Influences of soil, terrain, and crop growth on soil moisture variation from transect to farm scales

Soil and topography are widely recognized as important controls of soil moisture variation. Soil moisture distribution in a landscape has often been conceptualized as being controlled predominantly by soil properties during dry periods, and by topography during wet periods. However, this conceptualization does not explicitly consider plant growth, spatial scale, and soil depth. We investigated the influences of soil, terrain, and crop on soil moisture variation in an agricultural landscape at four spatial scales (entire farm of 19.5 ha, landform unit of 0.3–5.1 ha, plot size of 10 × 10 m, and slope transect of 40 × 2 m), in three depths (0.1, 0.4, and 0.8 m), and during two seasons (crop growing vs. non-growing seasons). The results showed that the controls on soil moisture variability varied by season, soil depth, and the degree of soil-topography variability at a given spatial scale. During the growing season, crop and soil exerted significant influences on soil moisture, especially at the 0.1- and 0.4-m depth; whereas during the non-growing season, terrain attributes showed more significant impacts on soil moisture variation, particularly at the 0.8-m depth where soils were generally wetter. In areas of steeper slope (> 8%), topography dominated over soil properties in controlling soil moisture variation, while in relatively flat areas (< 8% slope), soil properties dominated regardless of season and soil wetness. Generally, topographic influence on soil moisture increased with topographic variation (slope percent in this study). In hillslope with a greater variation in soil properties, soil's influence on moisture variation was apparent, but not in transects with relative homogenous soil properties. Overall, this study demonstrated the intertwined spatial complexity and temporal dynamics in soil moisture controls by terrain, soil, and crop across a typical agricultural landscape, the degree of which is a function of spatial scale, soil depth, and season.

Research Highlights
► Topography dominated over soil in controlling soil moisture variation in the steep sloping areas.
► Soil dominated over topography in controlling soil moisture variation in relatively flat areas.
► The influence of terrain on soil moisture variation increased in areas with less soil variability.
► Strong crop and soil influences were observed during the growing season at 0.1- and 0.4-m depths.