Research advances in eco-hydrological process and function

YongGang Yang , HongLang Xiao , ZuoDong Qin , Na Kang , CaiMei Li

1. Institute of Loess Plateau, Shanxi University, Taiyuan, Shanxi 030006, China

2. Cold and Arid Regions Environmental Engineering Research Institute, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

1 Introduction

Eco-hydrology is an emerging interdisciplinary field, gradually developed after the 1980s, and is in the forefront of hydrological research. Eco-hydrology is the interaction of plants and the hydrological process,and describes ecological patterns and eco-hydrological mechanisms (Zhao and Cheng, 2001). The research purpose of eco-hydrology is to increase the understanding of the coupling relationship between ecological and hydrological processes, and to provide a theoretical basis of restoring and establishing a healthy eco-hydrological environment (ecological engineering).There is a close relationship among ecological system pattern, ecological process change and hydrological process. The target of eco-hydrology research includes dry land, wetlands, forest, grasslands, mountains, lakes and rivers and is based on a theoretical relationship between plants and water with the scale throughout the study (Xiao and Cheng, 2006).

The research focus in the field of resource and environment is on the observation of hydrological processes. Ecological systems present a relationship between vegetation and environmental factors, while the hydrological process, as a main driving force of ecological system succession, controls vegetation structure and distribution patterns, and ecological process(Zhao and Cheng, 2001). International research into eco-hydrology is focused on the interaction among ice,frozen soil, precipitation, groundwater and the ecological environment (Petts and Bradley, 1997). The main task for future scientific development and management in the basin is to recognize the coupling relationship between ecosystem and hydrological process,and to integrate the technical system of ecological restoration (Cheng, 2009).

2 Research of eco-hydrological process

The structure and function of the ecosystem and its hydrology are both important components of research in this field. Analyzing relationships between hydrological processes and biological dynamics may also shed light on how ecological patterns and changes are interlinked, and dependent on hydrological mechanisms. The study of stability and interaction mechanism of hydrological process and the ecosystem can provide a theoretical basis for eco-hydrological layout and its dynamic balance maintenance, and provide a scientific explanation for the relationship between ecological succession and hydrological circulation changes (Wanget al., 2001). There are three important scientific problems on the study of eco-hydrological process as follows.

2.1 Research of eco-hydrological restoration

One important problem of eco-hydrological process research is eco-hydrological restoration. The study of eco-hydrological restoration deals with the effect on vegetation cover sensitivity, ecological effect of water behavior in its watershed, the physiological and ecological processes, and the performance of plants under drought conditions (Kanget al., 2008).Cairns (1980) elaborated on the main theoretical and practical problems in the process of damaged ecosystem restoration. Over the past 20 years, numerous studies have focused on remediation technology of the eco-hydrological environment, involving restoration and management of grasslands, mines, water and soil.Since the 1950s, China has provided long-term observation and comprehensive renovation research of degraded ecosystems, and numerous small watershed ecological restoration projects have proved successful.Research focused on the causes, solutions, technology methods of restoration and reconstruction, the selection of plant species, and the ecological effects of recovery and rebuilding. Special attention was given to the ecological restoration process while emphasizing the ecological succession theory and biodiversity restoration (Huanget al., 2003). The Shapotou Desert Research Station of Chinese Academy of Sciences have provided practical experience and data accumulation for ecosystem restoration in degraded arid regions, such as in areas with less than 200 mm precipitation (Kirkbyet al., 1996). Vegetation in different landscape zones effects rainfall, runoff and evaporation on many levels. Consequently, water resources are allocated, which influences the hydrological circulation process. Different landscape zones have different hydrological processes, from which some features of the landscape can be analyzed. The hydrological process controls many basic ecological patterns and ecological processes, especially basic vegetation distribution patterns, and is one of the main driving forces of ecological system succession. Therefore,vegetation dynamics and ecosystem succession can be controlled by hydrological process adjustment. Thus,research on hydrological process and ecosystems in the basin helps to discover eco-hydrological restoration mechanisms, such as the relationship between diversity change under different moisture conditions and the relationship between vegetation community succession and hydrological processes. This mechanism will be helpful for plant physiology and ecology under small scale stress conditions.

