LETTERS TO THE EDITOR Comment on “Theoretical Analysis of Soil and Plant Traits Influencing Daily Plant Water Flux on Drying Soils” by T.R. Sinclair. Agron. J. 97:1148–1152 (2005). Dear Editor: In the paper published in Agronomy Journal, Sinclair (2005) presented a derivation to describe plant water loss in response to decreasing soil volumetric water con- tent. There are three features of that paper on which comment is warranted. First, there was an error in the units conversion of one of the variables used in the der- ivation. We discuss the consequences of that error in regards to the conclusions presented by Sinclair (2005). The other two issues relate to the use by Sinclair (2005) of the derivation by Cowan (1965) to describe the hy- drostatic pressure gradient between the bulk soil and the root surface. The derivation of Cowan (1965) defined soil water potential and soil conductivity as exponen- tial functions of volumetric soil water content. Sinclair (2005) used the power functions presented by Clapp and Hornberger (1978) so there is the possibility of a dis- connect in using these different functions. Finally, the derivation of Cowan required an unstated assumption that the exponents in the soil water potential and soil conductivity functions are numerically equal. The robustness of this assumption across soil texture classes is examined. In Eq. [2] of Sinclair (2005), the original unit of cm water used by Cowan (1965) in describing soil water potential was converted to units of MPa. Unfortunately, the conversion was made by accounting for a factor of 1 000 instead of 10 000 (i.e., 1 cm equals 10 24 MPa). Consequently, Eq. [7] in Sinclair (2005) resulting from the derivation should be written as RT 5 (c soil 2c leaf )/[aE w /(10 000dK) 2c leaf ], [7] where RT is relative transpiration rate, c soil is soil water potential (MPa), c leaf is leaf water potential (MPa), a is a geometric variable based on root geometry (cm 2 ), E w is the upper limit for plant water loss rate (cm d 21 ), d is depth of soil water extraction (cm), and K is soil hydraulic conductivity (cm d 21 ). The correction in the above equation adds another order of magnitude to the denominator of the term [aE w /(10000dK)]. Conse- quently, the correction further enhances the original conclusion that this term in the relevant range of volu- metric water for plant water extraction is substantially less than c leaf . Hence, correcting the conversion error in this term further indicates that the variables a, E w , d, and K have little impact on the water extraction response and Sinclair’s Eq. [10] appropriately defines RT as a function of soil volumetric water content for most circumstances. A basis for Sinclair’s derivation was Cowan’s analy- sis of soil water extraction using equations describing soil water potential and soil conductivity as exponential functions of volumetric water content. Yet, Sinclair (2005) subsequently used in his derivation the power functions presented by Clapp and Hornberger (1978). If in fact the exponential functions depict a substantially different response of the soils to that of the power functions, there is an internal inconsistency in the derivation that needs to be resolved. To compare the two sets of func- tions, the estimates of soil water potential and soil con- ductivity calculated from the Clapp and Hornberger equations were regressed using the exponential function used by Cowan. In fact, there was very close agreement between the exponential functions and the power func- tions within the range of extractable soil water. Figure 1 gives the output from the Clapp and Hornberger (1978) for a sandy clay soil and the fit of these results by expo- nential functions. The correlation coefficient for fitting the Clapp and Hornberger results with exponential func- tions was .0.995 across all 11 soil texture classes and 0.18 0.22 0.26 0.30 0.0 0.5 1.0 1.5 y = 752.3 exp(-35.61 x) R 2 = 0.9982 Clapp - Hornberger Potential (MPa) 0.18 0.22 0.26 0.30 0.0 0.1 0.2 0.3 Volumetric Water Content Conductivity (cm d -1 ) y = 3.695 exp(58.8 x) R 2 = 0.9998 Fig. 1. Solid symbols in each graph are output from the Clapp and Hornberger (1978) power functions describing soil water potential and soil conductivity as functions of volumetric soil water content for a sandy clay loam soil. Representations of these data using ex- ponential functions were obtained by regression and shown as the solid line in each panel. Published in Agron. J. 99:1188–1189 (2007). Letters to the Editor doi:10.2134/agronj2007.0122L ª American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Reproduced from Agronomy Journal. Published by American Society of Agronomy. All copyrights reserved. 1188 Published online June 26, 2007