The hydrological influence of black locust ... - Wiley Online Library

HYDROLOGICAL PROCESSES, VOL. 6,241-251 (1992)

THE HYDROLOGICAL INFLUENCE OF BLACK LOCUST PLANTATIONS IN THE LOESS AREA O F NORTHWEST CHINA WANG YANHUI The Institute of Forestry, The Chinese Academy of Forestry, Beijing, China ABSTRACT Black locust (Robina pseudoacacia) has become one of the most important shelter species in the loess area of northwest China. This paper summarizes recent research concerning its hydrological influence, including canopy interception, litter absorption capacity, its effect on rainfall kinetic energy, infiltration rates, surface runoff, soil moisture, and evapotranspiration, and its role in soil conservation. Several predictive models are listed. O n the basis of existing results, optimum characteristics for an effective plantation are defined, and problems requiring further research are identified. KEY WORDS Black locust plantations Hydrological impact Interception Rainfall energy Infiltration Soil moisture Surface runoff

Evapotranspiration Water and soil conservation Loess area

INTRODUCTION Black locust (Rohina pseudoacacia) was introduced into China from Germany at the beginning of the 20th century. Since it readily adapts to harsh conditions and it can supply shelter, fuel wood, fodder, fertilizer, and honey, its value was rapidly recognized. It is one of the most successful tree species introduced since the 18th century and its area of cultivation is also the largest. It is now widely cultivated within a large region of China lying between latitude 23"-46" north and longitude 124'46" east although it is concentrated, between latitudes 32"-36" north and particularly in the loess area of northwest China (NWC). The loess area of NWC is characterized by sparse vegetation and serious problems of soil and water loss. Black locust plantations are widely used as shelter forests and with the expansion of soil and water conservation programmes its planting will further increase. Its function in restoring the ecological balance, in regulating the hydrological regime, in water and soil conservation, in watershed management, and sustainable development of the rural economy, will become increasingly important. In recent years, Chinese scientists have undertaken some preliminary research on its multiple benefits, but in this paper, emphasis will be given to summarizing recent research on its hydrological influence. All the research sites referred to in this paper are located in north Shaanxi Province and east Gansu Province. This is the central part of the loess region where losses of water and soil are at their most serious. In this region, the mean annual precipitation is 400-600 mm, and more than 60 per cent of this is concentrated into the rainy season which extends from June to September. Because of the low infiltration rate and reduced vegetation cover, summer storms often cause floods and serious losses of water and soil. In this region, annual erosion rates and runoff depths are typically 4000-10000 t km-2 year-' and 25-50mm year-' respectively. RESEARCH RESULTS Research on the hydrological influence of black locust plantations has mainly focused on the following seven aspects: rainfall interception, change of rainfall energy characteristics, infiltration, surface runoff, soil moisture, evapotranspiration, and soil conservation. 0885-6087/92/020241-11$05.50 0 1992 by John Wiley & Sons, Ltd.

Received 20 March 1990 Accepted 15 October I990

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Rainfull interception Precipitation, mainly comprising rainfall in this region, represents the input to the forest-water system. Where a vegetation cover exists, rainfall will be intercepted (canopy interception, stemflow, litter absorption) and both its quantity and quality will be changed, generating a significant influence on subsequent hydrological pathways. There are many factors affecting canopy inteception, including rainfall characteristics, meteorological conditions, and vegetation features. According to Liu Bingzheng et al. (1989), in the case of average rainfall and typical plantations with ages greater than eight years, for rainfall amounts of less than 10 mrn, 10-30 mm and more than 30 rnm, the interception will be 2.5 mm, 3.5 mm, and more than 3.5 mm but with a maximum of 7 mm, respectively. For a plantation with a canopy density of 0.3-0.5, 0.5 0.8, and denser than 0.8, the corresponding interception rates of annual rainfall will be 8.36 per cent, 10.67 per cent, and 15.96 per cent respectively. The rates observed by Yang Xinmin and Yang Wenzhi ( 1 989) were slightly lower, and this may be due to differences in vegetation conditions and other factors. Taking account of the physical mechanisms involved and making some simplifications, Wang Yanhui (1986) derived a simple model which takes account of the effects of canopy density and rainfall amount. Because of the lack of meteorological data during tests and the high and relative stable air humidity during rainstorms, the effects of meteorological factors on evaporation of intercepted rainfall were simplified in this model by assuming that the evaporation rate per unit canopy projected area was constant. The parameters were determined by fitting the model to measured data for different sized single trees:

