Meihuan YANG，Yongchong YANG

College of Surveying and Mapping Science and Technology，Xi'an University of Science and Technology，Xi'an 710054，China

Farml and is material base guaranteeing national grain security and sustainable development of social economy［1－2］．ZhaiWenxia and Han Binghua stated that reasonable control and improvement of farml and structure is helpful for coordination with industrial structure［3］．To value l and highly，use l and rationally and protect cultivated l and effectively is a national basic policy of China， and to ensure that China's farml and ator above the red line of120million hectares relies on scientific determination of farml and protection area in l and use planning［4］．The county-level l and use planning forms a connecting link between various levels of l and use planning，thus making prediction of farml and protection area in county-level l and use planning will be of great significance［5］．There are many methods to predict the farml and protection area［6－9］．Taking Mizhi County as an example and the year of 2005 as base period of planning，wemade a prediction of farml and demand in 2010 and 2020 using grain security method，supply-demand balance method， and trend extrapolation method．Then，we built a fixed weight combination model to make scientific summary of three prediction results．Finally，we predicted the farml and demand of Mizhi County in 2010 and 2020，in the hope of providing basis and reference for determination of farml and protection area in the l and use planning．

1 Overview of the study area and data source

1．1 Overview of the study areaMizhi County（E109°48′45″－E110°29′28″ and N37°39′26″－N38°05′22″）is a county in Yulin City of Shaanxi Province．It is59 km long from east to west and 47 km from north to south，having a total area of 1 173．80 km2．In the county，major l and form is the typical loess l andform．Located in north of hilly area of the Loess Plateau，Mizhi County has mild temperate semi-arid continental monsoon climate，with annual average temperature of8．6℃，extreme highest temperature of39．6℃，extreme lowest temperature of－31．8℃，mean annual hours of sunshine2 678．6 hours， and mean annual rainfall of440．9mm．Water system in the county belongs to Wuding River（now called Yongding River）system， and total volume of water resource is 71．66 million m3．In 2005，total population of Mizhi County reached 208 400，local GDP reached 599 million yuan，financial revenue reached 10．75 million yuan， and local financial revenue was up to 8．61 million yuan．farmers' per capita net income reached 1487 yuan and urban residents' disposable income reached 4 370 yuan．There are 7 towns and 6 townships：Yinzhou Town，Tao Town，Yangjiagou Town，Dujiashigou Town，Shajiadian Town，Yindou Town，Jijiacha Township，Qiaohecha Township，Gaoqu Township，Lizhan Township，Shilipu Township， and Guoxingzhuang Township under the administration of Mizhi County．

1．2 Data sourceNational economic and social development data of MizhiCounty were selected fromStatistical Yearbook of Yulin City and Statistical Yearbook of Mizhi Countyof 1997－2006， and statistical data provided by relevant statistical and specialized department．Basic data of l and use were selected from dataset of l and use changes of Shaanxi Province in 1996－2005．

1．3 Study methods

1．3．1Prediction of farml and protection area based on grain security．The prediction of farml and protection area based on grain security is the minimum farml and protection area guaranteeing regional grain security．Generally，it takes basic demand of local population for grain as objective and predicts area of farml and necessary for protection to satisfy certain self-sufficiency of grain［10］．The calculation formula is as follows：

1．3．2Prediction of farml and protection area based on supply-demand balance．This method starts from factors influencing changesof farml and area and takes satisfaction of demands of national economic development for various types of l and and realization of supply-demand balance as purpose［11］．Farml and increase is generally analyzed from l and development，l and consolidation and l and reclamation．Farml and decrease is generally analyzed from construction，ecological restoration，agricultural structural adjustment， and natural disaster， and the prediction of farml and protection area for supply-demand balance can be expressed in following formula：

whereSnsignifies the planned farml and supply capacity in the target year，S0signifies the farml and area of planning base period，S1is farml and area for construction purpose，S2is the farml and area for ecological restoration，S3is farml and area for agricultural structural adjustment，S4is farml and area damaged by natural disasters，S5is farml and area increased from l and use development，S6is farml and area increased from l and consolidation， andS7is farml and area increased from l and reclamation．

1．3．3Prediction of farml and protection area based on trend extrapolation method．Trend extrapolation is a forecasting technique that can be used to estimate both demand for and supply of human resources．This technique is based on what is called a time series－a set of observations measured at successive points in time（e．g．，weekly，monthly，yearly）．Usually the most significant underlying characteristic of any data is their trend，mathematically referred to as the slope or average rate of change over a time period．Trend extrapolation is to find out development rules of an event according to past and present data of the event，find out a suitable function curve changes， and accordingly predict future development of the event［12］．According to farml and change trend of MizhiCounty，we mainly adopted linear trend model and quadratic curve model to predict the farml and protection area of the target year．

