Regularity in Agro-ecosystem of north China*

Xie Xianqun and Tang Dengyin

Institute of Geography, Chinese Academy of Sciences

Beijing 100101

Abstract

The authors discuss the research connotation and strategy focused on “water movement and its regional differential regularity in agro-ecosystem of north China”. This paper presents the disclosure of the mechanism of water balance and water cycle process, and the establishment of crop productivity model in limited water environment condition of north China through combined experimental sites and network study of water movement and exchange regularity in agro-ecosystem in north China. It also gives evaluation on water movement and productivity sustainable development and its regional differential regularity in agro-ecosystem of north China.

Key Words: Agro-ecosystem, water movement, regional differential regularity.

1. Introduction

Water is the most active factor in the structure and function of an ecosystem. It is closely related to the formation of many kinds of ecosystems, and the occurrence and change of many kinds of physico-geographical process. Water conditions are playing a decisive role in the regional differentiation of humid, semi-humid, semi-arid, arid regions from east to west of north China( Northeast, North, Northwest China), and the corespondent formation of the forest, grassland, drought grassland and desert grassland ecosystems. Water is the most important carrier in the movement of solid materials and chemical materials within various ecosystems. Water is an indispensable factor in plant photosynthesis and growth process. Water keeps moving in ecosystem. Different water bodies, including atmospheric water, ground water, underground water and plant water cycle and transform each other constantly in the Soil-Plant-Atmosphere Continuum(SPAC). If their mutual relationship and its dynamic change process are identified, there will help make scientific explanation of the ecologic process and evolution phenomena of natural environment concerned water, and predict future eco-environment change. This will provide a scientific basis for correct calculation of water resources and their reasonable control and utilization.

The north China( Northeast, North, Northwest China) is the major part of agriculture area. Water resource is a main constraint to the sustainable development of agriculture. There exist water shortage, nutrition insufficiency and unreasonable water use in agriculture production in the Northwest China arid region, Loess Plateau semi-arid region, Northeast and North China semi-humid region. This not only limits the further increment of cereal production but also causes environment problems.

North China Plain is China’s major cereal production region, but its output still remain at middle-low level. To solve the water shortage problem and prevent soil saline, it is necessary to strengthen unified management of surface water and ground water.

The Loess Plateau is favorite for agriculture with very thick soil layer, but there is a shortage of water and fertilizers, the production is severely affected. Reasonable utilization of the precipitation and fertilizers, improvement of cultivation system will be a important way to promote the agricultural development in this region.

In Northwest China, there are plenty light and heat resources, the agricultural production can be greatly increased if the water problem can be solved properly.

To achieve the goal of cereal production of 500 billion kg by year 2000 and of 560 billion kg by year 2010 designed by the Central Government, it is very necessary to undertake fully and careful study of the key constraint factor- problem of the conflict between water supplies and needs. The discrepancy of water supplies and needs in the afore mentioned “three North” (Northeast, North, Northwest China) is caused by human activities and other natural factors. Therefore it is necessary to adjust and control human activities. The regularity of water movement and its transformation in the SPAC system in the ecosystem in north China, different human activities, such as different cultivation management and control means, and their impacts and function to this regularity should be carefully studied. This research is a wide-scope subject combing geography with biology of basic and application properties. It has penetrated a lot of subject fields, such as agronomy, physical geography, hydrology, meteorology, plant physiology, ecology. Various ecologic and environmental factors, water cycle, regularity of energy transformation and nutrition movement in SPAC system, and the crop-water relation which demonstrates the regularity must be studied thoroughly. These indicate the significance of water movement in agro-ecosystem in practice and initiative in the research of geography and ecology. Therefore the research on water movement and regional differential regularity has a clear and definite prospect of application and obvious theoretical and practical importance.

2. Progress of research on water movement

Research on crop-water relation, regional water balance and water transformation is a hot-spot of international research on agro-ecology. Many research reports on monitoring and forecasting physiological indicators of crop water shortage have been published by Water Conservation Laboratory of the US Department of Agriculture, North America Great Plain Center^[1-3]. A Crop Water Stress Index(CWSI) and a model of estimating regional water transformation were presented. De. Wit of Wagningen University of the Netherlands has published tens of series books on crop growth simulation model^[4-5]. Baker, Ritchie, Loomis of USA gained great achievements in research on crop production simulation model and its application^[6]. Root zone water dynamic balance simulation through water balance method are used in dealing with crop moisture absorption by these kinds of models. Water balance or water movement is a key component in these models.