Ic

=

I:,

(

1 - exp

(-

+ 0.051 T

the average depth of interception over the canopy projected area of a single tree during an individual rainfall event (mm); IT, = the absorption by leaves and twigs (mm); P = rainfall amount (mm); C , = canopy density (decimal); T = rainfall duration (h); AH = canopy depth (m); AT = time interval from the beginning of the rainfall event to the end of the previous one (d).

where I ,

=

This research also showed that the quantity of stemflow is mainly dependent on the rainfall amount. The stemflow model derived from measured data is as follows:

where S,

=

the average depth of stemflow over the canopy projected area of a single tree during an individual rainfall event (mm).

The above models can be used to estimate the amount of interception and stemflow in a given stand for a single storm or for a longer period, as long as the rainfall amounts and vegetation structure are given. The two equations provide a useful basis for building a model of forest hydrology or similar system models. Because the interception models proposed by other researchers are mainly simple linear regression equations, they will not be discussed in detail. Since the canopy is generally closed in this region, grass and shrub cover beneath the trees are usually sparse, and the ground cover is mainly composed of litter. The litter therefore frequently exerts an important influence on hydrological processes. Rainfall absorption is only one of such influences. Others will be considered later. The quantity of litter reflects the growth status of the stand and the litter retaining

HYDROLOGICAL INFLUENCE OF BLACK LOCUST PLANTATIONS

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properties of the forest floor (Liu Bingzheng et al., 1987). In general, the litter cover varies within the range 0.25-0-97 kg m-2. According to Liu Bingzheng et al. (1989), under natural conditions, the saturated absorption capacity of litter can reach to 2.93-3.76 times its air dry weight. However, laboratory measurements reported by Wang Yanhui (1986) included values of only 1.73. The differences may reflect contrasts in the degree of litter decomposition and the measurement technique. Further research has shown that the absorption capacity declines as ground slope increases. On slopes of 30°, 25", 20°, and 15", the relative absorption capacities are 1, 1.05, 1.15, and 1.28 respectively. It was estimated that the annual water retention of litter is 232-500 m3 ha-', i.e. a rainfall depth of 23-50 mm will be taken up by litter absorption, accounting for ca. 8 per cent of the annual precipitation (Liu Bingzheng et al., 1989). Canopy interception and litter absorption result in a decrease in the effective rainfall and the input to subsequent hydrological processes. Change in rainfall characteristics Canopy interception will lead to an uneven spatial distribution of rainfall beneath the canopy. Furthermore, stemflow will concentrate rainwater at the stem base, and may generate localized runoff. Rainfall reaching the canopy, will subsequently be divided into four parts, namely, interception loss, stemflow, direct throughfall, and canopy drip. Even though direct throughfall and canopy drip are commonly grouped together and termed throughfall, the two components differ in certain respects. The raindrop size distribution (SD) and raindrop kinetic energy (KE) of direct throughfall remain the same as those of rainfall in the open, whilst those of canopy drip are modified because of the effects of interception. The mean fall height of canopy drip as used in KE calculations is taken to be the centre height of the canopy (Wang Yanhui, 1986). In this location, the main factors affecting runoff generation are rainfall rate and rainfall KE, rather than rainfall depth. One reason is that runoff is mainly generated by rainfall excess because of the low infiltration rates (Zhao Renjun, 1984). Another reason is that raindrop impacts on bare soil surfaces will form a surface crust which will lead to a reduced infiltration rate (Liu Zhi et al., 1988; Wang Yanhui, 1986). In this context, the influence of vegetation on SD and KE are important aspects of research on the influence of forests on hydrological processes in this region. Research by Wang Yanhui (1986) showed that the SD of canopy drip is unaffected by the rate and type of rainfall in the open. Fitting the Best (1950) equation to measured data, the SD of canopy drip under black locust was represented by the relationship: F

where F

=1

-

enp(

-(&5)2'6126)

