Spatial correlation of soil moisture in small catchments and its relationship to dominant spatial hydrological processes

The geostatistical properties of soil moisture patterns from five different sites in Australia (Tarrawarra and Point Nepean) and New Zealand (three sites from the Mahurangi River Basin—Carranʹs, Claydenʹs and Satellite Station) are analysed here. The soil moisture data were collected using time domain reflectometry and consistent methods for all sites, thereby allowing comparisons to be drawn between sites without the complication of methodological differences. The sites have contrasting climatic and soils characteristics. Soil moisture in the top 30 cm of the soil profile was measured using time domain reflectometry on 6–8 occasions at each site. The variance and correlation structure of the patterns was analysed. Typical correlation scales lie between 30 and 60 m. We found that there was a seasonal evolution in the spatial soil moisture variance that was related to changes in the spatial mean moisture content at all sites. At the Australian sites there was also a seasonal evolution in the correlation length related to changes in the spatial mean moisture, but not at the New Zealand sites. The seasonal evolution of the correlation length in the Australian catchments is likely to be associated with a seasonal change in the processes controlling the soil moisture pattern. The more humid climate at the New Zealand sites leads to more consistent spatial controls over the year. Similarities between the correlation structure of the moisture and topographic indices representing lateral flow and topographically modulated evaporative forcing were found at Tarrawarra, Carranʹs and Claydenʹs. At Point Nepean the correlation structure of the soil moisture pattern is controlled by a larger (than the topography) scale variation in soils, properties and at Satellite Station a smaller scale source of variability is apparent in the data (although there were also topographical effects apparent, associated with valley features). The results demonstrate that the processes controlling spatial patterns can change between places and over time with catchment moisture status; however, when similar general conditions reoccur in a catchment, similar spatial patterns result. Soil characteristics and climate do provide a general pointer to what we might expect but the results also show subtleties specific to place.