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Water yield reduction due to forestation in arid mountainous regions, northwest China
International Journal of Sediment Research 25 (2010) 423-430
Water yield reduction due to forestation in arid mountainous regions, northwest China Pengtao YU1*, Yanhui WANG2, Xudong WU3, Xiaohong DONG4, Wei XIONG5, Gaowa BU6, Shunli WANG7, Jinye WANG8, Xiande LIU9, and Lihong XU10
Abstract Forestation has been encouraged worldwide due to increasing demand for forest products, and for its ecological benefits such as soil erosion control and sediment reduction. However, forestation reduces runoff, thus potentially aggravating water shortages in arid regions. In order to quantitatively estimate the possible water yield reductions caused by forestation in an arid region, a small watershed (the Pailugou watershed) in the Qilian Mountains of northwest China was chosen as a study area. The responses of hydrological dynamics to different forestation scenarios in the study area were simulated using the TOPOG model. The results showed that forestation could lead to a complete loss of runoff at the site scale. At the watershed scale, a 10% increase in forest coverage led to a runoff reduction of 25.6 mm, equivalent to 13% of the runoff in the un-forested watershed. However, due to climatological and topographical constraints, the potential forest distribution occupied only 46.3% of the watershed area, and runoff reduction was estimated to reach a maximum of 60% when the forest cover ratio increased from 0.41% to 46.1%. Actual forest coverage is 36% in the study area, thus the water yield will be reduced with any further increase in forest area. Our study suggested that a trade-off between the numerous benefits of forest coverage increase and its negative impact on water yield should be carefully addressed in arid regions with inherently severe water-shortage. Key Words: Arid region, Water yield, Forestation, Watershed scale, Site scale
1 Introduction Forestation has been encouraged worldwide, due to increasing demand for forest products and for its ecological benefits, such as soil erosion control (Yu et al., 2009), sediment reduction (Wang et al., 2008), hydrological regime regulation (Wang et al., 2008), and carbon sequestration (Winjum et al., 1997). However, forestation can cause a reduction in mean annual runoff of up to 44% in humid regions, based on a comparison between forest plots and grassland plots (Farley et al., 2005). The reduction is more notable in the semi-humid and semi-arid regions of China, where recent studies have indicated that runoff reduction can reach more than 50% after forestation (Sun et al., 2006; Wang et al., 2008; Yu et al., 2009; Bi et al., 2009). These studies suggest that forestation could adversely affect runoff yield in arid regions. Any reduction in runoff from mountainous areas in arid (desert) regions will seriously impact regional development, because mountain runoff is the primary water resource (Wang and Cheng, 2000). Nevertheless, mountainous areas in arid regions are still being forested, sometimes at increasing rates 1
Dr., 2 Prof. Dr., 4 Ms., 5 Dr., 10 Dr., Research Institute of Forest Ecology, Environment and Protection, The Chinese Academy of Forestry, Beijing, 100091, China, * Corresponding author, E-mail: [email protected] 3 Mr., 6 Ms., College of Ecology and Environmental Science, Inner Mongolia Agriculture University, Hohhot, 010018, China 7 Mr., 9 Prof. Dr., Academy of Water Resources Conservation Forests in Qilian Mountain of Gansu Province, Zhangye, 734000, Gansu, China 8 Prof. Dr., Tour College of Guilin University of Technology, Guilin, 541004, Guangxi, China Note: The original manuscript of this paper was received in May 2010. The revised version was received in Aug. 2010. Discussion open until Dec. 2011. International Journal of Sediment Research, Vol. 25, No. 4, 2010, pp. 423–430
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(Malagnoux, 2007), to reduce soil erosion. To date, most forest hydrological studies in arid regions have been carried out at the plot scale (for example, in the Qilian Mts., northwest China: Wang et al., 2008). Regional water resource changes after forestation were difficult to estimate from such plot scale measurements owing to the great physiogeographical heterogeneities in the mountains. Consequently, we know very little about the relationship between forestation and runoff changes in arid regions, which are the locations where water-shortages are most severe (Cheng and Zhao, 2008). Distributed eco-hydrological models have been used as powerful tools for estimating water yield changes at the watershed scale caused by vegetation dynamics. These models incorporate the spatial heterogeneity of environmental factors (Warrick et al., 1997; Krysanova et al., 2005; Wei et al., 2008). In this study, TOPOG, a distributed eco-hydrological model, was employed to simulate hydrological dynamics in a small watershed of the Qilian Mountains in northwest China. From these simulations, runoff variations caused by changes in vegetation type were analyzed. We aimed to quantitatively estimate the possible water yield reductions following increased forestation in this arid region of northwest China, and to identify the underlying mechanisms controlling water yield. 2 Methods 2.1 Study area The Qilian Mountains (97°24′-10°46′E, 36°43′-39°42′N) connect the Tibetan Plateau, the Mongolian Plateau and the Loess Plateau. The Qilian Mountains are the source of 56 rivers, and thus serve as an important water source area for the arid region of northwest China. Desert, grassland (Stipa-dominated steppe), forest (Picea crassifolia-dominated forest) and alpine vegetation form a vertical spectrum of vegetation belts from lower to higher altitudes in the Qilian Mountains. The study area was the Pailugou watershed (100°17′E, 38°24′N), a small watershed with an area of 2.74 km2 and elevation range of 2,650–3,770 m a.s.l.. It stretches across different vegetation belts (Table 1). Hydrological processes (precipitation, interception, evaporation, transpiration and surface runoff) were monitored in 40 study plots and the runoff was measured at the outlet of the Pailugou watershed (Wang et al., 2008). The measurements provided the possibility for parameter estimation and model validation. Table 1 Elevation (m a.s.l.) 2,650–3,000 3,000–3,300 >3,300
Vertical distribution patterns of soil and vegetation in the Pailugou watershed Aspect Vegetation type Soil type Sunny slope Grassland Haplic calsisol Shady slope Spruce forest Capcic luvisol Sunny slope Grassland and scattered shrubs Capcic luvisol Shady slope Spruce forest Capcic luvisol All slopes Sub-alpine shrub Hapludoll