Research on water cycle and movement in agro-ecosystem is greatly concerned in many international research programmes. Great attention has been giving by the International Hydrology Programme (IHP), International Geosphere-Biosphere Programme (IGBP), World Climate Research Programme (WCRP), Global and Energy and Water Cycle Experiment (GEWEX). Water and energy transportation process in SPAC system in different scale has been taken as important content in the research on the simulation of land-atmosphere interaction, earth climate and water circle interaction simulation. The physical process of plant and water cycle, i.e., Biosphere Aspects of the Hydrological Cycle (BAHC), has been listed as a core project. A research subject has been listed titled as “ The plant change impact on water and energy circulation”.

Since the IGBP proposed the core research project on BAHC, many USA and European countries undertook research on water movement in ecosystem from 1990’s. The Terrestrial Initiative in Global Environment Research (TIGER) has been enacted in UK. The impact of water and energy budget in ecosystem is a major research subject in this programme, including water and energy interaction, the establishment of the model of moisture movement in Soil-Vegetation-Atmosphere Transfer(SVAT) system, and the hydrological model on continental scale. The Terrestrial Ecosystem Research Network (TERN) has been set up in Germany, a key project of which is “ The control of energy and water exchange in the plant surface”. This project contains some subprojects on agro-plants evaporation measurements, research and development of Soil-Vegetation-Atmosphere Transfer (SVAT) model. Soil moisture models and SVAT model and regional models are used in the evaluation of energy budget impacts of global change .

In 1960’ , Professor Huang Bingwei, an academician of the Chinese Academy of Sciences, proposed the theory of energy and water balance in ground surface and its function in physical geography. The Chinese scientists started the experiments on water and energy balance in farmland. A lot of achievements has been gained in the field of water resources assessment, water-saving application, evapo-transpiration of farmland in North China Plain. However, a systemic and over-all and comprehensive research is yet to be undertaken, and existing measuring means and research methods are still simple and backward. Theoretical research has not made break-through in many key fields, e.g. research on water movement, transportation regulation, energy and material transportation and movement principle, especially the research on models regarding from point to large scale area in agro-ecosystem during the period of the Seventh Five-Year Plan period and the Eighth Five-Year Plan period, the Chinese Ecological Research Network (CERN) organised a research programme on plant-water relation and research on water transformation regularity in agro-ecosystem. Series papers on “ Research on Crop-water relation” has been completed, and a experimental model to measure and calculate evaporation of water consumption and crop water lack diagnosis and water transformation proposed for the first time^[7]. Major crop water requirement, consumption, water use efficiency in the typical regions of China have also been proposed. (as indicated in table 1) ^[8]

Table. 1. The Crop Water Requirement, Consumption and Water Use Efficiency in Different Ecological Regions of China.

Region	Crop item	Wheat	Maize	Soybean	Rice	Millet	Cotton
Song-Neng Plain Hailun station	Ea mm Et mm R mm Ea-R mm Y kg/mu WUE kg/mm	450-500 382 244 206-256 236.8 0.62	450-500 450 430 20-70 460 1.01	500 482 430 70 136 0.29
Lower reach of Liaohe region Shenyan station	Ea mm Et mm R mm Ea-R mm Y kg/mu WUE kg/mm		600-650 670 630 -30- -20 535 0.79		550-600 544 600 -50 517 0.95
Loess hilly region Ansai station	Ea mm Et mm R mm Ea-R mm Y kg/mu WUE kg/mm	450-500 420 250 200-250 56 0.14	500 468 470 -2 374 0.80	450 412 454 -4 95 0.23		450 433 410 40 170 0.39
Huanghuaihai Plain Fengqiu station Yucheng station	Ea mm Et mm R mm Ea-R mm Y kg/mu WUE kg/mm	450-500 400-500 330 120-180 320 0.82	400-450 400-450 470 -70- -20 650 1.48	450-500 350-400 470 -20- -30 0.34
Desert oasis region Fukang station	Ea mm Et mm R mm Ea-R mm Y kg/mu WUE kg/mm	550 170 380 270 0.49	500-550 550 170 330-380 700 1.28				500-550 450-500 170 330-380 143 0.30
Red-soil hilly region Yingtang station	Ea mm Et mm R mm Ea-R mm Y kg/mu WUE kg/mm	450-500 427 240 210-260 731 0.17			400-450 395 800 -400 300 0.80

Notes: Ea: water requirement, Et: water consumption, R: precipitation, Y: yield, WUE: water use efficiency

spring wheat represents wheat in Hailun station, buckwheat in Yingtan station,

summer maize represents maize and soybean in Huanghuaihai plain, summer soybean data measured in Yucheng station.