2 Results and analyses

2．1 Prediction of farml and protection area based on grain security

2．1．1Determination of related parameters．

（i）Per capita grain demand．According to the prediction ofNational Medium and Long Term Grain Security Plan（2008－2020），the per capita grain demand of the whole country will be 389 kg in 2010 and 395 kg in 2020．

（ii）Determination of total population．According to theOverall Urban Planning of Mizhi County（2009－2030）and the Outline for National Economic and Social Development Planning of Mizhi County，the total population of Mizhi County is 220 000 in 2010 and 240 000 in 2020．

（iii）Grain self-sufficiency．According to basic ideasof revision ofOutline of National Overall Planning on L and Useand the white paperGrain Issue in Chinaissued by Information Office of the State Council on October24，1996，the basic policy of China's grain issue is to realize self-sufficiency based on domestic resources， and the grain self-sufficiency of China should not be lower than 95%in normal years．Considering future grain security of Mizhi County and its strategic position of large agricultural county in Yulin City and combining statistical data of relevant departments，we determined that the grain self-sufficiency of Mizhi County is 95%in both 2010 and 2020，so it can basically realize grain self-sufficiency．

（iv）Per unit area yield of farml and．The per unit area yield，multiple crop index， and grain-crop ratio of MizhiCounty in 1997－2005 are listed in Table 1．

Table 1 The per unit area yield，multiple crop index， and grain-crop ratio of M izhi County in 1－2005

According to changes of per unit area yield since 1997，the per unitarea yield will be2450 kg／hm2in 2010 and 2724 kg／hm2in 2020．

（v）Multiple crop index．According to theOutline of the Eleventh Five-Year Guideline for National Economic and Social Development of Mizhi County and Outline of the Twelfth Five-Year Guideline for National Economic and Social Development of Mizhi County，MizhiCounty actively builds modern agriculture industrial system，stably develops grain planting industry，strengthens animal husbandry，energetically develops forest and fruit industry，improves vegetable planting industry，actively develops labor migration industry，energetically develops agricultural and sideline product processing industry．In the planning period，the farml and utilization efficiency will further rise．We predict that the multiple crop index of Mizhi County will be 100% and 115%in 2010 and 2020 respectively．

（vi）Grain-crop ratio．The grain-crop ratio is farml and area planting grain to the total sown area of farml and．According to data in Table 1，based on factors such as economic development and agricultural structural adjustment of Mizhi County，we determined that the grain-crop ratio of Mizhi County will be 70%in 2010 and 65%in 2020．

2．1．1Calculation of the farml and demand based on integrated grain production capacity．According to values of the above parameters，using the formula（1），the farml and demand of Mizhi County willbe47 405 hm2and 44 229 hm2in 2010 and 2020 separately．

2．2 Prediction of farml and protection area based on supply demand balance

2．2．1Prediction of farml and reduction．

（i）Construction．In 1997－2005，various types of construction such as urban and rural residential areas，traffic，water conservancy and industrial and mining construction of Mizhi County occupied 566 hm2farml and，the annual occupation of farml and is up to 62．8 hm2．Without considering demand of rapid economic development for farml and，the construction of Mizhi County will occupy 314 hm2farml and in 2006－2010， and it will occupy 972 hm2farml and in 2006－2020．Mizhi County always sticks to the principle of protecting and rationally utilizing every parcel of farml and and promoting conserved and intensive use of l and．Itgets rid of various difficulties and strictly implements the order of farml and area for new construction issued by superior government．Finally，it determined that the farml and of Mizhi County for construction will be 150 hm2in 2006－2010 and 340 hm2in 2006－2020．

（ii）Ecological restoration．Since the implementation of the policy of returning farml and to forest，Mizhi County actively responded to the callof the state for building beautiful north west area．In 1997－2005，the total farml and for ecological restoration reached 17435 hm2．With the ecological restoration focusing on quality and benefit since 2004，the ecological restoration enters steady implementation period and the farml and area for ecological restoration will become slower than in the past．

Table 2 Reduction of farm l and in M izhiCounty due to ecological restoration in 1997－2005

Farml and with slope greater than 25°ismajor area for returning to forest．In Mizhi County，the farml and with slope greater than 25°takes up 58．56%of the whole farml and area of Mizhi County．Thus，it is not proper to return all such farml and to forest．According to situations of recent years，with policy permits，Mizhi County arranged 500 hm2farml and for ecological restoration．In 2006－2010，the farml and for ecological restoration will be 2413 hm2；in 2006－2020，the farml and for ecological restoration will be 7 413 hm2．

（iii）Agricultural structural adjustment．In 1997－2005，agricultural structural adjustment of Mizhi Couty occupied 154 hm2farml and with annual reduction of 17．2 hm2．Since Mizhi County will focus on developing highly efficient ecological agriculture in future，the demand of agricultural structural adjustment for farml and will increase．It is predicted that agricultural structural adjustment will occupy 20 hm2farml and annually．We determined that the farml and reduction due to agricultural structural adjustment will be100 hm2in 2006－2010 and 300 hm2in 2006－2020．