At 2,580 m a.s.l., close to the outlet of the Pailugou watershed, mean annual air temperature is 0.5oC and mean annual precipitation is 435.5 mm. The annual pan evaporation is 1,051.7 mm. Mean annual relative humidity is 60%. The lapse rate of mean annual temperature is 0.58 oC/100m. Mean annual precipitation increases with elevation at a rate of 18.6 mm per 100 m (up to 3,300 m a.s.l.) and then decreases at a rate of 16.4 mm per 100 m above 3,300 m a.s.l. (Wang et al., 2008). The potential habitat for spruce forest in the Pailugou watershed is limited to shady slopes at elevations of 2,650–3,300 m a.s.l., an area accounting for only 46.3% of the overall watershed area. Patches of existing spruce forest are scattered in these habitats, and cover 36% of the Pailugou watershed. These are mature forests with an average canopy density of 0.7 and a density of 2,000 trees/ha. The average tree diameter at breast height (DBH) was 12.7 cm, and the average tree height was 8.3 m. An underdeveloped shrub and grass layer and a developed moss layer covered 33% of the ground in the spruce forest. 2.2 TOPOG model In the TOPOG model, the watershed was divided into hydrological units and the water flow trajectories were established from landform data comprising topographic contours, watershed boundaries, high points and saddle points. TOPOG excels in identifying micro landforms at fine scales and classifying them into individual hydrological units, since contours can describe landform in more detail than DEMs in which - 424 -
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some small, micro landforms are merged. Each hydrological unit identified by TOPOG was presumed to be homogeneous in terms of soil and vegetation. The main eco-hydrological processes, including vegetation growth, canopy interception, infiltration, vertical water movement in the soil profile, transpiration and soil evaporation were calculated in each hydrological unit. Lateral water movement (surface runoff and subsurface runoff) between neighboring hydrological units was also calculated. Evapotranspiration was described by a Penman-Monteith equation, and Richard’s equation was used to calculate water movement in the vertical soil profile. A full description of the TOPOG model is available on its website (www.per.clw.csiro.au/topog). The TOPOG model has been successfully applied to predict the hydrological effects of vegetation and land use changes such as crop rotation, forest harvesting and fire in tropical areas (Warrick et al., 1997; Hatton et al., 2002; Patrick et al., 2002), and to study the effects of soil moisture on vegetation growth (Silberstein et al., 1999). We improved the model in this study by separating precipitation into rainfall and snowfall using an air temperature threshold of 0 °C. When the mean daily air temperature was above 0 °C, the precipitation was classed as rainfall. Otherwise, it was classed as snowfall. Rainfall was routed and calculated according to the original TOPOG model. Some snowfall was assumed to be intercepted by the canopy, as follows. If daily snowfall was less than the capacity of canopy snow interception (Imax, mm), all the snowfall was assumed to be intercepted by the canopy. If daily snowfall was greater than Imax, the daily snow interception was set equal to Imax, given by I max = LAI ⋅ Rint (1) 2 2 where LAI is leaf area index (m /m ), and Rint is the coefficient for canopy snow interception (mm/LAI). The snow passing through the forest canopy was assumed to accumulate on the soil surface and start melting when the air temperature exceeded 0 °C. Daily snow melt rate (SMR, mm/d) is a function of mean daily air temperature (MAX, °C): SMR=D*MAX (2) (3) 0 ≤ SMR ≤ SNO where D is an empirical coefficient of snow melt (mm/°C) determined by the vegetation types and SNO is the amount of snow on the soil (mm). 2.3 Data input and scenario design The Pailugou watershed was divided into 5,879 hydrological units ranging in area from 24.0 to 5,449.1 m2. The vegetation type of each unit was determined from existing vegetation maps (1:2000 scale). Major soil and vegetation parameters, such as porosity (Table 2), were measured in 40 sample plots of size 20 m ×20 m. These parameters were then assigned to the hydrological units according to the unit vegetation type. Table 2 Parameters Area fraction (%) Soil type
Soil features and main vegetation types of the Pailugou watershed Vegetation types Spruce forest Shrub Grassland 36 23.8 39.9 Capcic luvisol Hapludoll Haplic calsisol
Soil depth (cm)
70
45
40
Saturated soil moisture (m3/m3)
0.67
0.50
0.58
Saturated conductivity (m/d) Maximum root depth (m)
4.00 0.70
1.26 0.45
1.49 0.40
In the Pailugou watershed, a standard meteorological station was installed at 2,580 m a.s.l. and meteorological parameters, such as air temperature, humidity and precipitation were measured according to the international meteorological observation standards. Daily climate data from this meteorological station collected in 2001 (annual precipitation 301.1 mm) and in 2002 (annual precipitation 411 mm) were used in this study to calculate the climatic parameters of each hydrological unit based on the unit elevation range and calculated lapse rate (Wang et al., 2008). Daily runoff data measured at the watershed International Journal of Sediment Research, Vol. 25, No. 4, 2010, pp. 423–430
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outlet were adopted for model calibration in 2001 and for model validation in 2002. Additionally, comparison between the simulated and measured transpiration of spruce trees was employed to test the model performance. Modeled interception was tested by comparing interception simulated by TOPOG and Eq. (4) above, with that calculated by an empirical equation for interception as follows: I = 0.4469 P 0.8016 (r2 = 0.69) (4) where I is the canopy interception (mm) per rainfall event with rainfall total of P (mm). This equation was established from measured interception data collected in spruce forests during 285 rainfall events in the Pailugou watershed (Zhang et al., 2000). Over the last several decades, numerous ecological programs have been launched and the area of forest has continued to increase in the arid regions of northwest China. Forest cover is planned to increase continuously, according to the China National Forestry Action Plan for Climate Change issued on Nov. 6, 2009. However, a forest cover of 100% can not be achieved on mountains in the arid region since patchy forests are limited to shady slopes. For example, the maximum potential forest fraction is 46.33% in the Pailugou watershed. Thus, 23 scenarios with forest cover ratios ranging from 0.41% to 46.33% were set (Table 3) in order to estimate forestation effects on runoff. Table 3
Scenario No.
