Descriptions of major crop systems, climate, topography, and soils
Agriculture is vitally important for China. China feeds 1.3 billion of people, or 19% of the world population, with only 7% percent of the planet's arable land, with about 75% of China's cultivated area is used for food crops (FAOSTAT, 2014). Rice, maize and wheat are the three major crops, and the production of these three crops accounts for more than 90% of China's total food production. In China, around 80% of crop production is allocated to human food, while 20% to animal feed (Kearney, 2010; Foley et al., 2011). With the rapid urbanization and growth of people's income and change of life style, the diet of Chinese people are changing from high carbohydrate foods to high protein and energy-dense foods, resulting in more demand for animal products (mainly pork and poultry), therefore more crop productions (Du et al., 2004; Zhai et al., 2014). This situation is in turn putting more pressure on agriculture. In total, China will need 776 Mt grain by 2030 to feed its own people, representing an increase of 36% from the food production in 2014 (Li et al., 2014). Hence, further increase and sustaining of food production has become a national priority.
Major crop systems
China is one of the largest rice producing and consuming countries, accounting for more than 40% of global rice production (Figure 1) and more than 30% of global rice consumption. Rice harvest area in China's accounts for about 25% of the county's cultivated area (FAOSTAT, 2014). Irrigated rice dominates in China, accounting for more than 99% of total rice harvested area (You et al., 2014). Between the two major types of rice, Indica rice accounts for three-quarters of the rice area while the remaining is Japonica rice. Indica rice varieties are generally grown in the south and Japonicas in the north. Hybrid rice varieties occupy about 50% of China's rice harvested area (Yuan, 2003). In south China, two to three rice crops can be grown within one year, whereas in north China only a single-season rice is grown.
The major problems confronting rice production in China are the narrow genetic background, overuse of fertilizers and pesticides, breakdown of irrigation infrastructure, oversimplified crop management, and a weak extension system (Peng et al., 2009). On the other hand, many studies have reported that rice yield has reached its plateau and remained unchanged for several years in China (Zhang et al., 2014; Wei et al., 2015; Xu et al., 2016), implying a small or even negligible yield gain in the near future in China. Therefore, alleviating yield stagnation and further raising rice production is vitally important to ensure food security in China.
Maize accounts for more than one-third of China's cereal production (Figure 2) and is responsible for 23% of the global maize output (FAO, 2012, Figure 1). Maize is grown in north and northeast China. Until the mid-1990s, China's maize grain yield increased in a near-linear fashion, but has stagnated at around 5 Mg ha-1 since 1995. Average annual growth rates declined from 5-8% in 1960-1980s to <1% in the past 20 years. In particular, the focus of China's demand for grains has shifted from staple grains including rice and wheat to maize, which is used primarily as animal feed (Meng et al., 2013).
Since 1991, China has been one of the top wheat producer in the world (Figure 1). A tremendous progress has been achieved in China's wheat production with average yields increased from less than 1 t ha-1 in 1949 to 5 t ha-1 in 2013, and a total production of 126 Mt in 2014 (Qin et al., 2015). The wheat-growing regions of China are divided into three major agro-ecological zones: northern China winter wheat region, southern China winter wheat region, and southern China spring wheat region. The northern winter wheat region is the most important wheat-producing area, with 60 to 70% of the total wheat production in China, followed by the southern winter wheat region with 25% (He, 2001).
Figure 1 Food (either rice, maize or wheat)production in the world and top three producing countries in the year 2015 (Source from USDA, http://apps.fas.usda.gov/psdonline/).
Figure 2 Tendency of harvested area, average yield, and total production of rice, maize and wheat in China. (Source from National Bureau of Statistics of China, http://www.stats.gov.cn/)
Climate, topography and soil
China has diverse topography, soil and climatic conditions. The entire country can be divided into nine major agricultural zones (Source from FAO, http://www.fao.org/ag/agp/agpc/doc/counprof/china/china1.htm):
As a vast plain, this zone has fertile land for agriculture and forest and plenty of water, but a relatively shorter duration of the growing season. Industry and communications are developed, and agriculture scale is large, but the population engaged in agriculture is low. It is the most important production base for cereals, soybean and sugar beet.