Water requirements of many crops refer to related references.

This research results provided an experimental basis for research on the relation between water cycle and productivity in the agro-ecosystem during the period of the Ninth Five-Year.

3. Connotation and strategy of the research on water movement and its regional differential regularity in agro-ecosystem

The direction of research on water balance and water cycle in agro-ecosystem, can be summarized as the research on water exchange in SPAC system and its theoretical significance and practical use. It is composed of following components: (1) to fix different elements of water balance in ecosystem; (2) to explain water cycle mechanism in different processes; (3) to determine the role of water in the process of crop growth and yield formation; (4) to explore the relation of water movement, transformation to the movement and transformation of energy and chemical matters (salt and nutrition) movement and change; (5) to probe the role of water in the structure and function of agro-ecosystem.

3.1. Water cycle research in agro-ecosystem

Figure 1 demonstrates the main content and purpose of agro-ecosystem research. It shows that contemporary water cycle research in agro-ecosystem is based on continued, systemic and dynamic view and quantitative method, i.e., taking SPAC system as a physically continuum, taking atmosphere water, surface water, plant water, soil water , ground water as a mutual related entity, undertaking research on different aspects of the transforming process and regularity of these “five waters” in farmland. This involves evaporation, transpiration, infiltration, runoff etc., establish a water cycle process model. In the initial stage, it is to address each single process, then combine different processes, at last explore water control mechanism in farmland based on the soil water-crop relation and internal relation with crop yield to provide a basis of theoretical and practical application of water management in farmland.

runoff

plant water

surface water

Figure 1. Water cycle process in agro-ecosystem

Figure 1 indicates that the research on water cycle in agro-ecosystem includes two active research fields worldwide, i.e. research on water relation in SPAC system, and research on water movement in unsaturated layer.

No matter the research on water balance or water cycle, three steps are designed: firstly, to gain accurate and typical data; secondly, to explore principle and mechanism; thirdly, to use the results for theory establishment and field practice.

3.2. Research on water cycle and water balance in SPAC system

According to afore mentioned theory of water cycle process, Kang Shaozhong proposed a frame of research on water cycle process and water balance^[9].

Figure 2 clearly indicates that precipitation, canopy water reservation, infiltration, soil moisture redistribution, water discharge and water absorption in crop root system, water transportation in plant trunk, water diffusion from crop leaves to air, at last water joining into atmospheric turbulence exchange and other water transportation process are taking place continuously in farmland and form water cycle process. According to figure 2, water budget

z_r

horizontal underrun R_ss

				zd

Figure 2. The water budget and exchange process in farmland^[9]

in a period of time in farmland is equal to water reservation difference in farmland. Therefore, water balance equation can be given. For soil and canopy above the active layer of crop root system layer:

(I + P + D_ec + D_es + S_G) - (E_s + T+ F_d + R_s + R_ss + E_slD + E_Cl)

= △W_s + △W_p (1)

the meaning of the above marks are the same as that of figure 2.

In the same time, water amount exchange follows the following relation expression:

P = P_IC + P_t + P_s (2)

S_r - T = △W_p (3)

F + S_G - R_ss - F_d – S - E_s = △W_s (4)

P_t + P_s + D_es + I - E_SID - R_s - F = 0 (5)

All elements in figure 2 such as precipitation, irrigation water, infiltration, soil moisture movement, soil moisture infiltration into groundwater, soil water evaporation, water absorption and transpiration and evaporation in plant root etc., are necessary to be monitored and studied. While undertaking the research on mathematical models of water potential situation and its transformation relation, the SPAC system can be divided into two parts including plant part above ground and soil part under ground. As to the spatial variations of different parts, an average value will be taken, disregarding the variables in different situation. And then, according to water amount balance equation and energy balance equation in farmland. and continuity equation of water flow movement, a water and heat exchange model can be established for the two layers to solve the problem of dynamic transforming process of the variables in different situations including leaves temperature, transpiration speed of the plant above the ground, and soil temperature, soil evaporation speed among plants of the part under ground, as well as farmland latent heat, air sensible heat etc.