Forest fraction (%)
Current S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 S23
36.00 36.15 36.67 38.08 40.89 42.15 43.45 45.95 46.03 46.12 46.33 35.00 33.00 30.70 26.36 23.58 17.34 16.90 12.42 8.58 5.89 3.46 1.61 0.41
Descriptions of the scenarios Scenario definition Elevation ranges in which the vegetation was changed (m a.s.l.) 2650–2700 2650–2750 2650–2800 2650–2850 2650–2900 2650–2950 2650–3000 2650–3050 2650–3100 2650–3150 2650–2700 2650–2750 2650–2800 2650–2850 2650–2900 2650–2950 2650–3000 2650–3050 2650–3100 2650–3150 2650–3200 2650–3250 2650–3300
Vegetation change -
Grassland on the shady slopes was changed to spruce forest
Spruce forest was changed to grassland
2.4 Calculation of potential runoff The potential runoff (R) of a hydrological unit in the watershed was calculated as follows: (5) R = P − ET − ΔW where P (mm) is the precipitation during a given period, ET (mm) is the evapotranspiration of this period simulated by TOPOG (as the sum of the interception of litter and canopy, transpiration, and soil evaporation), and ΔW is the increase in soil moisture in this period as simulated by TOPOG. Over a long period such as a year, ΔW can be assumed as zero. R=0 indicates that all the precipitation in the hydrological unit is consumed by the ecosystem through evapotranspiration. If R>0，evapotranspiration is less than precipitation, indicating that the unit generates - 426 -
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runoff. If R<0, then evapotranspiration exceeds precipitation, and not only the precipitation but also part of the runoff received from upper slopes is consumed by the ecosystem, indicating this hydrological unit contributes negatively to the runoff from the watershed. 3 Results
4.0
0
3.0
20
2.0
40
1.0
60
0.0
80
2001-1-1
2001-5-1
D aily p recip itatio n (m m )
D aily ru n o ff (m m )
3.1 Calibration and validation of TOPOG model The mean annual runoff of the Pailugou watershed was 98.8 mm, 82.3% of which occurred during the rainy season from June to October. Measured and simulated daily runoff from the Pailugou watershed in 2001–2002 showed good agreement (Fig. 1). Also, peak flows simulated by the TOPOG model matched well with measured peaks (Fig. 2). The absolute error of the daily runoff simulation was 0.02 mm on average. The maximum absolute error was 0.41 mm and the maximum relative error was 11%. These results demonstrate that the calibrated model was acceptable for simulating the daily and annual runoff in this watershed.
2001-8-29 2001-12-27 2002-4-26 2002-8-24 2002-12-22
M easured runoff Simulated runoff Fig. 1 Measured and simulated daily runoff from the Pailugou watershed in 2001 and 2002
M easured daily runoff (m m )
1.6 y=x 2
R = 0.9596
1.2
0.8
0.4
0 0
0.4
0.8
1.2
1.6
S imulated daily runo ff (mm) Fig. 2
Peak flows (measured, and simulated by TOPOG) in the Pailugou watershed during 2002
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The canopy interception simulated by TOPOG was close to that calculated by the empirical Eq. (4). For example, in spruce forest at an elevation of 2,730 m a.s.l., the interception simulated by TOPOG was larger than that calculated for all rainfall events in 2001 and 2002, but only by a maximum of 0.9 mm. The total canopy interception during the growing season (May–September) simulated by TOPOG was 2.7% larger than that calculated by Eq. (4). The transpiration simulated by TOPOG was 180 mm during the growing season in spruce forest at an elevation of 2,730 m a.s.l., which is close to the transpiration rate of 183.9 mm measured in the field. The above tests indicated that the calibration and validation of TOPOG were successful and that it could be used to simulate the ecohydrological processes in the Pailugou watershed. 3.2 Runoff comparison between different vegetation types The difference between the precipitation and total evapotranspiration, i.e., the annual potential runoff, simulated from spruce forests was negative for all elevations in the Pailugou watershed. At elevations lower than 3,000 m a.s.l., the simulated annual runoff from spruce forests was -38.0 mm on average, whereas it was less negative (-3.0 mm on average) at elevations higher than 3,000 m a.s.l.. This clearly demonstrated that the spruce forests on shady slopes absorbed part of the lateral water flow as well as generating no runoff from precipitation. The spruce forests growing at lower elevations absorbed more lateral water flow than those at high elevations. In contrast to spruce forests, the runoff from grassland varied from 61.6 mm to 192.2 mm with an average of 127.2 mm. The annual runoff from shrubland averaged 110.6 mm, ranging from 28.0 to 231.4 mm. Therefore, these results indicate that conversion of grass- or shrub-lands to forests could cause a complete loss of runoff at the site scale. 3.3 Effects of forest increase on watershed runoff When forest coverage was increased in the watershed, from 0.41% to 46.33%, the annual watershed runoff decreased from 196.6 mm/yr to 78.9 mm/yr under a mean annual precipitation of 455.7 mm. An increase of 10% forest cover led to a decrease in watershed runoff of 25.6 mm (Fig. 3), which accounts for 13.0% of the runoff from a watershed with no forest cover. This runoff reduction was mainly observed in summer (16.1 mm, 62.9%) and spring (6.3 mm, 24.5%), while only 10.0% was observed in autumn and 2.6% in winter.
