The Hydrological Basis far Water Resources Management (Proceedings of the Beijing Symposium, October 1990). IAHS Publ. no. 197,1990.

Hydrological effects of forests

YU YANNIAN General Hydrologie Service ofAnhui Province, 1 Shuguang Road, Hefei, 230022, Anhui, China

Abstract This paper uses observed data to quantitatively analyse the effects of forests on both water quantity and quality in the hydrological cycle for given basins. In addition, a judgement criterion concerning the long-debated topic as to whether the annual runoff tends to increase or decrease due to the forest, is presented here.

Effets hydrologiques des forêts

Résumé La forêt est un participant, un régulateur et un protecteur de la circulation d'eau dans le cycle hydrologique. Le présent article analyse d'une façon quantitative les effets quantitatifs et qualitatifs de forêt dans le mouvement équilibré des quantités d'eau en ce qui concerne sa circulation dans les bassins. L'auteur présente dans cet article les principes du jugement relatif aux phénomènes de l'augmentation ou de la diminution d'écoulement annuel provoqués par la forêt.

INTRODUCTION

Over 70% of the global surface is covered by water and nearly one third of the land surface by forests. The hydrological cycle in the hydrosphere depends on solar energy and gravity. The forest plays an important role in the maintenance of the dynamic balance of the physio-ecosystem, as well as in the exchange of material flux and energy flux. In the case of the hydrosphere, for instance, the forest, with its physiological and ecological behaviour, takes part in the course of the hydrological cycle among soil-ground surface-organism- atmosphere, so that it has become an active participant in the hydrological cycle of the hydrosphere and a loyal protector of the quantative balance of the various components in the hydrological cycle process in the basin. Therefore, the study of forest hydrological effects are of great significance in the cause of transforming and protecting the hydrological cycle and orienting the cycle to the benefit of the human race.

EFFECTS OF THE FOREST ON PRECIPITATION

Forests comprise the main part of vegetation. The biological yield of one hectare of forest is 20-100 times that of 1 ha of farmland or grassland. The total photosynthesizing leaf area of a tree is 5-10 times that of grassland

413

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plants. When chlorophyll photosynthesizes under the influence of solar energy, six C02 molecules and six ïlf) molecules are absorbed, producing one glucose molecule and releasing six 0 2 molecules. The process can be expressed by the equation:

6C02 + 6H20 solar energy

chlorophyll C6H120 + 602 Î (1)

Due to the extended floor, large leaf area, high biological yield and long growth period, forests exert a more significant influence on precipitation than other vegetation.

Like other plants, forests feed on soil moisture in order to maintain theirs life and improve the environment for existence. Forest root systems, trunks, leaves etc., absorb a lot of moisture from surface, soil and ground and turn it into bio-water. Then by way of respiration, transpiration, etc., the forest converts the bio-water into gaseous water, and releases it into the atmosphere so as to form the hydrological cycle of soil-plant-atmosphere, maintaining a good precipitation development mechanism (Fig. 1). When the

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Fig 1 Impact of forest on precipitation.

forest transpires moisture and releases it into the atmosphere, a large amount of solar energy is consumed. Thus, the temperature-humidity field in the forest region is such that the temperature is low and the humidity high. Air is likely to be saturated with water vapour. Thus, there exists a large difference between surface temperature and air temperature in the forest region and its surroundings. Hence, a local convergent updraught flow would easily generate orographic precipitation. The observed data have indicated that a forest can transpire 50% more moisture than an ocean evaporates, even if

415 Hydrological effects afforests

the ocean is located at a similar latitude and covers the same area as the forest. The air humidity above the forest is 10-20% higher than that above farmland. For example, 50% of the precipitation around the Amazon River basin is generated by vapour from the forest. Huangshan Mountain and Tianmu Mountain, both at the same latitude, represent another example; though Tianmu Mountain is located nearer to the ocean than Huangshan Mountain thus having the advantage of vapour input conditions, the normal annual precipitation at Tianmu Mountain is 8.7% lower than that at Huangshan Mountain because the forest coverage on the former is 16.2% lower than that on the latter. The amount of rainfall is apparently interrelated with the forest coverage, as exemplified in Huaibei (flat areas), Jianghuai (hilly areas), the drainage network along the Yangtze River, and the Dabieshan and Wannan mountain areas of Anhui Province. It results from the interaction of the atmospheric cycle and natural geographical conditions (Fig. 2).

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1 2 3 4 6 8 10 20 30 40 60 forest coverage ( % )

Fig. 2 Relationship of forest coverage with annual precipitation normal in Anhui.

EFFECTS OF THE FOREST ON ANNUAL RUNOFF

The effect of the forest on annual runoff is determined by forest coverage, crown density, forest crop composition and forest age. If these factors are similar, the effects of different forests would vary with geographic locations, climatic conditions and forested area (Fig. 3).

