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 content. 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 derivation. 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 hydrostatic pressure gradient between the bulk soil and the root surface. The derivation of Cowan (1965) defined soil water potential and soil conductivity as exponential 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 disconnect 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 MPa). Consequently, Eq. [7] in Sinclair (2005) resulting from the derivation should be written as