An nu al ru n off (m m )
25 0 20 0 15 0 10 0 50 0 0
10
20 30 Forest fraction (% )
40
50
Fig. 3 Annual runoff from the Pailugou watershed with different forest cover ratios simulated by TOPOG model
4 Discussion Our results indicated that runoff at the site scale will be reduced to zero when grasslands or shrublands are converted to forest in this arid region. This runoff reduction would be more severe than the reported equivalent global-averaged runoff reduction of 44% in humid regions (Farley et al., 2005) and than the –50% runoff reduction in the semi-humid and semi-arid regions of China (Sun et al., 2006). - 428 -
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The absolute runoff reduction rate of 25.6 mm per 10% forest increase found in this study was higher than the equivalent figure of 15.8 mm observed in the semi-humid region of the Liupan Mountains in central China (Yu et al., 2009), although the precipitation in the Pailugou watershed was 200 mm less than that in the Liupan Mountains. This indicates that the runoff reduction due to forestation would increase with decreasing precipitation or increasing aridity from semi-humid regions to arid regions, opposite to the trend of absolute runoff reduction in the humid region where absolute runoff reduction is greater when precipitation is higher (Farley et al., 2005; Sun et al., 2006). More studies should be carried out in semi-arid regions to ascertain whether this observed trend is general or local. The negative calculated annual runoff from the spruce forest site means that more water was taken up by the forest than was provided by precipitation. This was due to the site-scale vegetation distribution, where forest commonly grows on the lower slopes and was supplied by soil water from the upper slopes, as shown by our observation of higher soil moisture levels after strong rainfall events on the lower slopes in this watershed. Despite the substantial reduction, the forestation did not reduce runoff to zero, because only part of the watershed area in this arid and mountainous region can support forests. For example, forests can only grow on shady slopes with elevations of 2,600–3,300 m a.s.l. in the central Qilian Mountains (Wang et al., 2008). Taking the Pailugou watershed as an example, the maximum forest area fraction is 46.3% and the corresponding maximum annual runoff reduction is 60%. However, such a runoff reduction may have serious consequences in an arid region already suffering from scarce water, and should be accounted for when developing strategies to address erosion control. Conversely, some forestation is needed since forests and trees are the backbone of arid zone ecosystems, acting as an important habitat and shelter for wildlife and flora, as well as being important for human communities by producing a wide range of goods and services (Malagnoux, 2007). It also should be emphasized that runoff reduction is greatest in summer (June–August, 62.9%) and spring (March–May, 24.1%) when frequent floods occur due to heavy rains. From this point of view, the runoff reduction by forests will minimize flood damage and reduce soil erosion. A trade-off between forest services and water yield, based on the quantitative understanding of forest effects on watershed runoff, is thus important to policy-making in arid regions. 5 Conclusions At the site scale, forestation could lead to a complete loss of runoff from mountainous slopes in arid regions, since the forest absorbs more water than it receives from precipitation. At the watershed scale in this study, an increase in forest area of 10% in the watershed led to a runoff reduction of 25.6 mm, which accounted for 13% of the runoff of the watershed without forest. The runoff reduction was concentrated in summer (June–August, 62.9%) and spring (March–May, 24.1%). The maximum runoff reduction due to forestation was 118.5 mm (60% of the runoff in the watershed without forest), but runoff was not reduced to zero because the maximum forest fraction is 46.3% in the watershed. Although there is no complete loss of runoff at the watershed scale, the predicted runoff is nevertheless significant in an arid region that already suffers from severe water-shortage. Acknowledgements We gratefully acknowledge funding from the National Science Foundation of China (40671038, 40730631), the State Forestry Administration of China (200904056, 200904005), the Chinese Academy of Forestry (CAFYBB2007038), and the Key Laboratory for Forest Ecological Environment of the State Forestry Administration of China. References Bi H. X., Liu B., Wu J., Yun L., Chen Z. H., and Cui Z. W. 2009, Effects of precipitation and landuse on runoff during the past 50 years in a typical watershed in the Loess Plateau, China. International Journal of Sediment Research, Vol. 24, No. 3, pp. 352–364. Cheng G. D. and Zhao C. Y. 2008, An integrated study of ecological and hydrological processes in the inland river basin of the arid regions, China. Advances in Earth Science, Vol. 23, No. 10, pp. 1005–1012. Farley K. A., Jobbagy E. G., and Jackson R. B. 2005, Effects of afforestation on water yield: A global synthesis with implications for policy. Global Change Biology, Vol. 11, No. 10, pp. 1565–1576. International Journal of Sediment Research, Vol. 25, No. 4, 2010, pp. 423–430
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International Journal of Sediment Research, Vol. 25, No. 4, 2010, pp. 423–430
Water yield reduction due to forestation in arid mountainous regions, northwest China Pengtao YU1*, Yanhui WANG2, Xudong WU3, Xiaohong DONG4, Wei XIONG5, Gaowa BU6, Shunli WANG7, Jinye WANG8, Xiande LIU9, and Lihong XU10
