0927-B1
Zhiqiang Zhang[1] and Lixian Wang
Research on forest hydrology is the key to understanding the physical, biological and chemical processes in a watershed and is significant for watershed management. Forest-water relations and their implications in watershed management, therefore, are one of the key themes of international hydrology and watershed management research. Lack of long-term research data causes vigorous debates over the practical role of forests or reforestation in reducing flooding, annual water yield and soil erosion in China among scientists as well as decision-makers. This paper presents the historical development of forest hydrology research in China during the past several decades, the long-term forest ecological research stations, and major results of forest hydrological research in terms of forest canopy interception, forest floor interception, evapotranspiration, runoff and soil erosion. In addition, the future challenges of long-term forest hydrological and ecological studies, which are crucial to clarify the forest impacts on hydrological regimes under different geographic and forest conditions, are also outlined in the context of Chinese national reforestation programmes.
Hydrological processes at different scales play an important role in maintaining the biodiversity, landscape heterogeneity as well as fresh water supply for human and animals. Research on forest hydrology is the key to understand the physical, biological, and chemical processes in a watershed and has significance to watershed management. Therefore, forest impact on the water cycling and sediment has been the major subject in hydrology and water management research since the early 1960s.
Although China has a long history of observing the interactions between forest and water, modern systematic hydrologic research only started in the mid-1980s. The major driving force for studying forest hydrology was the periodic flooding (e.g. 1981 and 1998) in the Yangtze River that has claimed thousands of lives. However, lacking long-term research data causes vigorous debates over the practical role of forests or reforestation in reducing flooding and soil erosion in China among scientists as well as decision makers. For example, there is no consensus on how forest clear-cutting will affect peak-flow rates and annual runoff in different parts of the country. Intensive forest hydrological research, therefore, is still needed to clarify the forest and water relations with reference to different geographic regions and test existing principles governing forest hydrologic processes under different geographic and forest conditions as that past studies suggested forested watersheds in different geographic regions have distinct response to forest management.
Another driving force for forest hydrology research in China during very recent years was that of water shortage in northern part of China and some experimental evidence that forests reduce total water discharge at watershed scale in northern part of China(G. Shen & L. Wang, 2001; L. Wang & Z. Zhang, 1998). Recent research work carried out in China shown that mountainous forests has the maximum regulation function on hydrology in comparison with other vegetation type and evapotranspiration of forest vegetation consumes a large amount of water, especially in arid area (excluding high mountains in arid zones), and the water yield will decrease significantly with the increase of forest coverage rate(G. Shen & L. Wang, 2001). Facing several environmental problems in China, national-wide reforestation programs were started over twenty years ago. To evaluate the effects of forest plantation on hydrologic processes, many research works have been carried out also under forest plantation conditions. This paper presents the historical development of forest hydrology research in China during past several decades, the long-term forest ecological research stations, and major findings of forest hydrological research in terms of forest canopy interception, forest floor interception, soil moisture dynamics, evapotranspiration, runoff, soil erosion. The future challenges of long-term forest hydrological and ecological studies that are crucial to clarify the forest impacts on hydrological regime under different geographic and forest conditions are also outlined in the context of Chinese national reforestation programs.
Facing the worldwide problems of population growth, resources shortage, deforestation, land degradation and environmental pollution, ecological and environmental research was becoming a heated topic throughout the world after the World War ¢ò. Long-term ecological research was proved to be crucial to clarify the complicated interrelationships between organisms and their diversified and changing environments. Many long-term ecological research projects and stations were established such as American LTER, the West Germany Solling Project, Swedish Coniferous Project. Furthermore, long-term forest ecological research programs were also established in the development of global ecosystem research network such as IBP, IGBP, and BACH.