2.2 Simulation research of eco-hydrological process

Simulation research is another important direction of eco-hydrological process research. The complexity of hydrological conditions, and the heterogeneity and variability in temporal and spatial distribution of the elements influences hydrological behavior, making it difficult to directly quantify eco-hydrological changes.

Currently, the models used for eco-hydrological process research are: SPAC, SWAT, WAVES, SWIMV,and PATTERN, which are used to simulate the material and energy transfer process of soil-vegetationatmosphere interaction, and analyze eco-hydrological effects of vegetation (Kremeret al., 1996); RHES,XAJ, TANK, and SLT, are used for eco-hydrological simulation in the basin, especially for hydrological process simulation (Uso-domenechet al., 1995;Mulligan, 1996); RUTTER, MASSMAN, and PHILIP, are used for forest eco-hydrological process,and the influence of plants on water studies; and LUCID, is used to simulate how land use change affects the hydrological processes (Thornes, 1990;Cheng and Zhao, 2006; Xiaet al., 2007). The development of GIS (Geographic Information System)and RS (remote sensing) has greatly influenced the eco-hydrological models. The improvement of various high-precision spatial eco-environmental data and the popularization of high spatial and temporal resolution of remote sensing data made it possible to achieve eco-hydrological models based on GIS and RS, and the coupling relationship among GIS, RS and mathematical models helps to quantify eco-hydrological changes. Although eco-hydrology models have undergone improvements in recent years, they are mostly limited by scale, when they are applied in other scales, re-parameterization and precision problems appear. Although there have been numerous, separate ecological process and hydrological process models, they cannot be regarded as a eco-hydrological process model in a strict sense.Therefore, it is necessary to strengthen the coupled model research on eco-hydrological process. The traditional basin model lacks information required for modeling, thus it is a conceptual model. An important tool for future hydrology research is use of the hydrological tracer technique, especially the iso-tope tracer technique. This technique can provide more detailed information for the study of water flow in soils, hydrological model structure, and identification parameters.

2.3 Research on ecological water requirements

The study of ecological water requirements is an important research subject in eco-hydrology, and is influential in ecology, hydrology and water resource sciences. These requirements maintain a stable ecosystem structure and ecological function, and can protect the ecological environment from further deterioration, leading to a gradual improvement. The ecological water demand can be divided into two categories, namely vegetation and ecology (Siet al.,2005). At present, the study of ecological water demand is still at the initial stage, focusing mostly on the establishment of eco-hydrological models, parameter estimation and calculation system. The research progress of temporal and spatial variability and scale of ecological system is slow, and the study of correlation among hydrology, ecology and climate is still under exploration. Therefore, future research should focus on the spatial and temporal variability of ecosystems, growing season and characteristics of vegetation, and natural geography, thus ecological system water demand in particular conditions can be objectively evaluated. The research on formation mechanism is scarce (Fenget al., 2008). Thus, future research should apply a comprehensive analysis on the theoretical system of ecological water demand in terms of internal and external influencing factors and conduct systematic experimental research on different ecosystems to reveal the ecological water demand.Relatively speaking, eco-hydrology research in China started late but quickly developed in recent years,with tremendous progress in conceptual, theoretical mechanism, and estimation methods. Siet al.(2005)presented a quantitative understanding of the ecological water requirements of a desert riparian forest by studying: (1) water-heat transfer model for the soil-vegetation-atmosphere system; (2) ecosystem effect on the hydrological process; (3) ecological water requirements based on the eco-hydrological process; (4)evapo-transpiration and water consumption characteristics involving leaves, plants, forest and area. Based on the study of ecological water demand in the eco-hydrological process, Fenget al.(2008) calculated the demand downstream of the Heihe River by the water balance method. They also calculated the physiological water demand, the evaporation of phreatic water among trees and its patches during the growing season, and the evaporation of phreatic water in vegetation covered area during the non-growing season.There has been limited progress in water consumption and ecological water demand of oasis vegetation at the plant/community level in Ejina, and in the evaluation of ecological water demand for the entire oasis.However, the conversion methods of different scales among individual, community, ecosystem and oasis landscape needs to be improved. These aforementioned achievements have provided a theoretical reference for ecological environmental construction and the implementation of sustainable development strategy. In recent years, the study of ecological water requirements in China has focused on arid and semi-arid regions with contrasting supply and demand of water resources, and a relatively weak ecological environment. Thus, for future basin research on ecological water requirements, one needs to combine the study of ecological process and hydrological process with new theories and methods (Heet al., 2005).

