
3	6/6/81	109
4	2/10/57	98
5	5/24/58	83
6	3/5/63	74
7	3/6/67	72
8	12/22/70	70
9	5/28/56	67
10	8/11/84	63
11	4/30/66	63
12	1/22/59	61
13	3/21/62	59
14	3/19/63	58
15	7/9/85	54
16	1/29/70	54
17	4/28/58	53
18	11/28/85	51
19	3/5/64	49
20	8/16/75	49

 

Table 4.  HUBBARD BROOK, N.H., CLEARCUT & SEDIMENT (Kg/ha) (Kg/ha, or Kilogram/hectare, is about the same as pounds/acre)
 

Year	Control	Cut	Increase
1966	4	13.3	3.25 X
1967	31	67.2	16 X
1968	10	92.9	9.20 X
1969	13	195	15.00 X
1970	42	3.65	6.69 X
1971	5	97	19.40 X
1972	6	22.3	3.67 X
Totals	111	851	7.67 X