Coupled dynamics of photosynthesis, transpiration, and soil water balance. Part II: Stochastic analysis and ecohydrological significance

The coupled dynamics of soil moisture, transpiration, and assimilation are studied at the daily time scale by temporally upscaling the hourly time scale results obtained in a companion paper. The effects of soil and vegetation c,haracteristics on soil moisture dynamics at the daily time scale and the parameters characterizing the dependence of transpiration and assimilation on soil water content are analyzed and discussed. The daily leaf carbon assimilation is then coupled to a stochastic soil moisture model to obtain a probabilistic description of the carbon assimilation during a growing season. The rainfall regime, in terms of both frequency and amount of precipitation, controls the mean assimilation during a growing season that reaches a maximum for an intermediate range of daily rainfall probabilities, indicating the existence of a rainfall regime that is most effective for plant productivity. The analysis of the duration and frequency of periods of no assimilation provides a measure of plant water stress as a function of the soil, vegetation, and climate characteristics. The results are in good agreement with the dynamic water stress defined in Porporato et al. on the basis of the crossing properties of the stochastic soil moisture dynamics.