Investigation of Soil Moisture Variability Under Sod in East-Central Illinois

Long-term data collection of volumetric soil moisture under sod has been conducted in Illinois for more than 25 years. These data have been used in numerous applied and modeling studies in which the results are often related to regional conditions under a variety of surface covers. The actual level of representation of these data to nearby areas with different surface covers, however, is unknown. In 2006–2007, the Soil Moisture Under Sod Experiment was conducted at Bondville, Illinois to increase understanding of soil moisture variability across a very small area of seemingly uniform surface and near-surface conditions. Ten locations were chosen at random within a 5.9-hectare sodded field for twice-weekly neutron probe soil moisture observations over a period of more than 13 months. Measurements were taken at the surface and at 20-centimeter intervals down to 2 meters, precisely matching the historic Illinois depth observations. A detailed surface terrain analysis was conducted to consider impacts of surface slope or ponding potential on soil moisture attributes at each monitoring location across the very low-relief surface. The near-surface water table level at the field location was monitored. At the end of observations, soil property heterogeneity (e.g., soil porosity, bulk density, and soil color) was determined by digging trenches and extracting soil cores immediately adjacent to each monitoring site at all observation levels within the predominantly loess soil. Results indicate a strong temporal consistency at each site in trends of volumetric soil moisture at all depths throughout the experiment. However, inter-site spatial variability increased with depth, indicated by an average standard deviation of all temporal observations of 2.26% in the top 30 centimeters of soil and 5.19% in the 170- to 200-centimeter layer. Differences between the average field soil moisture at all primary randomly selected sites and the historic Bondville site was 2.39% and 6.51%, respectively. Variations in soil moisture in the lowest layers appear to be related to an intrusion of the water table. In addition, an apparent relationship was observed between soil moisture in deeper layers and surface terrain slope, and to a lesser extent with soil porosity and bulk density. Further, the near-surface soil moisture observations under sod in loess soils over one small region in Illinois are useful as ground truth for near-surface data observed with remote-sensing observations. The question of representativeness of soil moisture under sod to moisture under crop surface covers was addressed in a cursory manner. Differences similar in magnitude to those found under sod were observed. Soil moisture variability measured across this seemingly uniform field suggests that proper use of the historic Illinois dataset by future researchers related to adjacent areas may need greater attention. Most of Illinois is under an agricultural cover, not sod, as are the surface covers at all of Illinois’ soil moisture monitoring sites. Adequate data monitoring of terrain slope, soil profiles, and water table climatology under various major surface covers within a region is recommended prior to installing new soil moisture monitoring sites and before making useful assumptions concerning spatial representation that attribute individual soil moisture datasets to adjacent areas.