The plant water uptake is primarily determined by the switch Basic equation, which presents two approaches. In the “SPAC” (Soil Plant Atmosphere Continuum) approach (option: “Darcy based”), the plant and soil properties are explicitly considered and empirical functions for the plant resistance and for the soil rhizosphere resistance are used to calculate the water uptake rate. The other option “Pressure head response” is a simplified approach that is chosen by default if the time resolution is not within the day. In this latter approach simple response functions are used to estimate the water uptake from different soil layers. Water uptake in the “Pressure head response” approach is considered to be a fraction of the atmospheric demand of water, whereas in the “SPAC” approach the uptake is considered to be the result of different water potentials in the plant and the soil.
In the “SPAC” approach the default option is to consider the water uptake equal to transpiration and consequently there is no storage of water in the plant. Plant water storage during the day can optionally be simulated if the “SPAC” approach is used to calculate water uptake and another function is used to calculate transpiration. This third option is determined by the switch PlantWaterStorage. Some authors like Waring et al. (1979) indicated that, for forests, water in vegetation may contribute to a considerable amount of transpiration during short periods, and the variations in plant water within the day, i.e. plant water storage, could therefore be important to account for. If plant water storage is simulated, compensatory water uptake by roots due to water shortage in one soil layer, so called DemandRedistribution, cannot be accounted for.
In the following text these three different approaches (“dynamic SPAC approach”, “steady-state SPAC approach” and “Pressure head response”) are described in reversed order.
There are five switches that could be used depending on the context (=the options set by other switches).