(3)

= the

percentage of the accumulated volume of rainfall for which drop diameters are equal to or smaller than D; D = raindrop diameter (mm). Since the fall height of canopy drip is usually lower than the height needed to achieve terminal velocity, it is necessary to estimate values of KE per unit volume of canopy drip for different fall heights (Figure 1) (Wang Yanhui, 1986). The KE curve in Figure 1 can be subdivided into 4 segments represented by the following equations: Om < H 3m < H 6m < H 12m d H

< 3m, Ed, = (0.0309 + 0.10175H-')-' < 6m, Ed, = (0.03274 + 0-09085H-')-' < 12m, E d , , = 23.8(1 - exp(-0.3015(H + 0.91511))) < 30m, Ed, = 23.8(1 - exp( -0.14257(H + 18-6978)))

where, Ed, = KE per unit volume of canopy drip (J m-' mm-'); H = fall height of canopy drip (m).

(4)

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WANG YANHUI 24

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Figure I . The relationship between K E per unit volume of canopy drip and fall height

The K E per unit volume of rainfall in the open varies with the type (frontal or convective) and rate of rainfall. Taking the example of convective rainfall in eastern Gansu Province (Wang Yanhui, 1986), the relationship is: E where E I

= =

=

(5)

34.07131°.2947

K E per unit volume of convective rainfall in the open (J m-' mm-'); rainfall rate (mm min-').

The extent to which the rainfall K E in the open is reduced by the canopy is a very complex question. Different conditions will lead to different conclusions. Taking the above example, if we only consider the difference in K E per unit volume of rainwater between canopy drip and rainfall in the open ( A E ) , then: AE

=

Ed, - 34.07131°'2947

(6)

By simulating the case of a convective rainfall event, the influence of the canopy in decreasing KE relative to that in the open is demonstrated in Figure 2. This involved a general stand of 8-year-old black locust trees and a highly erosive convective rainfall which led to the maximum rate of surface runoff and soil erosion

E

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Figure 2. A simulated relationship between the degree ol reduction in the kinetic energy of rainfall beneath a black locust canopy and canopy density ( C d ) ,canopy thickness (AH), and canopy height ( H )

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within a two year period. In this simulation, while the influence of one specific factor was being evaluated, other factors were maintained at their natural values. The results show that, within the natural range of variation, if the canopy density is raised from 0.4 to 1-0,the percentage decrease of KE increases from 24 per cent to 48 per cent. If the canopy height is reduced from 9 m to 3 m, the decrease in KE will be increased from 35 per cent to 45 per cent. Only in the range below 3 m, does a decrease in canopy height lead to a sharp decrease in KE. For example, if the canopy height is 0.5 m, the percentage decrease will be increased to 64 per cent. The decrease of KE caused by varying canopy thickness (AH) shows very limited variation. According to this relationship, when a good ground cover cannot be maintained, forestry activities which are able to provide a low and dense canopy are effective measures for decreasing KE and surface runoff and preventing soil erosion. This is particularly the case when trees are combined with shrubs or managed as shrubby fuel forest (Wang Yanhui, 1986). The function of the litter layer in decreasing KE has not yet been directly measured but when the effective K E impacting the soil surface is being calculated, it is usually assumed to be equal to the product of throughfall KE and the bare ratio of the soil surface. The influences of throughfall and stemflow on water chemistry has not yet been studied. Infiltration Because trees increase the soil organic matter content and the ground cover, and the root systems increase soil porosity, the physical properties of forest soils will be improved, i.e. the content of water-stable aggregates, the soil porosity, and the infiltration rate will all increase. Liu Bingzheng et al. (1987) have measured infiltration rates under different aged forests and under farmland and grassland, with double-ring infiltrometers. The infiltration curves represented by his fitted equations are presented in Figure 3. From these it can be concluded that infiltration rates for the forest floor under black locust plantations are significantly higher than those for natural grassland and farmland. Wang Baitian and Yan Shuwen (1988) have also measured infiltration rates under a 6-12-year-old Black Locust, using a double-ring infiltrometer. They fitted the Philip equation to their measurements, i.e.