Inner Mongolia and Along the Great Wall
With a temperate climate, low precipitation and shortage of water, there is less arable and forest, but vast grasslands. The agricultural environment is very vulnerable because of frequent drought, windy weather and increasing desertification. The main agricultural sector is livestock, and crop and animal husbandry are mixed.
Yellow River, Huai River and Hai River
The climate is temperate monsoon with simultaneous rainfall and solar radiation. Water is relatively scarce. With vast plains, well-equipped agricultural machinery, good communications, a long history of cultivation and a high proportion of arable land, it is an important base for wheat, cotton, maize, groundnuts and fruits.
The topography is characterized by plateau and hills covered by loess; soil erosion is very severe. Solar radiation is plentiful, but water scarce. Agricultural structure is unitary with rainfed grain production. The commodity economy is weakly developed, however the potential for developing grassland farming and fruit culture is high.
Middle and Lower Reaches of Yangtze River
Solar radiation, temperatures and water are all favorable. The arable area is large and fertile. City density is quite high and industry is well developed. Agriculture is highly developed with a high total yield of agricultural products and a high commodity rate. It is the main integrated agricultural production area and the base for rice, cotton, oil crop, tea, silk, pigs and fish.
The climate is warm and humid. The terrain is dominated by hills, with little flat land. There is plenty of biodiversity and rare species. Agriculture is poor with extensive systems and low productivity. Grain production is for subsistence.
The topography is characterized by hills, scarce arable land and a long coastline. Most of the area is subtropical with plenty of precipitation; it is the only area suitable for tropical crops.
Gansu and Xinjiang
Land resources are typified by vast areas and low quality; there is much natural grassland but little forest and arable. Solar radiation and thermal resources are abundant, but water is very scarce. Desertification and salinization are very severe and the agricultural environment is very fragile. Grain and oil resources per capita are high. The yield of cotton, sugar beet, fruit and melon is high. Grassland husbandry is well developed.
This zone is characterized by rarefied air, high altitude, strong solar radiation and low temperatures. The area of natural grassland takes first place and forest takes second. Arable land is very rare and distributed in patches. Water is plentiful but unevenly distributed. The management of agriculture and animal production is extensive and production level is low.
Data Sources and Assumptions on rice crop (following GYGA protocols)
Harvested area of irrigated rice was retrieved from SPAM2000 (http://mapspam.info/).
Weather data and reference weather stations (RWS)
In total, 50 RWSs located mainly in northeast, north, central and south part of China were selected following the GYGA protocols (Grassini et al., 2015; van Bussel et al., 2015). The total area of the buffer zones formed around the RWSsaccounts for 48% national rice harvested area. Those RWSs belong to 16 CZs, covering 85% of national rice harvested area.
Weather data of 11 years (2004-2014), including maximum and minimum temperatures, wind speed, relative humidity and precipitation were collected from the Chinese Meteorological Administration. Solar radiation was obtained from the NASA-POWER project (http://power.larc.nasa.gov/).
Data of actual yield were collected from statistical books and online sources of national and provincial governments. Data of recent 2-5 years of the counties where selected RWS were located were used. For the double rice (including early and late season), only the average yield without separation of early and late season was available. Since the harvested area of early and late season rice is largely equal (Statistical data from Ministry of agriculture of China), the average of simulated Yp of the early and late season rice was used for double rice systems.
ORYZA v3 was used to simulate Yp. Standard parameters as compiled in ORYZA v3 were used, but some parameters were calibrated to make the simulation results more adaptive to the specific crop and environmental conditions in different regions in China.
Rice management practices
Most crop management practices within a climatic region are similar. Therefore, to unify and simplify the crop management practices of RWSs in the same region, six regions were classified by considering climatic suitability of different cropping systems and the locations of the GYGA climate zones based on Duan et al. 2012 & 2013. The single rice systems were grouped to north-east, north, central and west regions, while the double rice systems were grouped to central and south regions. In each region, we selected one (single season) or two (double season) dominant rice cultivars in that region to run the simulation. Rice management practices including sowing date, plant density, growth duration and phenological stage of local cultivars were collected. For both single and double rice systems in the central part of China, two years (2012-2013) unpublished high yield experimental data from Wuxue experimental station, Hubei province were used to calibrate the model. Field management practices for high yielding rice were followed for those regions. For other locations, we utilized the published experimental data or locally calibrated parameters of ORYZA v3/2000 from high yielding research papers which are supposed to practice high yielding managements.
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