Abstract

Quantification of soil hydraulic functions is essential to catchment studies. While soil water retention has been widely studied, a catchment-wide characterization of soil water retention parameters has not yet been commonly done. In this study, I report the spatial patterns of soil water retention parameters, obtained through in situ monitoring data, as a function of soil type, landform unit, and soil depth in the forested Shale Hills catchment in central Pennsylvania. Soil water matric potential and volumetric moisture content were collected in 2005-2010 at 61 sites throughout the catchment and at depths of 10, 20, 40, 80, and 100 cm. These data were fitted with the van Genucthen, Campbell, and Gardner soil water retention equations. Parameters from these curves were then analyzed in connection to soil-terrain attributes. Based on various statistical analyses, topographic wetness index, depth to bedrock, and curvature were found to have significant influence on soil water retention parameters across this catchment. The spatial relationship of the van Genuchten parameters across the catchment at 10, 20, 40, 80 and 100 cm depths were quantitatively analyzed and compared. An increase in spatial variance from near surface 20 cm to the deeper 80 cm was evident in saturated moisture content as the semi-variogram range increased from 14.3 meters to 31.2 meters. Moisture retention parameters were estimated across the catchment at all available depths with regression kriging using Bayesian statistics to optimize spatial model parameters. These maps would be used to inform hydrologic modeling and ecological studies for the Shale Hills catchment. A foundation for delineating functional units of the catchment with similar hydrological, pedogenic, and topographic properties (called Hydropedological Functional Units or HFUs) was established in this study through analyzing: 1) soil moisture profile storage maps spanning 2008 through 2010, 2) catchment-wide eletromagnatic induction surveys from wet and dry seasons, 3) maps of topographic variables, and 4) landscape-scale soil retention parameters and other basic soil properties. Five apparent HFUs were identified based on the spatially and temporally extensive datasets: 1) Regression-kriged maps of depth to bedrock and solum total moisture storage at saturation were used in a combined principal component and fuzzy c-means clustering to quantitatively delineate three hillslope HFUs; and 2) Two additional HFUs were delineated using slope value, elevation, and upslope contributing area. Total five HFUs were then compared with soil series map and landform units delineated using Park and vande Giesen’s method (2004). All three delineation methods were compared with observed soil moisture data. Results showed that the HFUs out-performed the soil series map and the landform units in depicting soil solum moisture storage in general linear models. This study shows that catchment-wide characterization of saturated moisture content can be integrated with topography and depth to bedrock to delineate Hydropedological Functional Units. Delineated HFUs generally predict soil moisture patterns more accurately than soil series and landform units. This research confirms that the delineation of sub-catchment units with soil depth, topographic variables, and soil properties can sufficiently separate a catchment into units with similar topography, soil properties, and hydrologic functions.

Citation

Baldwin, D (2011): Catchment-Scale Soil Water Retention Characteristics and Delineation of Hydropedological Functional Units in the Shale Hills Catchment. Master of Science, Soil Science, The Pennsylvania State University, p. 126.