I

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1 MI

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Time (minutes)

Figure 3. A comparison of infiltration rates under different vegetation covers

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WANG YANHUI

Based on the values of S and A obtained for each sampling point, when soil water content (v) is 0.25 g cm-3, the average values of S and A for 11 forest sites located on the lower part of a slope were 4.7212 and 0.2935 and the average values for 10 forest sites located at the upper part of a slope were 4.6622 and 0.6658. The equivalent values for farmland on the upper part of a slope were 3.1008 and 0.1220. The effects of Black Locust plantations in increasing infiltration rates are thus clearly demonstrated. Analysis of infiltration parameters for forests has, however, shown that the increase in infiltration rates under 6-7-year-old plantations is minimal. Obvious improvement only occurs under trees older than 10 years. Surfuce Runoff Since surface crusts easily form on bare soils under raindrop impact, and since most runoff is generated by rainfall excess in this region, surface runoff is controlled by both KE and rainfall rate. An index of rainfall erosivity, E J , , , which is similar to the E I , , index used in the Universal Soil Loss Equation (USLE) was used by Wang Yanhui (1986). Here, E , is the effective rainfall KE impacting the soil surface which is assumed to be the product of throughfall KE and the bare ratio of the soil surface, I , , is the maximum continuous 10 minute rainfall rate during a rainfall event. The selection of 10 min was based on a stepwise regression analysis with a range of time durations. The selection of 10 min also shows a close correspondence with the observation that most runoff is generated within a period of only a few minutes (Zhao Renjun, 1984). Research by Wang Yanhui (1986) showed that, although infiltration rates should theoretically vary inversely with antecedent soil water content, the sparse cover and increased raindrop impact on soils with a lower water content, where there is increased soil dispersion compared with that on soils with a higher water content, will lead to a greater crust forming ability. These two conflicting influences make the relationship between infiltration rate and antecedent water content complex. He indicated that when the antecedent water content of the 0-5 cm soil layer is less than 14 per cent, the runoff ratio is inversely related to water content; whilst when it is higher than 14 per cent, the relationship assumes a directly proportional form. This can be explained by the two conflicting effects noted above. Wang Yanhui ( I 986) analysed measurements of surface runoff from 15 different vegetated plots (black locust plantation, natural grassland, bareland) for two years by stepwise multiple regression. A wide range of possible controlling factors, such as rainfall, topography, vegetation, and soil were considered. The resultant regression equation for surface runoff was given as follows: R , = P,(0.1530

-

0.2041C

+ 0~000555l(l- P,.)EJIo)

(8)

where R , = surface runoff during a rainstorm event (mm); P, = effective rainfall (mm) i.e. the rainfall in the open minus canopy interception and ground cover (which is mainly composed of litter) absorption; C = ground cover ratio (decimal); P, = level soil preparation ratio (i.e. level terrace, level pitting) (decimal); E J , , = effective rainfall erosivity at the ground surface (J m - 2 mm min-'). Employing the rainfall event and plantation stand used previously for KE simulation, the influence of several parameters on surface runoff generation were simulated using equation (8). The results are shown in Figure 4. The results indicate that the main factor causing decreased surface runoff is the ground cover ratio. Other significant factors in order of importance, are soil preparation ratio, canopy density, and canopy height. The variation caused by changing the canopy thickness is very limited. It is therefore important to maintain a high ground cover ratio. Wherever a good ground cover cannot be maintained, it is recommended that trees should be planted on prepared soils and/or mixed with shrubs and/or shrubby vegetation managed as fuel forest, so as to form a low and dense canopy which will reduce surface runoff. Research on a 14-year-old black locust plantation undertaken by Yang Xinmin and Yang Wenzhi (1989) showed that, even though the runoff ratio for individual rainstorm events can reach as high as about 13 per cent, the annual surface runoff ratio is only 2--4 per cent. Research by Hou Xilu (1985b) showed that an


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c, c, P, 1

H (m)AH (m)

