Water transfer through Soil-Vegetation-Atmosphere system is one of the most important multi-interface processes of land surface. A good understanding those interface processes and their mechanism is helpful to determine and regulate the water requirements of an ecosystem. Toward these ends, more attentions have been paid by the Chinese scientists, however, most researches focus on production and application agriculture and forestry. From the mid 1970s to the mid 1980s, researches were mainly focused on soil moisture holding capacity, dynamics and numerical simulation of moisture movement. In the mid 1980s, the conception of Soil-Plant-Atmosphere Continuum (SPAC) was introduced into China, a series of research, with focus on farmland's ecosystem, on reciprocity among soil, plants and atmosphere, laws of moisture movement inside SPAC and fluxes' evaluation model have been conducted. For example, many articles were published, covering fields of energy-substance exchange in the ecosystem of farmland (Niu et al., 1987), relations between crop and moisture (Xie et al., 1992), measurement and calculation of farmland evapotranspiration (Zuo et al., 1991; Xie et al., 1991), farmland transpiration and crop water consumption (Cheng et al., 1993), energy-moisture balance and agriculture potential productivity (Zhao et al., 1992),moisture movement in SPAC system(Tan,1983；Luo,1986; Kang et al.,1990;Yu et al.,1992,1994,1995,1997; Liu et al.,1997,1999). In the mid and late 1990s, the above researches were deepened while much more attentions were paid to researches on laws of crop water consumption and water-saving agricultural technology (Dong, 1999). Meantime, the mechanism of water transfer through interface was enhanced and exciting outcomes were achieved, such as the transformation rule, micro-ecosystems of plant root ends and simulation of water movement across the leaf-atmosphere interface. Other significant progress has been achieved in the study of changing flow resistance in micro-ecosystems of plant root ends and leave-atmosphere systems, including resistance due to soil, soil-root interfaces, root systems, and air. Exploration of such resistance factors must consider their changing relationship with organisms of different genetic makeup, variable root system structure, soil-root interface clearance and changing conductivity under natural conditions. In resent years, water and carbon flux in SPAC and water cycle of Soil-Vegetation-Atmosphere with large scale has been emphasized.
The main activities of research on water transfer in Soil-Vegetation-Atmosphere can be outlined as two following aspects: (1) numerical simulation on water/heat transfer in SPAC and modeling coupled water/heat and carbon transfer process, which verified by field observation data; (2) water cycle of Soil-Vegetation-Atmosphere System at watershed/regional level. More details as follows.
A great deal of work has been done to simulate water/heat transfer through SPAC multi-interface based on the field experiments. For instance, Shao（1985）numerically simulated the macroscopic water uptake in Loess area; With the help of multiple nonlinear regression, Shao et al. (1997) developed a mathematical model of the crop root uptake; The macroscopic water uptake models widely used in modeling the water flow in soil-plant system are evaluated by using the precisely obtained evaptranspiration and soil water content data from the lysimeter and the root distribution data obtained in the wheat field nearby (Luo et al., 2000); Soil moisture, heat/temperature dynamics were simulated under mulching condition at farmland (Sui et al., 1992; Kang et al., 1993). By integrated use knowledge of soil dynamics, micro-meteorology, plant physics, several coupled water/heat transfer models in SPAC were established and were used for field water balance and crop water requirement evaluation. Yang et al. (1989) developed a model for coupled movement of moisture, vapor and heat in field and simulated soil evaporation by using the model. Lu (1996, 1998) simulated the effects of soil water condition on winter wheat growth at rainy area in South China; An improved dual-source model is proposed by which the canopy transpiration and underneath soul evaporation can be estimated analytically (Mo et al., 2000)； Based on the theory of water and heat flow transport in porous media proposed by Philip (1957) and de Vries (1958), mass lumped finite element method is used for developing numerical model of water movement and heat transfer in the inhomogeneous soil under mulched wheat straw (Shen, 1997); Ren et al. (1998) established a numerical model using the alternating direction implicit (ADI) finite difference method to study two-dimensional soil heat and water flow with a partial surface mulch cover; A numerical model for analyzing the heat and flow in the SPAC system under transparent polyethylene mulch is studied. A system consists of four layers including atmosphere, canopy, mulch and soil is established in the model (Wu et al., 2000); Mao et al. (1998) established a SPAC model having function of describing soil moisture, heat and transpiration dynamics during the crop growth period, and by means of the simulation results of water and heat transfer in winter wheat field, an empirical formula for estimating crop evapotranspiration from the reference evapotranspiration and soil water storage between soil surface to 1 m below is established as well (Mao et al., 2001). Wu et al. (1996) physically understand the evaporation–transpiration on SPAC and the water–thermal conditions of soil, a humidity-temperature equation of the canopy reflecting the coupled relation between the evaporation–transpiration form the foliage and the water–thermal conditions of the soil is deduced. And a coupled iteration method that can be used to quantitatively depict the coupled relation and to solve the evaporation–transpiration from the foliage and the water–thermal regime of the soil simultaneously is developed. On basis of the crop growth model MACROS(L1D), Luo et al. (2001) established a dynamic coupling model for crop growth and water balance in field is established with the help of the relationship between soil water conditions and crop growth, which has been used in simulating the water balance for winter wheat in rainy region of southern China, and in studying the processes of occurrence and fading of soil waterlog in this region. Zhang et al. (1999) using the crop stomatal conductance as an explicit parameter, a water deficit crop yield model is established based on the remote sensing information from NOAA-AVHRR, crop photosynthesis characteristic and meteorological data, and verified by the yield distribution of winter wheat in North China Plain.
