Simple model to estimate field-measured soil water limits

Engineering and modeling applications often require reasonable estimates of the upper and lower limits of plant extractable water. Laboratory measurem ents of these limits do not always coincide with field observations. Statis tical models to estimate soil water retention can be complicated and usuall y are not based on field measurements. The objective of this work was to de velop simple, generic equations to estimate the field-measured limits of th e soil water reservoir based on soil survey data such as texture and bulk d ensity. We used linear regression to estimate the gravimetric upper limit f rom sand and clay content; the volumetric upper limit can be estimated from the gravimetric upper limit and bulk density. We used non-linear regressio n to estimate the volumetric plant extractable water from sand content. The lower limit cart be estimated as the difference between the upper limit an d the plant extractable water. We adjusted the predictions for coarse fragm ents and organic C and we placed reasonable upper and lower boundaries to o ur estimates. The equation for the drained upper limit has a RMSE of 0.030 kg kg(-1) and uses two coefficients. The equation for the plant extractable water has a RMSE of 0.030 m(3) m(-3) and uses three coefficients. We teste d the simple model with an independent data set. The RMSE of our model was similar or better than that of a more complex statistical model.