3.3. Energy balance and transportation in agro-ecosystem

Energy balance research in SPAC system plays a very important role in water balance and water cycle study in agro-ecosystem. This is because certain element of energy balance (e.g., evaporation) itself is a very important element in water cycle and important process of water cycle. Energy balance and water cycle are very close to crop growth.

Energy balance and transportation in SPAC system constitute a very complicated system. It includes the solar radiation transportation and underlying surface radiation balance between land surface and atmosphere, and net radiation absorption of land surface and their redistribution process (including canopy transpiration latent heat consumption, air sensible heat and soil heat flux). All these are a necessary process on research on water-energy balance and cycle in farmland. They are also one of the most important submodels of the model of research on the relation between farmland water cycle and crop productivity.

Figure 3 demonstrates the energy balance and transportation in SPAC system :

Figure 3. Energy balance and transformation process within SPAC^[9]

3.4. Model of the relation of crops to water and nutrition

Model of the relation of crops to water and nutrition is a mathematical description of water change and different nutrition condition and their impacts to yield in crop growth process. The described process may be a part or full crop growth period which is used mainly in the prediction of crop yield in limited CWSI(Crop Water Stress Index) and different nutrition ratio. In fact, the description of the relation of crop to water and nutrition should be considered only as one of the submodels of crop growth model. Other submodels include crop growth and its relation with other environmental factors. A complete comprehensive model of crop growth is a complex SPAC model involving water flow and energy flow. The input items are (1) physical parameters(water conductivity, water content, soil moisture characteristic curve); (2) crop parameters (root system distribution, leaf area index, crop growth situation, crop water potential); (3) meteorological parameters(air temperature, humidity, wind speed, solar radiation); (4) photosynthesis parameters(CO₂ flux, photosynthesis active radiation); (5) nutrition level parameters including different composing types of water and nutrition. Major output items are: transpiration and evaporation (crop transpiration, soil evaporation), soil and crop water situation, fertilization situation and crop growth situation, crop biological yield and economic yield(dry weight of stem, root, leaves, seeds and production ). The conceptual model of relationship between water cycle and crop productivity in agro-ecosystem is shown in figure 4.

Figure 4. Model of relationship between water cycle and crop productivity in agroecosystem

3.5. Research on the regional differential regularity of water movement

Research on the regularity of water movement aims at seeking the regularity of spatial variation in a region and in different regions. Thus, the data on water environment gained by the CERN (Chinese Ecosystem Research Network) in agro-ecological stations in north China area for the past years should be fully used. And a further systemic and fixed station observation shall also be undertaken on different elements in water environment, then a dynamic submodel of element movement shall be established, and then a comprehensive model of meso-micro scale water transformation and movement shall be established. Based on this model, a typical model on regional scale in agro-ecosystem will be set up to seek the regional different regularity. At the same time, according to water environment’s evolution regularity of water element, water resource preservation, allocation feasibility and agricultural needs by national economy, water a prediction model of environment element will be established. This will give assessment to the tendency of water environmental variance and the situation of agricultural water needs/supplies in different dry or humid regions of the major cereal production regions in north China, and advance a proposal on water management countermeasure. All this is the final goal of that research. The research frame is given as follows:

Figure 5. Models of the research on water movement and its regional differential regularity in agroecosystem

The above figure shows that the unified observation on water movement and transformation among atmospheric water, surface water, plant water, soil water and ground water and their mutual exchange in agro-ecosystem and water movement-crop productivity relation are the core of this research. Firstly, based on the single station field comprehensive experiment, to put comprehensive observation into SPAC system to gain element data of single station on water amount exchange, to establish single station experimental model on farmland water amount transformation and water movement ; secondarily, to combine the field comprehensive experiment with mathematical model, and combine the single station experiment with regional and historical and existing ecological environment element such as hydrology, meteorology, soil, plant, etc., and crop yield, then to compare the water information network with spatial variations in different regions. Thirdly, to combine field comprehensive experiment with remote sensing and establish related experimental remote sensing model and GIS system for realizing the regional scale transformation.

It is important to point out , the above mentioned strategy of the research could not be completed in a short time, on the contrary, it would have to take a long time. In the near future, the exploratory research on experiment should be stressed to obtain accurate experimental data and establish typical farmland-scale experiment model, as well as the establishment and verification of submodel of water- productivity relation. The long term goal is to establish mathematical model through the research on scale transformation of point(micro-scale)-section(meso-micro scale)-entire area(macro-meso scale), or through remote sensing technology and establish GIS system, achieve the evaluation of regional water amount transformation regularity and its relation with agriculture productivity.

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