In summary, present studies focus on the relationship between vegetation and water in the ecological system, the eco-hydrological process and ecological patterns. Studies on the coupling relationship between hydrological process and landscape structure of the ecosystem in the basin are rare, and only a deep understanding of the eco-hydrological rules can be helpful for the construction of an ecological environment and management of water resources.

3 Research on eco-hydrological function

Research on basin eco-hydrological function examines hydrological mechanism of change and coupling relationship between land surface ecosystem pattern and hydrological process. This offers guidance for protection and improvement of natural landscape in fragile ecological areas, promoting environmental construction and water resource management (Zhao and Cheng, 2001). Change of ecosystem pattern and ecological process are closely related to the hydrological process. The basic eco-hydrological relationship is between stability of the hydrological process-ecosystem and coordinative mechanism of hydrological process-ecosystem. The coupling mechanism and rule of hydrological and ecosystem process is the key for the basin research (Wanget al., 2005). Basin eco-hydrology focuses on the coupling relationship between hydrological and ecosystem process, emphasizing the mechanism and process of scale transformation. Research focuses on correlations of ecosystem stability and water environment, ecosystem recover technology and comprehension on a basin scale,eco-hydrological influence of landscape pattern change,and the coordination of water-used relations between natural and artificial ecosystems. Basically,eco-hydrology examines how the hydrological process influences ecosystem distribution, structure, function and dynamics, and how feedback of biological pro-cesses influences the hydrologic cycle. In order to improve predictability of environmental change under the influence of natural and anthropogenic activities,and reveal the influence on water resources, social economy and the ecosystem, research is required on the relationship between vegetation and hydrological process from the angle of eco-hydrology (NRC, 1992).

Present research on the coupling relationship between multi-scale hydrological and plant models has made only preliminary progress. Limited research has been performed on basin wide ecosystem repair technology, ecological hydrologic effect of landscape pattern changes and the coordination of water-used relations between natural ecosystem and artificial ecosystems (NRC, 1992). In respect to the interaction between vegetation and hydrology, research involves hydrology mechanism of ecological structure and ecological processes, the temporal and spatial coupling of hydrological and ecological process, and most important, suitable saturated ecological water requirements. At present, research on eco-hydrological function mainly focuses on forest vegetation and hydrology. The function of soil and water conservation,and flood disaster resistance in forest ecology are universally accepted, yet there is limited research on different vegetation ecosystems, and far less on catchment eco-hydrological function in terms of hydrology. Different vegetation patterns have different impacts on basin hydrology. At present, most studies concentrate on the impact of basin runoff from a certain landscape pattern, which is not comprehensive.Thus, a basin wide systematic study should be instituted on vegetation zone composition and spatial distribution patterns (Songet al., 2005). At present the main methods employed in the study of hydrologic effect and eco-hydrologic function include vegetation NDVI index, "3S" technology and the eco-hydrological model. Cheng (2007) researched the ecological function of a meadow steppe in the Yellow River catchments by vegetation NDVI and landscape pattern indices (Wainwright, 1996; Liu, 2004). Based on "3S" technology and model simulation, Liet al.(2005) performed space analysis on vegetation spatial distribution pattern and its eco-hydrology function up-stream of Minjiang River. Combining Arc GIS and the hydrology model, Yanget al.(2004) researched forest landscape patterns and hydrologic characteristics in Dayekou and Haichao Ba basins, while other scholars performed similar researches by various eco-hydrological models (Xu, 2005; Zhanget al.,2007; Kanget al., 2008). The study of eco-hydrological function in different vegetation landscape zones is very rare, and no research has been done on eco-hydrological processes and function of different landscape zones in the basin by the combination of isotope technology, landscape patterns and the hydro-chemical method.