Figure 4. A simulated relationship between the degree of reduction in surface runoff associated with a black locust stand and canopy density (Cd), canopy thickness (AH), canopy height (H), ground cover ratio (C), and the degree of soil preparation (f,)

8-year-old Black Locust plantation can reduce surface runoff by 86-93 per cent compared with that on farmland. Because ground roughness will be increased by litter, the velocity of surface runoff on the forest floor will be greatly decreased. The precise effect will however vary with different flow and ground conditions. At present, this effect has not been rigorously examined, but there are some approximate simulation results. According to Zhou Hongqi (1982, unpublished), with a rainfall rate of 2.1-2.6 mm min-', and a ground slope of 15", the velocity of surface runoff on a bare surface was 5.0 m min- ', while that on a forest floor covered by litter was reduced to 1.05 m min-', i.e. the velocity was reduced by 79 per cent. According to other research, with a littler layer 3 cm thick and a ground slope of 25", the velocity on a forest floor was 0.05 m min-', whilst the velocity on farmland with the same slope reached 4.60 m min-', i.e. the velocity on forest floor was only 1 per cent of that on farmland (Shandong Province Institute of Forestry, 1975). At present, most research concerning the influence of black locust plantations on surface runoff has been limited to the scale of the runoff plot. The few investigations undertaken at the small watershed scale have not been fully reported. As a result, quantitative assessments of the influence on watershed runoff and on watershed flooding as well as on watershed runoff convergence are not yet available.

Soil water content In northwest China, the water table is very deep. The primary source of soil water is precipitation. Soil water content exhibits a well-defined seasonal variation in response to the seasonal variation of precipitation. According to Yang Xinmin and Yang Wenzhi (1989), soil moisture levels reach a minimum during the period from June to July when the mean water content in the 0-200 cm layer is only 9.3 per cent, representing a water depth of 241.5 mm. The rainy period which lasts from the third ten days in June to the second ten days in September is the period of replenishment. The long-term average replenishment during this period is only 43.6 mm, representing 13.5 per cent of the rainfall during the same period. The infiltration depth is less than 250 cm. Because of the complex topography and the spatial variation of evapotranspiration, the distribution of soil water in a watershed is very uneven (Hou Xilu, 1985a). Most runoff is generated during the rainy season and the water content of the surface layer is the major controlling factor. Wang Yanhui (1989) has investigated the pattern of soil water depletion during the rainy season on bareland, grassland, and stands of 8-12-year-old black locust. The parameter K , in the equation: W

= (W, -

Wmi,)K'

+ Wmin

(9)

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WANG YANHUI

whcre W W,

water depth contained in a soil layer (mm); water depth (mm); Wmin= the minimum water depth (mm), usually taken as the value for the wilting point; t = time elapsed (d), =

= initial

has been used to represent the soil water depletion. The average K for different conditions, such as vegetation type, position on a slope, and soil depth (0-5 cm, 5 10 cm, 10 20 cm, 20-40 cm) have been calculated and are presented in Figure 5. From Figure 5 it is clear that the most important effect on soil water depletion is exerted by slope position. The difference in K caused by slope position is often larger than that caused by vegetation type. The difference in K between forest land and natural grassland is small, and the pattern of variation of K us. depth is also similar. One significant feature of K for vegetation covered sites is that I< is relatively high in the surface layer and relatively low in the deeper layer, giving a low gradient slope to the relationship, while K values for bare land exhibit contrasting behaviour. Eoapotranspiratinn

The physical mechanism of evapotranspiration is very complex and it responds to many controlling factors. It is very difficult to describe its dynamic variation precisely. At present, the main purpose of the research on black locust plantations is to evaluate the mutual relationship between afforestation and the soil water balance, because the soil water deficit is the most important factor affecting the survival and growth of trees. Yang Xinmin and Yang Wenzhi (1989) have investigated the evapotranspiration regime of a 14-year-old black locust plantation using the water balance method. The result showed that the annual evapotranspiration varied between 402.7 and 632-4 mm with an average of 524.6 mm, while the annual precipitation varied between 360.3 and 702.7 mm with an average of 531.6 mm. The ratio of annual evapotranspiration to annual precipitation varied between 0.84 and 1.12, with an average of 0.99. Evapotranspiration represents the major water output from the black locust plantation ecosystem. Using the high frequency weighing technique, Zhou Niangen and Zhou Jialin (1983) have studied the transpiration rate of 12-year-old black locust during the growing season. It was shown that black locust transpires both day and night defining a single peak curve. The peak appears at 12:OO and the period of high rates extends from 1O:OO to 16:OO and the period of minimum rates extends from 0O:OO to 04:OO hrs. According to the author's conversion of the original data, the maximum rate is 0.42 mm hr-'. It was also shown that the seasonal pattern of daily transpiration exhibits a double-peak curve. One peak occurs during