Abstract Forestation has been encouraged worldwide due to increasing demand for forest products, and for its ecological benefits such as soil erosion control and sediment reduction. However, forestation reduces runoff, thus potentially aggravating water shortages in arid regions. In order to quantitatively estimate the possible water yield reductions caused by forestation in an arid region, a small watershed (the Pailugou watershed) in the Qilian Mountains of northwest China was chosen as a study area. The responses of hydrological dynamics to different forestation scenarios in the study area were simulated using the TOPOG model. The results showed that forestation could lead to a complete loss of runoff at the site scale. At the watershed scale, a 10% increase in forest coverage led to a runoff reduction of 25.6 mm, equivalent to 13% of the runoff in the un-forested watershed. However, due to climatological and topographical constraints, the potential forest distribution occupied only 46.3% of the watershed area, and runoff reduction was estimated to reach a maximum of 60% when the forest cover ratio increased from 0.41% to 46.1%. Actual forest coverage is 36% in the study area, thus the water yield will be reduced with any further increase in forest area. Our study suggested that a trade-off between the numerous benefits of forest coverage increase and its negative impact on water yield should be carefully addressed in arid regions with inherently severe water-shortage. Key Words: Arid region, Water yield, Forestation, Watershed scale, Site scale
1 Introduction Forestation has been encouraged worldwide, due to increasing demand for forest products and for its ecological benefits, such as soil erosion control (Yu et al., 2009), sediment reduction (Wang et al., 2008), hydrological regime regulation (Wang et al., 2008), and carbon sequestration (Winjum et al., 1997). However, forestation can cause a reduction in mean annual runoff of up to 44% in humid regions, based on a comparison between forest plots and grassland plots (Farley et al., 2005). The reduction is more notable in the semi-humid and semi-arid regions of China, where recent studies have indicated that runoff reduction can reach more than 50% after forestation (Sun et al., 2006; Wang et al., 2008; Yu et al., 2009; Bi et al., 2009). These studies suggest that forestation could adversely affect runoff yield in arid regions. Any reduction in runoff from mountainous areas in arid (desert) regions will seriously impact regional development, because mountain runoff is the primary water resource (Wang and Cheng, 2000). Nevertheless, mountainous areas in arid regions are still being forested, sometimes at increasing rates 1
Dr., 2 Prof. Dr., 4 Ms., 5 Dr., 10 Dr., Research Institute of Forest Ecology, Environment and Protection, The Chinese Academy of Forestry, Beijing, 100091, China, * Corresponding author, E-mail: [email protected] 3 Mr., 6 Ms., College of Ecology and Environmental Science, Inner Mongolia Agriculture University, Hohhot, 010018, China 7 Mr., 9 Prof. Dr., Academy of Water Resources Conservation Forests in Qilian Mountain of Gansu Province, Zhangye, 734000, Gansu, China 8 Prof. Dr., Tour College of Guilin University of Technology, Guilin, 541004, Guangxi, China Note: The original manuscript of this paper was received in May 2010. The revised version was received in Aug. 2010. Discussion open until Dec. 2011. International Journal of Sediment Research, Vol. 25, No. 4, 2010, pp. 423–430
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(Malagnoux, 2007), to reduce soil erosion. To date, most forest hydrological studies in arid regions have been carried out at the plot scale (for example, in the Qilian Mts., northwest China: Wang et al., 2008). Regional water resource changes after forestation were difficult to estimate from such plot scale measurements owing to the great physiogeographical heterogeneities in the mountains. Consequently, we know very little about the relationship between forestation and runoff changes in arid regions, which are the locations where water-shortages are most severe (Cheng and Zhao, 2008). Distributed eco-hydrological models have been used as powerful tools for estimating water yield changes at the watershed scale caused by vegetation dynamics. These models incorporate the spatial heterogeneity of environmental factors (Warrick et al., 1997; Krysanova et al., 2005; Wei et al., 2008). In this study, TOPOG, a distributed eco-hydrological model, was employed to simulate hydrological dynamics in a small watershed of the Qilian Mountains in northwest China. From these simulations, runoff variations caused by changes in vegetation type were analyzed. We aimed to quantitatively estimate the possible water yield reductions following increased forestation in this arid region of northwest China, and to identify the underlying mechanisms controlling water yield. 2 Methods 2.1 Study area The Qilian Mountains (97°24′-10°46′E, 36°43′-39°42′N) connect the Tibetan Plateau, the Mongolian Plateau and the Loess Plateau. The Qilian Mountains are the source of 56 rivers, and thus serve as an important water source area for the arid region of northwest China. Desert, grassland (Stipa-dominated steppe), forest (Picea crassifolia-dominated forest) and alpine vegetation form a vertical spectrum of vegetation belts from lower to higher altitudes in the Qilian Mountains. The study area was the Pailugou watershed (100°17′E, 38°24′N), a small watershed with an area of 2.74 km2 and elevation range of 2,650–3,770 m a.s.l.. It stretches across different vegetation belts (Table 1). Hydrological processes (precipitation, interception, evaporation, transpiration and surface runoff) were monitored in 40 study plots and the runoff was measured at the outlet of the Pailugou watershed (Wang et al., 2008). The measurements provided the possibility for parameter estimation and model validation. Table 1 Elevation (m a.s.l.) 2,650–3,000 3,000–3,300 >3,300
Vertical distribution patterns of soil and vegetation in the Pailugou watershed Aspect Vegetation type Soil type Sunny slope Grassland Haplic calsisol Shady slope Spruce forest Capcic luvisol Sunny slope Grassland and scattered shrubs Capcic luvisol Shady slope Spruce forest Capcic luvisol All slopes Sub-alpine shrub Hapludoll
