| China is a country with extreme short of water resources,and the huge amount of agricultural water consumption and low efficiency of water using has greatly affected China's sustainable development.The irrigation is an improtant measure to improve agricultural production,clarifying the spatial variability of soil moisure content(SWC)in the field can accurately monitor changes of SWC,and is particulary important for improving precision irrigation.Taking the farmland in Guanzhong region as an example,this paper studied the spatial variability of SWC under different soil layers,different sampling intervals,different vegetation cover and different growth periods by combining classical statistics and geostatistics.At the same time,the temporal stability of SWC were explored by Spearman rank correlation coefficient and relative deviation method.In the end,the reasonable sampling method of field scale was initially proposed.The main research results showed that:(1)The spatial variation of SWC in field scale was weak and moderate.When SWC was within the range of 11.7%-20.1%,SWC was negatively correlated with spatial variability.Sampling interval significantly affected the calculation accuracy of the spatial variability of SWC.The coefficient of variation of SWC between the east-west direction spacing of 27 m and the north-south direction spacing of 18 m.With increasing sampling density,the contour change of SWC at the field scale was 21 points.When the sampling spacing was 18 m in the east-west direction,9 m in the north-south direction,SWC at field scale had a high spatial distribution correlation with SWC in the middle position being 3%-5%higher than the surrounding.(2)The vegetation types had small effects on the spatial variability of SWC.Coefficients of variation(CVs)of 11.71%-13.29%,9.16%-11.74%and 8.18%-11.75%in a corn field,peach orchard and persimmon orchard,respectively.The deeper the soil layer was,the smaller the spatial variability of SWC.The SWC of the field exhibited strong spatial dependence,and the spatial variability of SWC was mainly caused by structural factors(soil properties).(3)With the increase of depth,the temporal stability of the SWC at the field scale increased,and the spatial pattern of SWC in the soil layer of 40-60 cm was weaker than that in the soil layer of 0-20 cm.The mean value of R~2,ME and RMSE between the soil moisture content at the selected measuring points in each soil layer and the regional average soil moisture content is 0.91,0.08%and 1.46%,respectively.These points can be used as representative measurements,in order to estimate the average value of regional soil water content.Most of the SWC at different sampling times showed a significant positive correlation.For rain-fed crops,the soil moisture content in the rainy season in summer is relatively low,and the temporal stability of soil moisture content is mainly affected by external factors.(4)The minimum reasonable number of sampling points for SWC at the field scale is 4,and the points should be arranged in a quadrangle.If the SWC monitoring depth is?40 cm in the vertical direction,the sampling point soil depth should be selected from the surface layer(0-20 cm)soil;when the SWC monitoring depth>40cm,the sampling point should be arranged in the middle of the SWC monitoring depth.Field verification in the corn field,peach orchard and persimmon orchard showed that the relative error between the average SWC by the combination of sampling methods in the horizontal and vertical directions and the average SWC of the field area are mostly within 10%for the different SWC monitoring depth.It showed that the recommended sampling point method and the number of sampling points were basically feasible and the sampling accuracy was high. |
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