4 Application of isotope progress in eco-hydrology

The isotopic composition of different water is relatively stable, which can be used to judge the cause of water and different water source division. The isotope application on basin hydrological processes can effectively prevent the distortion of simulated natural conditions, thus explaining the natural hydrological characteristicle. Stable isotope technology has been used in many aspects, including water cycle, rainfall-flow process line segmentation, groundwater recharge source and update cycle, temporal and spatial change of soil moisture and root water absorption (Keith,2000; Mulet al., 2008). This application can also determine plant water source and utilization in communities such as desert, forest, river bank and coast(Zhanget al., 2007). Stable isotope technology has been applied to related eco-hydrological research on the temporal and spatial changes of water use efficiency, influencing factors and plant water source,concerning vegetation ecosystems of different regional forest, grassland, desert and farmland (Maand Liu,2007). Research on vegetation water source, soil moisture dynamic change and plant root water absorption and release, and plant water use efficiency have benefited by use of comparing hydrogen and oxygen isotopes of plants, soil, water sources under natural conditions (Trattonet al., 2000; Zhaoet al.,2008; Caoet al., 2009). Isotope technology has proven its advantages and application prospect in the eco-hydrological field, concerning plant water-use efficiency of various vegetation types, temporal and spatial changes, differences of various life forms, difference of efficiency in different environmental gradients, influencing factors and relations between the plant water-use source and water-used efficiency(Zhaoet al., 2008). Isotope technology will provide future potential and application on eco-hydrological research.

5 Research prospect

Research on the coupling relationship between hydrological processes and ecosystem is very weak,and how glaciers, snow and frozen soil affect the influence of runoff remains unclear. Future studies should be based on field observation stations and field eco-hydrological observation and experiments, combined with isotope technology, water chemical and landscape pattern analysis. Research objects should include glaciers, snow, frozen soil, cold desert, swamp,scrub, forest, meadow and steppe in alpine areas in order to discuss the hydrological processes and eco-hydrological function in different landscape zones on the basis of runoff segmentation. This research is mainly carried out as the following:

1) Research the temporal and spatial variation rule of glacier and snow melt water, frozen soil, precipitation and groundwater in the source area, mountain runoff, the combination of isotope technology and water chemical method in the dry season, along with ice, snow melt and rainfall during the flood peak period.

2) Fusion of various methods such as isotope technology and eco-hydrological models to discuss hydrological process in different landscape zones,carry out basin wide hydrological process research of groundwater–soil–vegetation function layers, and study the function of the hydrologic cycle in different landscape zones.

3) Observe water sources, water ratio and eco-hydrological function of some typical landscape zones such as glacier belt, alpine cold desert, swamp meadow, alpine scrub, meadow steppe and mountain forest. Explore the interaction between hydrologic processes and ecosystem, water conservation and ecosystem storage mechanism. Perform research on the coupling relationship between hydrologic processes and ecosystem, how the hydrological process affects the distribution, structure, function and dynamics of the ecosystem, and how the feedback of ecological process affects the hydrological process.

The aforementioned research reveals the function and significance of different landscape zones in the hydrological process and ecological environment maintenance, while promoting effective control and regulation for eco-hydrological processes and eco-hydrological function. This research also provides a scientific basis and reference for some key scientific problems including basin ecological water demand,vegetation restoration, ecological environment construction, basin water resources evaluation, management and water benefit improvement, and ensures the sustainable utilization of the water resources in the basin.

This research is supported by the Projects International Science & Technology Cooperation Program of China (2012DFA20770), National Natural Science Foundation of China (41201043), Natural Science Foundation of Shanxi Province (2012021026-3), and Science and Technology Major Projects of Shanxi Province (20121101011).

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