At 2,580 m a.s.l., close to the outlet of the Pailugou watershed, mean annual air temperature is 0.5oC and mean annual precipitation is 435.5 mm. The annual pan evaporation is 1,051.7 mm. Mean annual relative humidity is 60%. The lapse rate of mean annual temperature is 0.58 oC/100m. Mean annual precipitation increases with elevation at a rate of 18.6 mm per 100 m (up to 3,300 m a.s.l.) and then decreases at a rate of 16.4 mm per 100 m above 3,300 m a.s.l. (Wang et al., 2008). The potential habitat for spruce forest in the Pailugou watershed is limited to shady slopes at elevations of 2,650–3,300 m a.s.l., an area accounting for only 46.3% of the overall watershed area. Patches of existing spruce forest are scattered in these habitats, and cover 36% of the Pailugou watershed. These are mature forests with an average canopy density of 0.7 and a density of 2,000 trees/ha. The average tree diameter at breast height (DBH) was 12.7 cm, and the average tree height was 8.3 m. An underdeveloped shrub and grass layer and a developed moss layer covered 33% of the ground in the spruce forest. 2.2 TOPOG model In the TOPOG model, the watershed was divided into hydrological units and the water flow trajectories were established from landform data comprising topographic contours, watershed boundaries, high points and saddle points. TOPOG excels in identifying micro landforms at fine scales and classifying them into individual hydrological units, since contours can describe landform in more detail than DEMs in which - 424 -
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some small, micro landforms are merged. Each hydrological unit identified by TOPOG was presumed to be homogeneous in terms of soil and vegetation. The main eco-hydrological processes, including vegetation growth, canopy interception, infiltration, vertical water movement in the soil profile, transpiration and soil evaporation were calculated in each hydrological unit. Lateral water movement (surface runoff and subsurface runoff) between neighboring hydrological units was also calculated. Evapotranspiration was described by a Penman-Monteith equation, and Richard’s equation was used to calculate water movement in the vertical soil profile. A full description of the TOPOG model is available on its website (www.per.clw.csiro.au/topog). The TOPOG model has been successfully applied to predict the hydrological effects of vegetation and land use changes such as crop rotation, forest harvesting and fire in tropical areas (Warrick et al., 1997; Hatton et al., 2002; Patrick et al., 2002), and to study the effects of soil moisture on vegetation growth (Silberstein et al., 1999). We improved the model in this study by separating precipitation into rainfall and snowfall using an air temperature threshold of 0 °C. When the mean daily air temperature was above 0 °C, the precipitation was classed as rainfall. Otherwise, it was classed as snowfall. Rainfall was routed and calculated according to the original TOPOG model. Some snowfall was assumed to be intercepted by the canopy, as follows. If daily snowfall was less than the capacity of canopy snow interception (Imax, mm), all the snowfall was assumed to be intercepted by the canopy. If daily snowfall was greater than Imax, the daily snow interception was set equal to Imax, given by I max = LAI ⋅ Rint (1) 2 2 where LAI is leaf area index (m /m ), and Rint is the coefficient for canopy snow interception (mm/LAI). The snow passing through the forest canopy was assumed to accumulate on the soil surface and start melting when the air temperature exceeded 0 °C. Daily snow melt rate (SMR, mm/d) is a function of mean daily air temperature (MAX, °C): SMR=D*MAX (2) (3) 0 ≤ SMR ≤ SNO where D is an empirical coefficient of snow melt (mm/°C) determined by the vegetation types and SNO is the amount of snow on the soil (mm). 2.3 Data input and scenario design The Pailugou watershed was divided into 5,879 hydrological units ranging in area from 24.0 to 5,449.1 m2. The vegetation type of each unit was determined from existing vegetation maps (1:2000 scale). Major soil and vegetation parameters, such as porosity (Table 2), were measured in 40 sample plots of size 20 m ×20 m. These parameters were then assigned to the hydrological units according to the unit vegetation type. Table 2 Parameters Area fraction (%) Soil type
Soil features and main vegetation types of the Pailugou watershed Vegetation types Spruce forest Shrub Grassland 36 23.8 39.9 Capcic luvisol Hapludoll Haplic calsisol
Soil depth (cm)
70
45
40
Saturated soil moisture (m3/m3)
0.67
0.50
0.58
Saturated conductivity (m/d) Maximum root depth (m)
4.00 0.70
1.26 0.45
1.49 0.40
In the Pailugou watershed, a standard meteorological station was installed at 2,580 m a.s.l. and meteorological parameters, such as air temperature, humidity and precipitation were measured according to the international meteorological observation standards. Daily climate data from this meteorological station collected in 2001 (annual precipitation 301.1 mm) and in 2002 (annual precipitation 411 mm) were used in this study to calculate the climatic parameters of each hydrological unit based on the unit elevation range and calculated lapse rate (Wang et al., 2008). Daily runoff data measured at the watershed International Journal of Sediment Research, Vol. 25, No. 4, 2010, pp. 423–430
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outlet were adopted for model calibration in 2001 and for model validation in 2002. Additionally, comparison between the simulated and measured transpiration of spruce trees was employed to test the model performance. Modeled interception was tested by comparing interception simulated by TOPOG and Eq. (4) above, with that calculated by an empirical equation for interception as follows: I = 0.4469 P 0.8016 (r2 = 0.69) (4) where I is the canopy interception (mm) per rainfall event with rainfall total of P (mm). This equation was established from measured interception data collected in spruce forests during 285 rainfall events in the Pailugou watershed (Zhang et al., 2000). Over the last several decades, numerous ecological programs have been launched and the area of forest has continued to increase in the arid regions of northwest China. Forest cover is planned to increase continuously, according to the China National Forestry Action Plan for Climate Change issued on Nov. 6, 2009. However, a forest cover of 100% can not be achieved on mountains in the arid region since patchy forests are limited to shady slopes. For example, the maximum potential forest fraction is 46.33% in the Pailugou watershed. Thus, 23 scenarios with forest cover ratios ranging from 0.41% to 46.33% were set (Table 3) in order to estimate forestation effects on runoff. Table 3
Scenario No.
