EFFECTS OF SIZE AND SAMPLING GRID ON THE QUALITY OF APPARENT SOIL ELECTRICAL CONDUCTIVITY MAPS

ABSTRACT The apparent soil electrical conductivity (ECa) is an attribute commonly used for the characterization of the spatial variability of soil, but its determination by handheld sensors consumes considerable time and labor. The reduction in the number of sampling points allows minimize them but can result in increased uncertainty of interpolated maps. Thus, the goal of this study was to identify the best spacing and number of ECa measurements, to guarantee the quality of maps generated in three vineyards. The ECa values were obtained using a handheld sensor in different sampling grids. The data were submitted to descriptive statistical and geostatistical analyses. The relative deviation and Kappa coefficient of agreement were used to assess the similarity of generated maps. The reduction in the number of points and increase in the size of the sampling grid reduced the quality of maps and this reduction was greater when the spacing increased in the direction of the terrain slope. A minimum limit of 100 sampling points should be considered for the sampling planning to generate ECa spatial distribution maps, with a more conservative approach when regarding the increase in spacing in the direction of the terrain slope.摘要：表观土壤电导率（apparent soil electrical conductivity，ECa）是表征土壤空间变异的常用属性，但采用手持传感器测定该指标往往耗费大量时间与人力。减少采样点数虽可降低投入成本，但会导致插值地图的不确定性升高。为此，本研究旨在确定最优的ECa测量间距与采样数量，以保障三座葡萄园所生成空间分布地图的质量。研究通过手持传感器在不同采样网格下获取ECa数值，随后对数据开展描述性统计与地统计分析，并采用相对偏差与Kappa一致性系数评估所生成地图的相似性。结果表明，减少采样点数与增大采样网格尺寸均会降低地图质量，且当采样间距沿地形坡度方向增大时，该质量降幅更为显著。本研究建议，在开展ECa空间分布地图的采样规划时，应至少设置100个采样点；若采样间距沿地形坡度方向增大，则需采取更为保守的采样策略。