Aggregating available soil water holding capacity data for crop yield models

The total amount of water available to plants that is held against gravity in a soil is usually estimated as the amount present at -0.03MPa average water potential minus the amount present at -1.5 MPa water potential. This value, designated available water-holding capacity (AWHC), in this study is a very important soil characteristic that is strongly and positively correlated to the inherent productivity of soils. In various applications, including assessing soil moisture status over large areas, it is necessary to group soil types or series as to their productivity. Current methods to classify AWHC of soils consider only total capacity of soil profiles and thus may group together soils which differ" greatly in AWHC as a function of depth in the profile. This paper describes a general approach for evaluating quantitatively the multidimensional nature of AWHC in soils. Data for 902 soil profiles, representing 184 soil series, in Indiana were obtained from the Soil Characterization Laboratory at Purdue University. AWHC for each of ten 150-mm layers in each soil was estimated, based on soil texture and parent material. A multivariate clustering procedure was used to classify each soil profile into one of 4, 8, or 12 classes based upon ten-dimensional AWHC values. The optimum number of classes depends on the range of AWHC in the population of soil profiles analyzed and on the sensitivity of a crop to differences in distribution of water within the soil profile. This multivariate clustering approach better describes the moisture supplying capacity of soils than the simple univariate approach which uses only total AWHC of the soil profile. We conclude that this multivariate approach is a significant advancement in depicting the dynamic nature of soil moisture and represents a general quantitative approach for classifying soils for crop yield models.