The influence of the forest on annual runoff lies mainly in changing the runoff yield and concentration conditions so as to produce a compensated and regulated effect. In other words, the forest absorbs a greater part of the precipitation, soil flow and ground flow. It supplies evaporation and transpiration and also recharges into the river as dry season runoff. If the

1—Huaibei 2—Jianghui 3—Dabieshan Mtn Area 4—Drainage Net Along the

Yangtze River 5—Wannan Mtn Area X.—mean value in whole Anhui

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annual runoff

Fig. 3 Impact of forest on annual runoff.

effect on transpiration or evaporation efficiency surpasses the effect on compensation runoff efficiency, and the transpired moisture is not able to develop local precipitation, then the annual runoff would decrease. This phenomenon is quite evident in arid regions. On the contrary, if the effect on transpiration efficiency is lower than that on compensation efficiency, which occurs in humid regions, the annual runoff would increase. Comparative monitoring was carried out in five basins in the Loess Plateau along the middle reaches of the Yellow River in China. The experimental results from five contrasting areas indicated that the annual runoff coefficients in four forest regions decreased by 23-50% compared with non-forest regions. In only one of the four regions did the coefficient increase, and that by 9.1%. In five other experimental regions within the stream systems of Xiangjiang, Minjiang, Tojiang, Tangbaihe, and Hanjiang etc., in the Yangtze River Valley, the annual runoff coefficient for forest regions increased by 33-218% compared with non-forest regions. Figure 4 illustrates the correlation of forest coverage to annual runoff in Anhui Province.

However, forest compensation and regulation activities improve the annual variation of runoff and yearly runoff distribution in arid regions and in humid regions equally. This means that runoff variations between flood years and drought years and the degree of runoff distribution within a year is reduced according to the extent of forest coverage.

EFFECTS OF FORESTS ON FLOODS

Interception

When rainfall begins, it is intercepted by the leaves of the crown canopy. For example, the total needle length of a 100 year old growing pine may be

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f o r e s t coverage ( % ) Fig. 4 Correlation of forest coverage with annual runoff coefficient normal.

estimated at 250 km. The leaf surface area of a hardwood forest may be perhaps 75-100 times larger than its shadow area. Hence the forest can intercept rainfall very well. The interception effect is conditioned by crown density, crop composition, forest age and growth stage (Fig. 5).

The larger the amount of leaf storage per unit area the more rainfall is needed to wet the leaves and the greater the interception effect. In the case of forest crops of similar compositions, precipitation intensity would be inversely proportional to the interception amount, and when the precipitation intensity reaches a limiting value, the interception amount tends to its limiting value as well. This relationship can be expressed by the following equation:

I^IJI-VQHK/IJR]} (2)

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f o r e s t age ( y e a r s )

200

jFïg. 5 Relationship between forest age and interception amount (crown density = 1).

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where / is the specific interception amount; / is the maximum amount; K is a coefficient which varies with tree species (largest for needle-leaved trees; medium for small, broad-leaved trees; and smallest for large, broad-leaved trees; fluctuating between 0.15 and 0.20); and R is the amount of rainfall.

Infiltration

In forest regions, the ground is usually covered by a thick layer of vegetable remains. Loose soil structure, good aeration and rich microbial activity produce excellent water absorption and enhance the function of infiltration. The infiltration capacity varies with crown density, thickness of forest litter and the scope of root osmosis. Data have proven that the amount of water absorbed by the soil covering in a David Poplar forest may be equivalent to 3.16 times more than the covering itself. With the formed tabular pine forest, it may be 2.2 times. The initial infiltration capacity for forest land is 3.4 times that of grassland, 4 times that of farmland and 10 times that of solid soil. Figure 6 illustrates the experimental data obtained in China.

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Fig. 6 Relationship of crown density to infiltration capacity.

Lag of flood concentration time

The increase in surface roughness of forest land leads to prolongation of the concentration time of flood on slope areas. Therefore, peak discharge is reduced and the period of flood rising is increased. From the viewpoint of hydraulics, in the case of slope confluence or in the case of channel concentration, the concentration flood velocity is determined by water surface gradient, hydraulic radius, surface roughness and other hydraulic factors. As roughness in a forest region is 10 to 100 times greater than that in a non-forest region, the forest plays an important part in retarding flood peak, reducing peak discharge and preventing potential flood disaster. Figure 7

419 Hydrological effects afforests

presents the relationship between forest coverage and mean annual maximum discharge of the main rivers in China. The basins have similar characteristics but different forest coverage.

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100 10 20 30 40 60 100 200 300 500 1000

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Fig. 7 Relationship between forest coverage and annual maximum discharge normal for different drainage areas.