China started its long-term ecological research project much later than those of international communities. Several forest ecosystem experimental stations were established in the late 1950s to early 1960s. These stations contributed much of earlier ideas of forest and water relations in China. After the Cultural Revolution in China, new ideas and methods were introduced into Chinese long-term forest ecosystem research with special emphasis on nutrient and energy flow at ecosystem scale. For instance, Changbai Mountain forest ecosystem station was established in 1979, and joined the MAB program in the same year. The station becomes the world-famous forest ecosystem research base with the well-preserved typical mountain forest ecosystem of Eastern Asia. In 1982, Chinese Ministry of Forestry started forest ecosystem study project in China and gave continuous financial support to 11 long-term stations. In 1992, Department of Science and Technology of the Ministry of Forestry chaired working conference of forest ecological research in China and decided to publish two books about Chinese forest ecosystem study. In 1994, these two books were published “Long-term Research on China’s Forest Ecosystem” and “In Situ Research Methods of Forest Ecosystem”(X. Zhou, 1994; X. Ma, 1994). The first book published numerous research papers in CFERN stations and the second set up the unified and standard experimental methods for CFERN stations. In 1993, the Forest Hydrology for the first time published in China(X. Ma, 1993).
The Chinese Academy of Sciences also started to establish Chinese Ecosystem Research Network(CERN) in 1988. CERN stations was selected in existing stations throughout China and composed of 29 stations. Of which 7 stations are forest ecosystem stations.
There are currently 20 long-term forest ecosystem research stations located in the different part of the country representing different forest types and different climatic zones. Of which 13 stations administered by the State Forestry Administration(formerly Forestry Ministry) composed of the Chinese Forest Ecosystem Research Network(CFERN)(X. Zhou, 1999) and the other 7 operated by Chinese Academy of Sciences as forest ecosystem stations of Chinese Ecosystem Research Network(CERN)(Secretariat of CERN Science Committee, 1997). In addition, there are also many temporary research stations in the different regions of China dealing with protection forest plantation impacts on the hydrological behavior(Zh. Zhang, 2001). These temporary research stations were set up to carry out the national research projects in connection with the national forestry programs such as Natural Forests Protection Program, Protection Forest Planting in the “Three North” and Lower Reaches of Changjiang River, and Agricultural Used Slope-land Conversion to Forest Use etc.
As an integrated part of forest ecosystem study, forest hydrology in most of such stations is addressed(S. Liu et al., 1996; L. Wang and Zh. Zhang; 1998). After the first Forest Hydrology book published in 1993(X. Ma, 1993) and long-term forest ecosystem research stations established in China, with the support from Chinese State Forestry Administration(formerly Ministry of Forestry), Chinese Ministry of Science and Technology and National Science Foundation of China, forest hydrology study in China developed very quickly during the past 20 years. Research methods include both watershed study and processes study. Watershed study mainly addresses the problem of the forest influence on the discharge regime by using watershed experimental method. In addition to traditional experimental watershed study, nested watersheds experiments in recent year were emphasized with special attention in order to study the scale issues in forest hydrology. Processes study, in other hand, is concentrated into different part of hydrological processes within the forest ecosystem as separated parts such as forest canopy interception, forest floor interception, forest soil infiltration and soil moisture dynamics, soil water movement in lateral direction, forest impact on the different runoff components, forest ecosystem evapor-transpiration, and forest impact on soil erosion. During the recent years, the great efforts have been made to synthesis processes study results into watershed scale(Zh. Zhang et al., 2000).
Forest canopy is the first interface of forest ecosystem on which forest impacts hydrological cycle within the system. Generally speaking, forest canopy interception is varied with both the forest ecosystem type, the development stage of the system and the precipitation pattern in a given regions. Canopy interception of temperate coniferous forest is normally between 20-40%(Gash et al.,1980; Rutter et al.,1971;Teklehaimanot et al.,1991; Viville et al.,1993)¡£It was experimentally determined that the canopy interception of different forest ecosystems in different geographic regions of China falls between 11.4%-34.3% and variation coefficients between 6.68%-55.05%(G. Wen & Sh. Liu, 1995). Averaged annual interception amounts to 134.0 to 626.7mm from north to the south.
Forest floor interception and storage variations are of significance to the water and energy transportation between under-canopy air and mineral soils. Capacity of water interception and storage of forest floor influences soil water replenish and plant water availability. In addition, forest floor is much porous than mineral soil and the water intercepted easier to be evaporated than soil moisture. It was estimated that the forest floor evaporation could reach 3%-21% of the forest evapotranspiration(Teklehaimanot et al., 1991). Forest floor interception capacity could be 2- 4 times of their own weight(Sh. Liu, 1996). Table 3 shows the forest floor biomass of different forest systems.
