Modeling and Measuring Root Water Extraction and Transpiration Reduction.

Understanding physical mechanisms of root water extraction is important for modeling crop transpiration and its reduction under dry or saline conditions. During the last few years, our research group and cooperators developed a model for describing root water extraction and transpiration rates as a function of soil hydraulic conditions, rooting characteristics and atmospheric demand. A single root model was developed based on the soil matric flux potential and its gradients around a water extracting root, allowing the prediction of the onset of limiting hydraulic conditions and the shape of the reduction function in the falling rate phase. The model was then upscaled to the root system scale, dealing with the macroscopic heterogeneity, i.e., differences in soil hydraulic status and rooting density between soil layers. The prediction of transpiration reduction as a function of soil water status was incorporated in the agrohydrological model SWAP. Experiments were and are being performed in pots, split-pot lysimeters and under field conditions, with common bean, soy bean, sorghum and rice. In these experiments, environmental parameters were monitored to test the model. The model appeared to predict a higher tolerance to drought than experimentally verified. This was thought to be due mainly to root system architecture heterogeneity. A root system efficiency factor was included, coping for this mesoscopic heterogeneity, i.e., the effects of uneven root distribution within layers.

  A new modeling approach to deal physically with this mesoscopic heterogeneity showed that the modeling of transpiration reduction is sensitive to mesoscopic heterogeneity, therefore the mismatch between predicted and observed values of transpiration may be partly due to this heterogeneity. Other factors like soil-root contact, root activity and a higher than assumed limiting root water potential are other factors under investigation.