INFLUENCE OF THE FOREST ON SOIL CONSERVATION

Sediment arises from erosion. Generally, soil erosion is maintained in balance with soil formation due to the weathering of parent rocks. River sediment is caused by natural erosion and mostly by human activity. Figure 8 indicates that the volume of generated sediment is an important factor of erosion intensity and erodibility which is conditioned by land use. When land resources are used rationally, the degree of erosion will be reduced and erosion durability of soil strengthened. The forest makes positive con­tributions to soil conservation. Firstly, it mitigates topsoil erosion by wind, water, gravity, etc. Secondly, it provides the surface with a litter covering, strengthening the erosion durability of the surface soil and lessening the erodibility. The process confirms the following principles: soil is washed away by rainfall and sediment is carried away by the flow, in agreement with the laws of hydraulics.

According to the Law of Falling Bodies, the speed of rainfall, which leads to washing off of the topsoil, is directly proportional to the square root of the vertical distance fallen by the rainfall. The washing force is referred to the crown canopy only when the raindrops fall to the ground through the

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Fig. 8 Processes of sediment production.

crown canopy. Thus the distance fallen is greatly reduced. The flow concentration velocity of a sloping surface, V, is directly

proportional to the square root of the slope gradient, S, and inversely proportional to its roughness, n. Forest vegetation decreases S but increases n. Hence the degree of erosion of the slope from concentration of flow in a basin is lessened and the sediment discharge reduced.

If the kinetic energy of water Kg represents wash-off capacity, then Kg = V2/2 , it is directly proportional to V2.

The amount of soil erosion, i.e. the total sediment discharge of a basin, R, is directly proportional to F5 and the size of silt particles carried away by the flow is directly proportional to V6.

The forest can retard erosion action, and in addition, improve soil erosion durability. This is due to the following reason: the roots of the forest and vegetation and other plant cover build up a net-like system beneath the topsoil, which generates a soil-protective cover, keeping the soil from being directly washed away by rainfall. The infiltration increases so that concentration time is retarded. The sediment flow decreases the sediment content, when it goes through infiltration by the forest litter covering. Furthermore, the forest humus intensifies microbial activity, improving soil aeration. All of this is of benefit to soil and water conservation. Figure 9 illustrates the relationship between average annual discharge and annual sediment discharge from Dabieshan Mountain area and Huangshan Mountain area, in Anhui Province. The figures clearly demonstrate the great role played by the forest in preventing water loss and soil erosion.

INFLUENCE OF THE FOREST ON WATER QUALITY

Water balance is based on the water cycle and water quality balance. Forests are located mostly in mountainous areas, where rivers originate. Therefore, the forest exerts the following almost unique effects on the regulation and

421 Hydrological effects afforests

2 3 4 5 10 20 30 50 100

annual average discharge (m3 s'1 }

Fig. 9 Relationship between annual average discharge and annual average sediment discharge with forest coverage as parameter.

improvement of water source quality: (a) the forest regulates the balance of atmospheric 0 2 and maintains the

oxygen cycle by means of photosynthesis, thus intensifying the self-purification of air;

(b) the forest diminishes poisonous and harmful gases, dust loads etc., in the air by adsorption, assimilation, biochemical activity and sterilization, thus reducing pollutants dissolved and carried by rainfall in precipitation;

(c) part of the pollutants in rainfall is dissolved and removed after being affected by the biochemical action of crown canopy, leaves, branches and stems; and

(d) the rainfall which falls on the ground is assimilated and filtered by the litter covering and roots so as to be purified. Figure 10 illustrates these processes. It was found from data measured abroad that soluble materials in the

valley flow from non-forested mountain slopes amount to 16.9 t km"2, while the content in the flow from forests is only 6.4 t km"2. According to average annual levels of the ions Ca2+, Mg2+, Na+, K+, SO2;, Cl", C03 and HCO'y and the degree of water mineralization measured for many years in rivers from Anhui Province, the author has calculated weighted arithmetic means for

Yu Yannian 422

precipitation a i r puri­fication

biochemic purification

oxygen supply in photoyn thesis

< forest action > X

transpiration intercept! oil <S f i l t ra t ion

—I f i l t ra t ion by fo res t - l i t t e r

f i l t ra t ion by soi l & roots

purified flowing into rivers

Fig. 10 Processes of water purification by forest.

different regions in the Province, which appear to correlate well with forest coverage (Figs 11 and 12).

Because the observed data are limited, the effect of forests in preventing pollution in water bodies could not currently be quantitatively analysed, but it is a subject worth studying in the future.

The forest participates in and protects the water balance in the hydrological cycle. It not only participates in but also regulates the equilibrium of quantity, heat, energy and quality. Therefore, it is very important to protect the forest in order to improve the water balance in the hydrological cycle and

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Fig. 11 Relationship between forest coverage and annual total ion content normal.

423 Hydrological effects afforests

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Fig. 12 Relationship between forest coverage and annual mineralization degree normal.

increase the benefits for mankind. The role of forests can never be replaced by that of any other structure. In the development of water resources and the regulation of basins, we could obtain twice the result with half the effort by weighting structural measures equally as importantly as biological measures.