Forest ecosystem evapotranspiration
Evaportranspiration study is a key to analyze the forest influence on water yield at watershed scale. Water yield increase after forest conversion into grassland is believed to be the result of evaportranspiration decrease. In the early 1960s, China started forest evaportranspiration studies(D. Xu, 1989). Most studies show that evapotranspiration of forest ecosystem(including canopy interception) amounts to 40-80% of annual precipitation(Sh. Liu, 1996). Methods adopted in China to measure forest evaportranpiration include micro-climatic method(energy balance-Bowen ratio, aerodynamics, Penman-Monteith and eddy correlation), hydrological balance method, and physiological method(X. Ma et al,1994). Energy balance-Bowen ratio method is the most popular one and approved to be good in terms of accuracy. Lysmeter was also used to observe the forest evaportransporation.
Forest affects watershed runoff in various ways including runoff generation, runoff component, peak flow rate, total discharge and the quality of the runoff. The accumulated effects of forest conditions and various watershed management measures could be reflected in the watershed discharge. Forest influence on peak flow rate and total discharge is one of the most concerns to scientists and decision-makers in China.
Water yield increase after forest cutting is generally observed for many field studies at worldwide scale(Hewlett, 1971; Hibbert, 1983; Hornbeck and Adams et al, 1993; Stednick, 1996). However, there are different reports regarding the forest changes on the annual discharge at watershed scale in China. In most cases that annual water discharge increases following the forest coverage decline. However, in the high mountain of subtropical region of Miyaluo the annual water discharge was decreased dramatically up to 433.8mm after clear-cutting Abies forests. It was summarized in a latest report that forest increase in dry-land area will decrease the annual discharge while for the humid areas there are no significant influences and the difference were contributed to the different evaportraspiration in these two geographical regions(G. Shen and L. Wang, 2001). In the humid tropical and subtropical region, vegetation recovery is much faster than that of temperate region. As the result, water discharge increase following cutting could be adjusted by the vegetation recovery and this trend of hydrologic phenomena could not last longer time like the case in temperate region. However, different hydrological years can be resulted in the different water yield regime even under the same treatment.
Lacking database causes forest peak-flow rate much more controversial. Major controversies are related with the flood magnitude and area of the watershed. It is commonly believed in China that forest could reduce peak flow rate at small watershed scale and in case of the plentiful pre-event rainfall such effect will be declined dramatically.
Soil erosion, sediment transportation and deposition into the reservoir can cause great loss of capacity and economic value. At the same time, soil erosion also is resulted in the water quality degradation. It is estimated the annual economic loss of on-site and off-site damages due to soil erosion is over $10 billion in the United States of America(Sun et al.,1997). Forest plantation can reduce the soil erosion due to the peak flow and runoff yield reduction. Table 3 indicates the sediment yield monitoring results for three years from 1992 to 1994. Forest plantation harvesting was resulted in sediment yield increase by 40.58 to 523.08%. From energy point of view, forest ecosystem is a dissipating system of rainfall impact and surface runoff shear stress by means of canopy, forest floor, and tree stems interceptions. From quantity point of view, interception of different layers and increased infiltration due to the root system decrease the overland flow volume and then soil erosion.
A major challenge to forest hydrologic research in China is represented by the frequently questioning to the water regulating roles of forests. In seeking the causes of big floods in Yangtze River in 1998, some scientists believed that forest cutting in upper and middle reach basin of the river was one of the major causes of the event while others denied the relations between the forests and the flood. As the national scale reforestation activities development the hydrologic roles of the forests will be continued of great concerns among scientists, decision-makers and publics. Does forest reduce the peak flow discharge and increase the low flows of the watersheds? To what extent is it effective? Does forest mitigate water-related disasters and to what extent? How does forest impact the annual discharge or water yield of a watershed, does it increase the water yield or decrease the water yield as the forest coverage increased? What are the differences of hydrologic response to forest management and reforestation in the different geographic regions? To answer the question of water yield changes following forestry activities is very important in the context of national wide reforestation efforts specifically in the water resources shortage areas. There are different reports regarding the forest roles on the annual water yield and peak flow discharges in China. Two representative ideas in China on this regard include that forest increase in dry-land area will decrease the annual discharge while for the humid areas there are no significant influences. Most explanations to this difference are evaportranpiration differences in these two geographical regions.