Forest fraction (%)
Current S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 S23
36.00 36.15 36.67 38.08 40.89 42.15 43.45 45.95 46.03 46.12 46.33 35.00 33.00 30.70 26.36 23.58 17.34 16.90 12.42 8.58 5.89 3.46 1.61 0.41
Descriptions of the scenarios Scenario definition Elevation ranges in which the vegetation was changed (m a.s.l.) 2650–2700 2650–2750 2650–2800 2650–2850 2650–2900 2650–2950 2650–3000 2650–3050 2650–3100 2650–3150 2650–2700 2650–2750 2650–2800 2650–2850 2650–2900 2650–2950 2650–3000 2650–3050 2650–3100 2650–3150 2650–3200 2650–3250 2650–3300
Vegetation change -
Grassland on the shady slopes was changed to spruce forest
Spruce forest was changed to grassland
2.4 Calculation of potential runoff The potential runoff (R) of a hydrological unit in the watershed was calculated as follows: (5) R = P − ET − ΔW where P (mm) is the precipitation during a given period, ET (mm) is the evapotranspiration of this period simulated by TOPOG (as the sum of the interception of litter and canopy, transpiration, and soil evaporation), and ΔW is the increase in soil moisture in this period as simulated by TOPOG. Over a long period such as a year, ΔW can be assumed as zero. R=0 indicates that all the precipitation in the hydrological unit is consumed by the ecosystem through evapotranspiration. If R>0，evapotranspiration is less than precipitation, indicating that the unit generates - 426 -
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runoff. If R<0, then evapotranspiration exceeds precipitation, and not only the precipitation but also part of the runoff received from upper slopes is consumed by the ecosystem, indicating this hydrological unit contributes negatively to the runoff from the watershed. 3 Results
4.0
0
3.0
20
2.0
40
1.0
60
0.0
80
2001-1-1
2001-5-1
D aily p recip itatio n (m m )
D aily ru n o ff (m m )
3.1 Calibration and validation of TOPOG model The mean annual runoff of the Pailugou watershed was 98.8 mm, 82.3% of which occurred during the rainy season from June to October. Measured and simulated daily runoff from the Pailugou watershed in 2001–2002 showed good agreement (Fig. 1). Also, peak flows simulated by the TOPOG model matched well with measured peaks (Fig. 2). The absolute error of the daily runoff simulation was 0.02 mm on average. The maximum absolute error was 0.41 mm and the maximum relative error was 11%. These results demonstrate that the calibrated model was acceptable for simulating the daily and annual runoff in this watershed.
2001-8-29 2001-12-27 2002-4-26 2002-8-24 2002-12-22
M easured runoff Simulated runoff Fig. 1 Measured and simulated daily runoff from the Pailugou watershed in 2001 and 2002
M easured daily runoff (m m )
1.6 y=x 2
R = 0.9596
1.2
0.8
0.4
0 0
0.4
0.8
1.2
1.6
S imulated daily runo ff (mm) Fig. 2
Peak flows (measured, and simulated by TOPOG) in the Pailugou watershed during 2002
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The canopy interception simulated by TOPOG was close to that calculated by the empirical Eq. (4). For example, in spruce forest at an elevation of 2,730 m a.s.l., the interception simulated by TOPOG was larger than that calculated for all rainfall events in 2001 and 2002, but only by a maximum of 0.9 mm. The total canopy interception during the growing season (May–September) simulated by TOPOG was 2.7% larger than that calculated by Eq. (4). The transpiration simulated by TOPOG was 180 mm during the growing season in spruce forest at an elevation of 2,730 m a.s.l., which is close to the transpiration rate of 183.9 mm measured in the field. The above tests indicated that the calibration and validation of TOPOG were successful and that it could be used to simulate the ecohydrological processes in the Pailugou watershed. 3.2 Runoff comparison between different vegetation types The difference between the precipitation and total evapotranspiration, i.e., the annual potential runoff, simulated from spruce forests was negative for all elevations in the Pailugou watershed. At elevations lower than 3,000 m a.s.l., the simulated annual runoff from spruce forests was -38.0 mm on average, whereas it was less negative (-3.0 mm on average) at elevations higher than 3,000 m a.s.l.. This clearly demonstrated that the spruce forests on shady slopes absorbed part of the lateral water flow as well as generating no runoff from precipitation. The spruce forests growing at lower elevations absorbed more lateral water flow than those at high elevations. In contrast to spruce forests, the runoff from grassland varied from 61.6 mm to 192.2 mm with an average of 127.2 mm. The annual runoff from shrubland averaged 110.6 mm, ranging from 28.0 to 231.4 mm. Therefore, these results indicate that conversion of grass- or shrub-lands to forests could cause a complete loss of runoff at the site scale. 3.3 Effects of forest increase on watershed runoff When forest coverage was increased in the watershed, from 0.41% to 46.33%, the annual watershed runoff decreased from 196.6 mm/yr to 78.9 mm/yr under a mean annual precipitation of 455.7 mm. An increase of 10% forest cover led to a decrease in watershed runoff of 25.6 mm (Fig. 3), which accounts for 13.0% of the runoff from a watershed with no forest cover. This runoff reduction was mainly observed in summer (16.1 mm, 62.9%) and spring (6.3 mm, 24.5%), while only 10.0% was observed in autumn and 2.6% in winter.
