Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved. Influence of Organic Matter on the Estimation of Saturated Hydraulic Conductivity Attila Nemes,* Walter J. Rawls, and Yakov A. Pachepsky ABSTRACT typically involved measurements of soil hydraulic prop- erties in repacked soil columns. The opposite effect has, Estimation of soil hydraulic properties by pedotransfer functions however been estimated by Nemes et al. (2005) and (PTFs) can be used in many applications. Some of existing PTFs estimate saturated hydraulic conductivity (K s ) of the soil, using organic Rawls et al. (2005) who used soil hydraulic PTFs as a matter (OM) content as one of the input variables. Several authors tool. Nemes et al. (2005) performed scenario studies, have shown an increase in K s with increasing OM content, a soil and simulated the effect of soil properties, such as OM property that presumably improves soil structure. We used three popu- content on certain soil water balance components. They lar PTFs to examine the relationship between OM content and K s . We simulated the increase of the OM content of a Chromic also used data originating from the U.S., Hungary, and the European Cambisol (Dystric Haplustept). Estimated K s was lower HYPRES database, to develop additional PTFs with the Group for higher OM contents with a PTF based on a Hungar- Method of Data Handling (GMDH). It appears that existing PTFs ian data set. Rawls et al. (2005) estimated effective po- negatively correlate K s with OM content for some soils. We found rosity (φ e ) using data from the USDA-NRCS National indications of negative relationship between OM content and K s with Soil Characterization database (Soil Survey Staff, 1997) the newly developed PTFs both for directly estimated K s , and for K s estimated via the effective porosity of the soil, using a generalized and used the relationship between K s and φ e suggested Kozeny–Carman approach. It is not straightforward to define the by Ahuja et al. (1984) and Rawls et al. (1998). They de- exact range of soils with the inverse relationship between OM and veloped a figure, which showed cases when their PTF K s . The range appeared to be data set dependent, but it was extensive predicted lower K s for higher OM content for certain within the valid input range of each PTF. soil textures. Soil hydraulic PTFs are, in most cases, not specifically developed to address one particular problem, but are H ydraulic conductivity is one of the essential in- developed from a larger data collection to potentially puts to most simulation models used in soil and provide information to many studies. The underlying data- land research. When such data is needed for large areas bases usually report on soil hydraulic properties deter- of land, estimations using PTFs offer a competitive alter- mined on undisturbed soil samples. A PTF user will native to the cumbersome and costly direct measure- obtain predictions that reflect the inter-correlations of ments. Data on soil texture (sand, silt, and clay content) data in the underlying database. Subsequent application and bulk density (D b ) are the two most commonly used of PTF estimates in simulation models without knowing inputs to such PTFs. Some authors however include the the nature of such correlations may lead to inexplicable OM content in the list of inputs, since OM is known to results and possibly to incorrect or inefficient decisions. affect the hydraulic properties of the soil. It is often This study aims to examine the effect of changes in assumed that greater OM content in the soil will result OM content on the estimation of K s . Existing PTFs are in higher saturated hydraulic conductivity (K s ). The ra- first examined and additional PTFs are developed from tionale behind such assumption is that better soil aggre- three different data sets. Two approaches are applied gation is linked to greater OM contents (e.g., Beare et to obtain estimates of K s . We estimate K s directly, and al., 1994), OM content and D b tend to be negatively also use the modified Kozeny–Carman approach, as de- correlated (e.g., Adams, 1973; Rawls et al., 2005) and scribed by Ahuja et al. (1984), to characterize soils for therefore OM content and porosity are thought to be which we found the inverse relationship between OM positively correlated. Greater porosity is supposed to and K s . lead to greater hydraulic conductivity. Several authors have shown in their experiments that MATERIALS AND METHODS such is the case for their soils (e.g., Auerswald, 1995; Published Pedotransfer Functions Mbagwu and Auerswald, 1999; Lado et al., 2004). These studies were specifically designed to examine the rela- We have searched through the international literature to tionships between a number of soil hydraulic properties identify PTFs that predict K s from a set of soil physical data and soil aggregation on limited number of soils, and including OM content as one of the predictors. Three sources, namely Vereecken et al. (1990), Wo ¨ sten et al. (1999), and A. Nemes and W.J. Rawls, USDA-ARS, Hydrology and Remote Wo ¨ sten et al. (2001) that meet the above criteria were iden- Sensing Lab., 10300 Baltimore Ave., Bldg. 007, BARC-West, Belts- tified. ville, MD 20705; Y.A. Pachepsky, USDA-ARS, Environmental Micro- Vereecken et al. (1990) has developed PTFs from a data bial Safety Lab., Powder Mill Road, Bldg. 173, BARC-East, Beltsville, set from Belgium. The following formula has been derived to MD 20705. Received 13 Feb. 2004. *Corresponding author (anemes@ estimate K s : hydrolab.arsusda.gov). Abbreviations: CPM, complexity penalty multiplier; D b , bulk density; Published in Soil Sci. Soc. Am. J. 69:1330–1337 (2005). Soil Physics GMDH, Group Method of Data Handling; K s , saturated hydraulic conductivity; OM, organic matter; PTF, pedotransfer function; φ, total doi:10.2136/sssaj2004.0055  Soil Science Society of America porosity; φ e , effective porosity;  33 , water content at -33 kPa matric po- tential. 677 S. Segoe Rd., Madison, WI 53711 USA 1330 Published online June 28, 2005