Long-term monitoring of forest changes on the hydrological regime such as annual water yield, peak-flow, low-flow are critical for testing existing principles regarding forest and water relations. For instance, it is believable of forest impacts on the peak discharge only when flood return periods or frequency, precipitation regime, and watershed areas are clearly stated. Furthermore, only long term monitoring of the watershed hydrology can the influences of soil, topography, climate on the water discharges be separated from that of forest influences.
From the watershed management point of view, under the most conditions in China there are many different measures to be taken and the land use are diversified, therefore, the forest management or reforestation will be have accumulated effects with other land-uses and measures on the watershed processes. To evaluate the hydrological effects of forest management or reforestation measures at watershed scale in the area without field experimental data, physically based distributed processes watershed hydrological model development with the GIS support are turned to be useful tool. However, there are no any such models are available in China although there were some efforts to physically model one of the forest hydrological processes at plot scale(Zh. Zhang,1999, 2000).
To model the forest hydrology at watershed scale, forest impacts on the runoff generation mechanisms is another research challenge in China. Runoff generation mechanism is one of the key research focuses in the hydrological study on which more and more attention is concentrated. The complexity of hydrological processes such as scale dependence and nonlinear behavior resulted from the widespread spatial heterogeneity and temporal variability of hydrological environment and fluxes makes it very difficult, given that without detailed understanding of the physical processes, to extrapolate the information from one area to another or from one scale to another. Current generation of physically-based, distributed parameter models could accommodate the assessment of hydrological impacts of forest vegetation changes to some degree in comparison with traditional approaches. However, the confidence of such models and the scale issues highlights current and future research needs in the forest vegetation influences on the runoff generation and delivery mechanisms. Three major research methods being adopted are hydrometry, tracer/isotope tracing, and dynamic calculation at the spatial scale from hillslope to watershed. The current understanding of forest vegetation influences on the runoff generation could be summarized as ¢Ùrunoff is controlled by variable source area mechanisms for forested watershed, ¢Úmajor runoff components for forested watershed are saturated surface runoff, subsurface runoff, and groundwater runoff, ¢Ûco-existing and shifting of different runoff generation mechanisms, and ¢Üpreferential flow is a key factor in controlling the runoff generation for forested watershed. However, in China there are seldom research works addressing this particularly important aspect of forest hydrology(Zh. Zhang, 2001).
Investigation of the interacting relationships between ecological processes and hydrological processes at watershed scale is also one of the challenges of forest hydrology and watershed management research in China. Following the traditional method of hydrological studies, the forest effects on watershed hydrology mainly are resorted to rainfall and runoff measurements and black-box modeling. However, physiological behavior, biomass accumulation, species changes of forest stands, regeneration and succession, pheneology, and decomposition of forest floor are all closely related with watershed hydrology. To integrate forest ecological processes and hydrologic processes to watershed scale runoff responses is critical to in-depth investigation of hydrologic roles of forests.
Regional scale forest hydrological modeling is also a challenging issue for the hydrology and watershed management study in China. Forest management and reforestation at large scale intended to protect surface waters and watershed environment are assumed to be relevant over an entire region. The findings of watershed forest hydrologic experiments always are claimed representative within the encompassing region. To evaluate the hydrological effects of regional reforestation, tentative research was started five years ago. However, the underlied methodological problems were not well tackled. In the most situations, forest hydrologic roles were quantified as linear behavior from watershed scale to the regional. To extrapolate from watershed hydrology to the regional scale, the complex interaction of both biotic and abiotic factors that influence hydrological dynamics must be considered. Up to date, there is no relevant research in China although the importance of the research was widely recognized.
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| [1] Deputy Head, International
Cooperation Development, Beijing Forestry University, Qinghua East Road
35, Haidian District, Beijing 100083, P. R. China. Email: [email protected] |