USING AVAILABLE WATER-CONTENT WITH THE ONE-PARAMETER MODEL TO ESTIMATE SOIL-WATER RETENTION

The Gregson et al. one-parameter model (GHM) is based on the log-log f orm of the soil water retention curve, below the air-entry value of ps i, ln psi = a + b In theta, where a and b are the intercept and slope, respectively. A strong linear -relationship observed between a and b was expressed as a = p + qb. Given this relationship, the GHM was deri ved as In psi = p + b (In theta + q). Given p and q values for a soil or group of soils, only one value of the psi(theta) relationship needs to be known to calculate the only unknown parameter in the model - b, and hence, the entire psi(theta) function. Typically, theta at the -3 3 kPa matric potential (theta-33 kP.) is used as the known psi(theta) value. Here we provide a regression relationship between b and the ava ilable water content (AWC) to estimate b, since in many eases the AWC is available in the USDA soil survey reports, whereas theta-33 kpa is not. Using the b thus estimated in GHM gives only slightly larger erro rs in calculating the water content at different potentials than when using theta-33 kPa. Further we show that the intercept (a') and slope (b') of a log-linear model, In psi = a' + b' theta, are also linearly related and an alternate form of the one-parameter model (LLM) can be derived, In psi = p' + b' (theta + q'), which uses AWC directly. The e rrors with this model are comparable to GHM. Unfortunately, LLM requir es individual soil p' and q' values and, because of more scatter in th e intercept - slope relationship, pooled p' and q' values for a group of soils are not as effective in LLM as they are in GHM.