JIA-2018-09

2101 WANG Di et al. Journal of Integrative Agriculture 2018, 17(9): 2096–2106 Accordingly, all sampling units were also divided into five strata based on the stratification of their FRG. The spatial autocorrelation level among crop acreages included in the sampling units of each stratum was also determined using the global Moran’s I index. As with the CPI, we computed the spatial autocorrelation level among crop acreages included in the sampling units of each stratum at each of seven sampling unit sizes. Using the sampling unit size of 1 000 m×1 000 m as an example, Fig. 7 shows the spatial distribution of the sampling units stratified by their FRG. 2.8. Stratification on the average ground slope of the sampling units The ASTER GDEM data were processed through tessellation, projection conversion and slope calculation, and the average ground slope within each sampling unit was then determined. The average values of ground slope were sorted from the smallest to largest, and then divided into five strata based on the rank: (1) 0.34–0.87%; (2) 0.87–1.01%; (3) 1.01–1.23%; (4) 1.23–1.82%; and (5) 1.82–11.64%. All sampling units were also divided into five strata based on the stratification of ground slope. As with the CPI, seven sampling unit sizes were used, and the spatial autocorrelation level among crop acreages included in the sampling units of each stratum at each sampling unit size was computed. Using the sampling unit size of 1 000 m×1 000 m as an example, Fig. 8 shows the 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 126°20´E 126°30´E 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 44°50´N 44°40´N 44°30´N 44°20´N 44°50´N 44°40´N 44°30´N 44°20´N 126°20´E 126°30´E N 0 10000 20000 m County boundary Irrigable land plots Paddy land plots Dryland plots Fig. 3 Spatial distribution of cultivated land in Dehui County, Jilin Province, China. 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 126°20´E 126°30´E 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 44°50´N 44°40´N 44°30´N 44°20´N 44°50´N 44°40´N 44°30´N 44°20´N 126°20´E 126°30´E N 0 10000 20000 m County boundary Maize Rice Fig. 4 Spatial distribution of maize and rice in Dehui County, Jilin Province, China. 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 126°20´E 126°30´E 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 44°50´N 44°40´N 44°30´N 44°20´N 44°50´N 44°40´N 44°30´N 44°20´N 126°20´E 126°30´E N 0 10000 20000 m County boundary High: 255 Low: 47 ASTER GDEM (30 m) Fig. 5 Land elevation in Dehui County, Jilin Province, China. 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 126°20´E 126°30´E 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 126°20´E 126°30´E 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 126°20´E 126°30´E 126°10´E 126°0´E 125°50´E 125°40´E 125°30´E 125°20´E 125°10´E 126°20´E 126°30´E 44°50´N 44°40´N 44°30´N 44°20´N 44°50´N 44°40´N 44°30´N 44°20´N 44°50´N 44°40´N 44°30´N 44°20´N 44°50´N 44°40´N 44°30´N 44°20´N N Maize Rice N 0 10000 20000 m 0 10000 20000 m County boundary 1st class maize 2nd class maize 3rd class maize 4th class maize 5th class maize County boundary 1st class maize 2nd class maize 3rd class maize 4th class maize 5th class maize A B Fig. 6 Spatial distribution of sampling units stratified by the crop planting intensity of maize and rice on a 1000 m×1000 m grid.