Spatial patterns of soil moisture under forest and grass land cover in a suburban area, in Maryland, USA

Knowledge about the effects of changing environmental factors on stability of spatial structure of soil moisture is important in measurement design and spatial and land-atmosphere modeling, but was rarely considered. We examined the effects of soil temperature and moisture on spatial structure of daily soil moisture at 10-cm and 20-cm depth covering forest and grass sampling points (31 and 11, respectively) over 10 months with a maximum between-point distance of 130 m. For average seasonal conditions, the peak variance (sill) was 4.8–7.5%2, spatial correlation scale was 0–56 m, and temporal scale was 5–90 days depending upon season, vegetation type and depth. Changes in soil temperature and moisture moderately affected parameters of the spatial structure, the variance (heterogeneity) and correlation length (continuity), and the effects were higher at 10-cm depth compared to 20-cm depth. Increase in soil temperature by 1 °C increased the correlation length by about 0.6 m only at the upper depth. Heterogeneity of soil moisture was affected by interaction of variability of soil temperature and moisture so that the peak variance changed by as much as 5%2 and 0%2 under changing soil temperature and by 6%2 and 3%2 under changing soil moisture over almost a year at 10-cm and 20-cm depths, respectively. The spatial structure was relatively stable under changing soil temperature and moisture as the correlation length would not change considerably under small (e.g., inter-annual) in contrast to large (inter-seasonal) variations in soil temperature and intra-annual changes in variance were also not high.

► Soil temperature and moisture moderately affected spatial structure of soil moisture.
► Soil microclimate influence was higher at 10-cm compared to 20-cm depth.
► Inter-seasonal variations in spatial continuity were high at 10-cm depth.
► The spatial heterogeneity increased but continuity decreased with increasing depth.
► Temporal scale of spatial structure varied between a few days and 3 months.