Illinois Soil Moisture under Sod Experiment
Authors: Scott Robert W. , Krug Edward C. , Burch Stephen L. 
Published: December 2009

Long-term data collection of volumetric soil moisture under sod has been conducted in Illinois for more than 25 years. Numerous applied and modeling studies have been undertaken with these data, often relating results to regional conditions under a variety of surface covers. However, the actual level of representation of these data to nearby areas with different surface covers is unknown. In 2006/07, 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-ha 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-cm intervals down to 2 m, precisely matching the historic Illinois depth observations. A detailed surface terrain analysis was conducted to consider effects on soil moisture attributable to surface slope or ponding potential 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 (i.e., 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 in intrasite trends of volumetric soil moisture at all depths throughout the experiment. However, intersite spatial variability increased with depth, indicated by an average standard deviation of all temporal observations of 2.26% in the top 30 cm of soil and 5.19% in the 170–200-cm layer. Differences between the average field soil moisture at all primary randomly selected sites and the historic Bondville site were 2.39% and 6.51%, respectively. In addition, an apparent strong relationship was observed between soil moisture in deeper layers and surface terrain slope, and to a lesser extent with soil porosity and bulk density.

The question of representativeness of soil moisture under sod to adjacent surface covers was not answered with this work, but the large differences measured across this seemingly uniform field suggest that proper use of the historic Illinois dataset by future research related to adjacent areas may need greater attention. Most of Illinois is under an agricultural cover, not sod. Adequate data monitoring of surface terrain slope, soil profiles, and water table climatology under various major surface covers within a region may be necessary prior to the installation of new soil moisture monitoring networks and before useful assumptions concerning spatial representation can be made that attribute individual soil moisture datasets to adjacent areas. These results highlight the importance of a strict globally unified protocol for soil moisture network design and data collection in support of quality in situ global soil moisture assessment, a primary goal of the International Soil Moisture Working Group of the Global Energy and Water Cycle Experiment.