The research process on water cycle in Soil-Vegetation-Atmosphere System at watershed or regional scale much more concentrated in regional evapotranspiration; Water yield/stream runoff response to the forest coverage; And regional Soil-Vegetation-Atmosphere interaction.
Li et al. (2000) utilized the Penman formula to calculate the regional evapotranspiration capacity in the upper reaches of the Yellow River and analyzed the changing trends of the climatic factors in the region such as evapotranspiration capacity, sunshine duration, air temperature and air saturation deficit, mainly focusing on study of impact of these factors on evapotranspiration capacity. They thought the evapotranspiration capacity in the upper reaches of the Yellow River appeared an increasing trend year by year, at an increasing rate of 3.25 mm each year. As the main influencing factor, sunshine duration was increasing at a rate of 3.6 hours each year. Meanwhile, temperature also presented a yearly rising trend of 0.4℃/10a, and the air saturation deficit also increased at a rate of 0.02 hPa/a. Therefore, the author held that the increase in sunshine duration, air temperature, and saturation deficit enhanced the grassland evapotranspiration capacity. And the resulted increase in evapotranspiration and the decrease of precipitation would directly cause the decrease of the flow discharge and desertification expansion in the upper reaches of the Yellow River. Li et al. (2000) accomplished the study on the observation and experiment at Aksu Water Balance Station of CAS the north edge of Taklimakan Desert and the analysis on daily evaporation change process in some arid, semi-arid and humid areas. Experiment and analysis on non-water-permeable underlying surface evaporation were also carried out. Liu (1999) studied the water and energy exchange between earth and atmosphere and the water and heat transmission simulation issues in the desert regions.
China has carried out catchments researches on ecological functions of forest water cycle. Watersheds under research vary from less than one square kilometer to over thousands of square kilometers. The researches were mainly about the relations between changes of runoff in watershed, including influences of vegetation coverage changes on annual runoff and its seasonal distribution, flood volume, process of flood, changes of runoff combination etc. Speaking from experiments of small catchments covering China's frigid temperate zones, temperate zone, sub-tropic zone and tropic zone and researches on larger catchments such as the Yellow River and Yangtze River, most conclusions showed that reduction in forest coverage would increase annual runoff of rivers to different degrees (Liu, 1996). However, conclusion drawn from the comparative studies on small catchments of forests of Miyaluo Mountains in West Sichuan Province, fir area in upper reaches of Minjiang River and the four major drainage basins of the Yangtze River was that: annual runoff of forest areas was larger than forestless areas or areas with sparse forest (Ma, 1987,1993; Huang et al., 1989). Comparative studies on dynamic changes with time of runoff in forest areas and that in forestless areas or sparse forest area showed that forest could regulate distribution of runoff and increase dry season flow discharge as well as reduce flood discharge and peak flood discharge and suspend or extend time of conflux. But some researches concluded that forest coverage and the runoff of dray season had obvious negative interrelations or forests had no prominent influence on flood characteristics (Zhou et al., 1993).According to CNC-IGBP (2001), the researches on the interaction of soil-vegetation -atmosphere in semi-arid grassland in Inner Mongolia, “Key Projects of National Sciences Foundation” has made following significant scientific breakthroughs: Completed a comprehensive mesoscale experiment on the interaction between soil-vegetation-atmosphere in the semi-arid grassland in mid-latitude, it is the first time that the observation experiment database for semi-arid grassland in mid-latitude has been set up. It is the first time that the observing data of mid-latitude grassland typical land surface/vegetation types and soil, water and relative atmospheric precipitation of the corresponding period were obtained. We found that when abnormal precipitation happened in 1998, the water held in surface stratum was leaning small, which was the result of water filtering towards deeper stratum through calcareous crust. That amends the conclusion that for near 20 years, scientific circle has thought that the regional rainfall cycle was limited to surface stratums less than 2 m deep and gained a new understanding on long term scale characters of semi-arid grassland water balance in mid-latitude. Revealed the characteristic of the water cycle of regional surface/vegetation composition structure. Combination of models and observations showed that sand transmission caused by the region's winter monsoon circulating structure was the main reason for the forming of regional sand zone. Generation of imbalanced surface structure of sand zone/grassland was the reason for overwhelming convection rainfall in the region caused by summer monsoon. It provided guidance for the region's desertification and climate changes, impact of human activities and regional protection by combining observation and analysis of sand zone and grassland with the above researches on models. Long-term observation and regional researches on soil quality and vegetation structure of grassland under different grazing intensities concluded the conceptual model of community eco-degradation and restoration/succession, providing base for establishing numerical model. According to the research, the process of carbon assimilation and dissimilation caused by human activities had serious results, for example, farming causes greatly decreasing of organic carbon in soil while common grazing hah no obvious affection. It showed that farming is extremely unsuitable for the region. Multi-time phase measurements in a whole year were carried out for the first time on the fluxes of N2O, CH4 and CO2 in different grasslands in the region, and typical source and sink data were obtained. Further understanding of the complexity of relations between temperature and water elements, and new understanding on the microorganism process of controlling N2O emission were realized. Observation and model researches obtained basic feature of boundary layer structure and land surface fluxes of semi-arid steppe of different types of grasslands at different growth stages, and set up some effective methods of identifying surface process parameters. These methods have been applied in model researches. A set of methods has been preliminarily formulated that retrieve relevant quantitative parameters within the areas by using the information obtained from satellites and radar. At present, the researches on the following scientific subjects are carried out the development of SVAT model with the involvement of human activities.
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