Influence of three-parameter conversion methods between van Genuchten and Brooks-Corey functions on soil hydraulic properties and water-balance predictions

The Brooks-Corey functions are commonly used in hydrologic models, with par ameters obtained by fitting the functions directly to measured soil water r etention data or by conversion methods from the van Genuchten functions whi ch are continuous across the domain of matric suctions. Problems in fitting the BC functions directly to the retention data motivated use of the conve rsion methods. However, differences in converted parameters could significa ntly influence model predictions. We compared the direct fitting method and the conversion methods of Lenhard et al., Morel-Seytoux et al., and van Ge nuchten using measured water retention data during drainage and determined the influence of these methods on hydrological predictions when the convert ed parameters were used in the root zone water quality model. The conversio n methods had significant influence on predictions of water retention, hydr aulic conductivity, runoff, and evapotranspiration, with the observed level of significance (p less than or equal to 0.006) much lower than the test l evel of significance (alpha = 0.05). The method of Morel-Seytoux et al. ina dequately described measured water retention data (p = 0.027), whereas the other two methods adequately described the data at relatively high suctions (p greater than or equal to 0.687), deviations occurred around the air-ent ry suction. The method of Lenhard et al. best reproduced the characteristic s of the Brooks-Corey functions (p greater than or equal to 0.31) and could be used to obtain the Brooks-Corey parameters simply and reproducibly.