An nu al ru n off (m m )
25 0 20 0 15 0 10 0 50 0 0
10
20 30 Forest fraction (% )
40
50
Fig. 3 Annual runoff from the Pailugou watershed with different forest cover ratios simulated by TOPOG model
4 Discussion Our results indicated that runoff at the site scale will be reduced to zero when grasslands or shrublands are converted to forest in this arid region. This runoff reduction would be more severe than the reported equivalent global-averaged runoff reduction of 44% in humid regions (Farley et al., 2005) and than the –50% runoff reduction in the semi-humid and semi-arid regions of China (Sun et al., 2006). - 428 -
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The absolute runoff reduction rate of 25.6 mm per 10% forest increase found in this study was higher than the equivalent figure of 15.8 mm observed in the semi-humid region of the Liupan Mountains in central China (Yu et al., 2009), although the precipitation in the Pailugou watershed was 200 mm less than that in the Liupan Mountains. This indicates that the runoff reduction due to forestation would increase with decreasing precipitation or increasing aridity from semi-humid regions to arid regions, opposite to the trend of absolute runoff reduction in the humid region where absolute runoff reduction is greater when precipitation is higher (Farley et al., 2005; Sun et al., 2006). More studies should be carried out in semi-arid regions to ascertain whether this observed trend is general or local. The negative calculated annual runoff from the spruce forest site means that more water was taken up by the forest than was provided by precipitation. This was due to the site-scale vegetation distribution, where forest commonly grows on the lower slopes and was supplied by soil water from the upper slopes, as shown by our observation of higher soil moisture levels after strong rainfall events on the lower slopes in this watershed. Despite the substantial reduction, the forestation did not reduce runoff to zero, because only part of the watershed area in this arid and mountainous region can support forests. For example, forests can only grow on shady slopes with elevations of 2,600–3,300 m a.s.l. in the central Qilian Mountains (Wang et al., 2008). Taking the Pailugou watershed as an example, the maximum forest area fraction is 46.3% and the corresponding maximum annual runoff reduction is 60%. However, such a runoff reduction may have serious consequences in an arid region already suffering from scarce water, and should be accounted for when developing strategies to address erosion control. Conversely, some forestation is needed since forests and trees are the backbone of arid zone ecosystems, acting as an important habitat and shelter for wildlife and flora, as well as being important for human communities by producing a wide range of goods and services (Malagnoux, 2007). It also should be emphasized that runoff reduction is greatest in summer (June–August, 62.9%) and spring (March–May, 24.1%) when frequent floods occur due to heavy rains. From this point of view, the runoff reduction by forests will minimize flood damage and reduce soil erosion. A trade-off between forest services and water yield, based on the quantitative understanding of forest effects on watershed runoff, is thus important to policy-making in arid regions. 5 Conclusions At the site scale, forestation could lead to a complete loss of runoff from mountainous slopes in arid regions, since the forest absorbs more water than it receives from precipitation. At the watershed scale in this study, an increase in forest area of 10% in the watershed led to a runoff reduction of 25.6 mm, which accounted for 13% of the runoff of the watershed without forest. The runoff reduction was concentrated in summer (June–August, 62.9%) and spring (March–May, 24.1%). The maximum runoff reduction due to forestation was 118.5 mm (60% of the runoff in the watershed without forest), but runoff was not reduced to zero because the maximum forest fraction is 46.3% in the watershed. Although there is no complete loss of runoff at the watershed scale, the predicted runoff is nevertheless significant in an arid region that already suffers from severe water-shortage. Acknowledgements We gratefully acknowledge funding from the National Science Foundation of China (40671038, 40730631), the State Forestry Administration of China (200904056, 200904005), the Chinese Academy of Forestry (CAFYBB2007038), and the Key Laboratory for Forest Ecological Environment of the State Forestry Administration of China. References Bi H. X., Liu B., Wu J., Yun L., Chen Z. H., and Cui Z. W. 2009, Effects of precipitation and landuse on runoff during the past 50 years in a typical watershed in the Loess Plateau, China. International Journal of Sediment Research, Vol. 24, No. 3, pp. 352–364. Cheng G. D. and Zhao C. Y. 2008, An integrated study of ecological and hydrological processes in the inland river basin of the arid regions, China. Advances in Earth Science, Vol. 23, No. 10, pp. 1005–1012. Farley K. A., Jobbagy E. G., and Jackson R. B. 2005, Effects of afforestation on water yield: A global synthesis with implications for policy. Global Change Biology, Vol. 11, No. 10, pp. 1565–1576. International Journal of Sediment Research, Vol. 25, No. 4, 2010, pp. 423–430
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