USE OF THE ROOT CONTACT CONCEPT, AN EMPIRICAL LEAF CONDUCTANCE MODEL AND PRESSURE-VOLUME CURVES IN SIMULATING CROP WATER RELATIONS

A simulation model ''DanStress'' was developed for studying the integr ated effects of soil, crop and climatic conditions on water relations and water use of field grown cereal crops. The root zone was separated into 0.1 m deep layers of topsoil and subsoil. For each layer the wat er potential at the root surface was calculated by a single root model , and the uptake of water across the root was calculated by a root con tact model. Crop transpiration was calculated by Monteith's combinatio n equation for vapour flow. Crop conductance to water vapour transfer for use in Monteith's combination equation was scaled up from an empir ical stomatal conductance model used on sunlit and shaded crop surface s of different crop layers. In the model, transpirational water loss o riginates from root water uptake and changes in crop water storage. Cr op water capacitance, used for describing the water storage, was deriv ed from the slope of pressure-volume (PV) curves of the leaves. PV cur ves were also used for deriving crop water potential, osmotic potentia l, and turgor pressure. The model could simulate detailed diurnal soil -crop water relations during a 23-day-drying cycle with time steps of one hour. During the grain filling period in spring barley (Hordeum di stichum L.), grown in a sandy soil in the field, measured and predicte d values of leaf water and osmotic potential, RWC, and leaf stomatal c onductance were compared. Good agreement was obtained between measured and predicted values at different soil water deficits and climatic co nditions. In the field, measured and predicted volumetric soil water c ontents (theta) of topsoil and subsoil layers were also compared durin g a drying cycle. Predicted and measured theta-values as a function of soil water deficits were similar suggesting that the root contact mod el approach was valid. From the investigation we concluded: (I) a mode l, which takes the degree of contact between root surface and soil wat er into account, can be used in sandy soil for calculation of root wat er uptake, so that the root conductance during soil water depletion on ly varies by the degree of contact; (II) crop conductance, used for ca lculation of crop transpiration, can be scaled up from an empirical si ngle leaf stomatal conductance model controlled by the level of leaf w ater potential and micrometeorological conditions; (III) PV curves are usable for describing crop water status including crop water storage.