Study On Spatial-Temporal Correlation Of Water Conservation Values In The Farmland Ecosystem Based On ESDA-GIS

To carry out the study on water conservation service value of the farmland ecosystem not only can recognize the importance of farmland ecosystem, enrich the theory system of ecosystem service function, but also can positively promote water and soil loss prevention, reasonably farmland resource development, farmland protection, and improvement of the farmland ecosystem production capacity. Based on above information, this study amended the existing farmland ecosystem model, established new water conservation value measurement model of the farmland ecosystem, and measured the water conservation value of farmland for each county(district, city) in Chengdu plain economic zone from 1998 to 2012. It also brought in Exploratory Spatial Data Analysis(ESDA) to explore space-time change rule of the water conservation value of farmland ecosystem for Chengdu plain economic zone. Results are concluded as follows:(1)From the aspect of temporal variation, there was a trend from sharp reducing to slow decreasing of farmland ecosystem water conservation value for Chengdu plain economic region from 1998-2012 year. The interannual variation rule is: 1998 year﹥2003 year﹥2012 year, 52.72×108 yuan was reduced in the 15 years in total, the value variation rate was 19.07%, and the annual average decreasing rate reached 1.50%. In which, the water conservation value of farmland ecosystem was reduced from 276.49×108 yuan/a to 234.95×108 yuan/a from 1998 to 2003 year, annual variation rate was 3.31%. Water conservation value of farmland ecosystem was reduced from 234.95×108 yuan/a to 223.77×108 yuan/a from 2003 to 2012, annual variation rate was 0.54%. From the aspect of time-sequence evolution process, the transformation between each type was obvious, certain variation also existed in aggregation structure of each time phase, the aggregation became more and more obvious. The water conservation value of farmland ecosystem for 20.63% counties(district, city) reduced rapidly(rapid changing type). The number of study region of rapid changing and slow changing decreased obviously, and the aggregation became more random. Certain salutatory migration phenomena exited in each distribution, and number of rapid changing and slow changing decreased gradually. The number of slow changing type added continually, and concentrated in north east and west of the study region.(2)From the aspect of spatial differentiation, the total water conservation value of farmland ecosystem took Chengdu as the center, and the south east as the starting point, which presented clockwise decreasing distribution characteristics. The total water conservation value rate of farmland ecosystem took Chengdu as center, and the west, north and east presented constant decreasing distribution characteristics. From the overall spatial pattern, the spatial aggregation trend of farmland water conservation value added constantly, which presented apparent spatial positive correlation and featured as follows: the water conservation value in the farmland ecosystem of high(low) tends to be adjacent to the water conservation value in the farmland ecosystem of high(low). From local spatial pattern, the change of “High-High” region in 3 time phase was not apparent, but the aggregation distributed obviously. No change occurred to the “Low-Low” region in 2003 and 2012, however, the quantity was increased to 11 in 2012 year as compared that of 9 in 1998 year. No change occurred to the “Low-High” region, which presented sporadically distribution.(3)Combining with time and space to analyze the variation rate of farmland ecosystem water conservation value, conduct overlay analysis to the annual decreasing rate and its autocorrelation change rate of the farmland ecosystem water conservation value, which can obtain 5 time-space changing models of farmland ecosystem water conservation value: “stable- weak correlation” model, “stable- positive correlation” model, “decrease-weak correlation” model, “decrease-negative correlation” model and “decrease-positive correlation” model. These 5 models made the abstract time and space become visible, and furthest reflect the time-space variation characteristics and spatial correlation of the farmland water conservation value.(4)From the overall, water conservation value of farmland for each county(district, city) presented different degree decreasing trend, the water conservation function of farmland decreased relatively. The overall spatial autocorrelation coefficient for each county(district, city) in Chengdu plain economic zone, Moran’s I=0.4775, presented strong spatial positive correlation, and its spatial distribution showed certain spatial aggregation and not completely random. Among it, obvious aggregation county(district, city) distributed in the east Chengdu plain economic zone, covering part of Deyang, Mianyang, Suining, Chengdu municipal districts, and county(district, city) in the area around Chengdu showing annular distribution, including Xindu district, Pi county, Wenjiang district and Shuangliu county.