（iv）Farml and reduction due to natural disasters．Mizhi County is located in typical hilly area of the Loess Plateau．The ground surface is torn to pieces，ridges and loess hills cross with each other．Gullies are widespread．The water loss and soil erosion is serious．In winter，it is cold and the icebound season is long．Natural disasters are of frequent occurrence．In 1997－2005，farml and reduced about 237 hm2due to natural disasters，with the annual reduction of farml and up to 33．9 hm2．Considering variability of farml and reduction due to natural disasters，the farml and reduction area is 25 hm2annually according to normal years．Therefore，we determined that the farml and reduction area due to natural disasters in 2006－2010 will be 125 hm2and 375 hm2in 2006－2020．

（v）Total reduction of farml and area in the planning period．According to the above prediction，the reduction of farml and area in Mizhi County is mainly resulted from construction，ecological restoration，agricultural structural adjustment， and natural disasters．The specific reduction of farml and area is listed in Table 3．

Table 3 Prediction of farm l and reduction in M izhi County in the planning period（unit：hm2）

2．2．2Prediction of farml and increase．According to l and consolidation planning of Mizhi County in 2010－2020，the farml and increase due to l and consolidation will be 2454 hm2in 2006－2020，farml and increase due to l and reclamation will be 67 hm2， and farml and increase due to l and development will be 111 hm2．In total，the farml and increase will be 2631 hm2with annual increase of175 hm2．

2．2．3Prediction of farml and protection area．According to analysis of farml and resource potential and factors for increase and reduction，we obtained the farml and increase and reduction situation of Mizhi County in 2010 and 2020（as listed in Table 4）．

From Table4，the farml and increase will be 877 hm2in 2006－2010 and the reduction areawill be 2 875 hm2， and the predicted farml and area is 37 615 hm2；the farml and increase will be 2 631 hm2in 2006－2020，the reduction area will be 8 428 hm2， and the predicted farml and area will be 33 826 hm2．

Table 4 Prediction of farm l and area of M izhi County based on supp ly capacity of farm l and（unit：hm2）

2．3 Prediction of farml and protection area based on trend extrapolation methodAccording to farml and changes in 1997－2005，we simulated farml and change model using linear trend model and quadratic curve model in the multinomial prediction model，as shown in Fig．1．We obtained the farml and change model of Mizhi County：

Linear trend mode：y＝－2451．2x＋5×106

Quadratic curve model：y＝－243．1x2＋970444x－109

Using the linear trend model，we calculated and obtained farml and area of Mizhi County will be 31 969 hm2and 19511 hm2in 2010 and 2020 respectively．

Using the quadratic curvemodel，we calculated and obtained the farml and area of MizhiCounty in 2010 and 2020 will be29 889 hm2and 17 045 hm2．This prediction result has great difference from agricultural development plan，so it is not included into the statistical data．

2．4 Prediction of the farml and protection area based on combination methodThe above three methods have various characteristics，but they also have limitation and applicable conditions．If using a single method，the error is great．Therefore，we built a combined prediction model to make more scientific，reasonable and accurate summary of prediction，so as to further increase accuracy，suitability and effectiveness of predicted value．Major points of the combined model lie in determination of single weight coefficient．In this study，we use the fixed weight combined model．The formula is as follows：

whereA1，A2andA3are prediction results of the above three methods， andK1，K2andK3are weight coefficient of the above three methods．

Comprehensively considering great-leap-forward development of Mizhi County and its l and supply capacity and agricultural development mode，we assigned weight to the above three prediction results．We predicted and calculated farml and area of MizhiCounty in 2010 and 2020．The farml and area of MizhiCounty will be40 967 hm2and 36 556 hm2in 2010 and 2020 respectively．

Table 5 Prediction results of farm l and area of M izhi County in 2010 and 2020（unit：hm2）

3 Conclusions

（i）Using the methods based on grain security，supply-demand balance and trend extrapolation，we made prediction of the farml and protection area of MizhiCounty．Using method based on grain security，the prediction result of farml and demand of MizhiCounty will be 47 405 hm2and 44 229 hm2respectively；using methods based on supply-demand method，the prediction result of farml and protection area of Mizhi County will be 37 615 hm2and 33 826 hm2in 2010 and 2020 respectively；using the linear trend model based on trend extrapolation，we obtained the farml and area of Mizhi County will be 31 969 hm2and 19511 hm2in 2010 and 2020 respectively；using quadratic curve model，we obtained the farml and area of Mizhi County will be 29 889 hm2and 17 045 hm2in 2010 and 2020 separately．

（iii）Summarizing the three prediction methods，we established a fixed weight combined model andmade a prediction of the farml and protection area of Mizhi County．Finally，we obtained the farml and area of Mizhi County will be 40 967 hm2and 36 556 hm2in 2010 and 2020 separately．

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