10-

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Figure 5. The variation of K with soil depth for different vegetation types and slope positions


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the last 10 days of June and another during the first 10 days of August. The period extending from the second 10 days in June to the first 10 days in August is the period of highest values (Figure 6). Soil conservation Because of the serious soil erosion problem in northwest China, the influence on soil loss represents a very important part of the research related to the hydrological impact of black locust plantations. Only a brief review will be provided in this paper. In addition to the process modifications mentioned above, there are other effects through which soil erosion can be reduced. These include the fixing of the soil by the root system, increasing the content of soil organic matter, and decreasing the erodibility of the soil. Liu Bingzheng et al. (1984, 1987) have investigated the importance of the root system and have measured the resistance to soil wash using a rainfall simulator device. It was found that the wash resistance of forest land is higher than that of grassland and farmland. The wash resistance is mainly governed by the content of thin roots ( < 2 mm in diameter) and increases with forest age. It was also found that if the content of thin roots in the 0-50 cm soil layer reaches 0.06 g 100 cm-3 (i.e. that generally associated with an 8-year-old plantation), the wash resistance will be significantly increased but, thereafter, further increase in the thin root content will not lead to a further significant increase in wash resistance. The soil reinforcement effect of the root system was also measured by Yang Weixi et al. (1988) using a method based on the extraction of a single root. Their measurements and calculations demonstrated that the reinforcement associated with black locust forest is three times that associated with Chinese pine (Pinus tabulaeforrnis Carr.). The degree of reinforcement increases with forest age, but the rate of increase will reduce when the age reaches 15 years. Wang Yanhui (1986) has undertaken detailed research on the effects of black locust plantations on soil loss. Many controlling factors were considered, such as those in Equation 8 and the potential energy of runoff. Stepwise multiple regression analysis was used to derive a soil loss equation of the form:

where S , = soil loss during a rainstorm event (g m-2); E p = mean potential energy of surface runoff (J m-2). By simulating the soil loss associated with the same rainfall events and the same plantation stand used in the simulation of KE and surface runoff, it was found that the relative importance of the factors affecting soil loss is the same as for surface runoff (Figure 7). It was also shown that an effective shelter forest of black locust established for water and soil conservation in this area should have a ground cover ratio higher than 0.5 and a canopy that is nearly or completely closed. In this situation, soil loss will not exceed low levels (100-500 t km-') even under heavy rainstorms.

Date

Figure 6. The seasonal variation of daily transpiration from a black locust plantation

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W A N G YANHUI

.......................... ...................y : :

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Figure 7. A simulated relationship between the degree of reduction in soil loss associated with a black locust plantation and canopy density (Cd), canopy thickness (AH), canopy height (H). ground cover ratio (C), and the degree of soil preparation (P,)

Research undertaken by Hou Xilu (1985b) showed that the annual soil loss from a 6-9-year-old black locust forest was only 1-2 per cent of that from farmland. SUGGESTIONS AND DEDUCTIONS In summary, the results of a wide range of research investigation undertaken in the loess area of northwest China have demonstrated both quantitatively and qualitatively that black locust plantations can substantially modify hydrological processes. As most of this research was undertaken in recent years, much of it is still exploratory in nature. In the future, attention should be directed to general problems which include: 1. The need for a more systematic and physically-based approach to research investigations and data analysis. 2. The need to investigate aspects about which relatively little is known, these include water quality, water budgets, evapotranspiration, soil water variation and the influence of watershed scale (including runoff convergence, flooding, runoff ratio, and runoff composition). 3. The need for research on old plantations and on forest areas, in order to obtain a more complete evaluation. Based on the research outlined above, the features required for an effective tree cover to prevent water and soil loss may be summarized as follows: 1, A high ground cover ratio. This should be maintained at more than 0.5. During the early stages of forest development, the lower ratio can be increased by preserving or cultivating the ground cover, or compensated for by soil preparation. 2. A low and dense canopy. The canopy should be nearly or completely closed. When a higher ground cover cannot be easily maintained, the canopy should be as low as possible and include shrubs or shrubby growth managed as fuel forest.


