I've run into a problem with one of the PTFs described in the WEAP documentation, as follows:


"Particle size Organic matter (Jabloun and Sahli 2006)": silt[%], clay[%], and sand[%] and organic matter[g/kg]

This model is defined by three specific equations to estimate water content at saturation, field capacity and wilting point.

qSAT = 0.7264(Si) + 0.2026(Sa) - 0.0083(Si^2) - 13.75491/Sa - 7.7387 Ln(Sa) + 2.2103 Ln(OM) - 0.0043(Cl)(Si) + 0.0051(Cl)(Sa) - 0.0047(Si)(Sa) + 53.4646
qFC = 0.2239(Cl) - 57.95441/Si - 11.69741/Sa + 6.90031/OM - 3.5324 Ln(Sa) + 24.0966 Ln(OM) + 0.0031(Cl)(Sa) - 0.1886(Cl)(OM) + 36.7918
qWP = -181.7238(Cl) - 183.5092(Si) - 182.4525(Sa) - 0.0048(Cl^2) + 0.0114(Si^2) - 0.0031(Sa^2) + 128.78961/Cl - 83.0451/Si + 6.52931/Sa + 9.18951/OM + 27.4919 Ln(OM) + 0.0043(Cl)(Si) - 0.2411

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Take a loam with 15% clay, 40% silt, 45% sand, and 2% organic matter.

The Soil Profiles Wizard gives qSAT = 42.53, qFC = 41.58, and qWP = 15.15.



qSAT = 0.7264 * 40 + 0.2026 * 45 - 0.0083 * (40^2) - 13.75491/45 - 7.7387 * ln(45) + 2.2103 * ln(2) - 0.0043 * 15 * 40 + 0.0051 * 15 * 45 - 0.0047 * 40 * 45 + 53.4646
qFC = 0.2239 * 15 - 57.95441/40 - 11.69741/45 + 6.90031/2 - 3.5324 * ln(45) + 24.0966 * ln(2) + 0.0031 * 15 * 45 - 0.1886 * 15 * (2) + 36.7918
qWP = -181.7238 * 15 - 183.5092 * 40 - 182.4525 * 45 - 0.0048 * (15^2) + 0.0114 * (40^2) - 0.0031 * (45^2) + 128.78961/15 - 83.0451/40 + 6.52931/45 + 9.18951/2 + 27.4919 * ln(2) + 0.0043 * 15 * 40 - 0.2411

Evaluating the equations given in the help file gives qSAT = 42.53, qFC = 41.58, and qWP = -18233.

The equation in the documentation matches the original paper. I can't find an erratum for this paper, but something is clearly wrong. Is there a corrected equation available?