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3. A sufficient age. With an age in excess of eight years, the wash resistance will be markedly increased; with an age greater than 10 years the infiltration capacity will be greatly increased; and with an age in excess of 15 years, the root reinforcement on forest land will reach a maximum. ACKNOWLEDGEMENTS

I would like to acknowledge the assistance of Professor Liu Changming of the Chinese Academy of Sciences, and Professor D. E. Walling of the University of Exeter, in making valuable suggestions for revising the manuscript. REFERENCES Best, A. C. 1950. ‘The size distribution of raindrops’, Quarterly Journal of the Royal Meteorological Society, 76, 16. Hou Xilu 1985a. ‘Movement of soil moisture and disposition of species of trees’, Bulletin of Soil and Water Conservation, 5(4), 9- 12. (in Chinese) Hou Xilu 1985b. ‘Observation and analysis for runoff and mudsand in different types of vegetation’, Bulletin of Soil and Water Conservation, 5(6), 35-37, 50. (in Chinese) Liu Bingzheng, Wang Youmin, and Chen Dongli 1984. ‘Study on soil wash resistance in Robinia forest’, Journal of Northwestern College of Forestry, 1(1), 83-94. (in Chinese) Liu Bingzheng, Wang Youmin et al. 1987. ‘A preliminary study on effect of improvement of Black Locust plantation’, Journal of Northwestern College of Forestry, 2( I), 48-57. (in Chinese) Liu Bingzheng, Wang Youmin, and Si Yubing 1989. ‘Research on the hydrological effect of the Robinia pseudoacacia stands’, Forest Science and Technology, 6, 14-16. (in Chinese) Liu Zhi and Jiang Zhongshan 1988. ‘Study on natural raindrop impacting effects on loess crusting of upland’, Bulleiin of Soiland Water Conservation, 8(1), 62-64. (in Chinese) Shandong Province Institute of Forestry 1975. Black Locust, The Publishing House of Agriculture, 1-12. (in Chinese) Wang Baitian and Yan Shuwen 1988. ‘Study on the relationship of soil hydraulic conductivity in artificial locust forest and its environmental factors on Loess Gullied-hilly Region’, Acta Conservationis Soli et Aquae Sinica, 2(2), 49-57. (in Chinese) Wang Yanhui 1986. ‘A quantitative study on the benefits of Black Locust on water and soil conservation in the Eastern Loess Area in Gansu Province’, Journal of Begins Forestry University, 8( l), 35-52. (in Chinese) Wang Yanhui 1989. ‘A study on the rule of depletion of soil water in rainy season’, Acta Conservationis Soli et Aquae Sinica, 3(2), 81-89. (in Chinese) Yang Weixi, Zhao Tingning, Li Shengzhi, and Zhang Wei 1988. ‘Study on root reinforcement of two tree species in Loess Plateau’, Acta Conservationis Soli et Aquae Sinica, 2(4), 38-44. (in Chinese) Yang Xinmin and Yang Wenzhi 1989. ‘A preliminary study on the soil water balance ofartificial forestland in loess-hilly region’, Scientia Siluae Sinicae, 25(6), 549-553. (in Chinese) Zhao Renjun 1984. Simulation of watershed hydrology - Xian An River model and Northern Shaanxi model, The Publishing House of Water Conservancy and Electric Power, 55-70. (in Chinese) Zou Niangen and Zhou Jialin 1983. ‘The preliminary report on the research of transpiration rate of Black Locust’, Shaanxi Forestry Science and Technology, 3, 1-5. (in Chinese)