ECOHYDROLOGY Ecohydrol. 3, 155–165 (2010) Published online 23 October 2009 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/eco.79 On the importance of accurate depiction of infiltration processes on modelled soil moisture and vegetation water stress Salvatore Manfreda, 1 * Todd M. Scanlon 2 and Kelly K. Caylor 3 1 Dipartimento di Ingegneria e Fisica dell’Ambiente, Universit` a degli Studi della Basilicata, Potenza I-85100, Italy 2 Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA 3 Civil and Environmental Engineering, Princeton University, Princeton, NJ 08544, USA ABSTRACT The description of soil moisture dynamics is a challenging problem for the hydrological community, as it is governed by complex interactions between climate, soil and vegetation. Recent research has achieved significant advances in the description of temporal dynamics of soil water balance through the use of a stochastic differential equation proposed by Laio et al. (2001). The assumptions of the Laio et al. model simplify the mathematical form of the soil water loss functions and the infiltration process. In particular, runoff occurs only for saturation excess, the probability distribution function (PDF) of which is well- represented by a simple expression, but the model does not consider the limited infiltration capacity of soil. In the present work, we extend the soil moisture model to include limitations on soil infiltration capacity with the aim of understanding the impact of varying infiltration processes on the soil water balance and vegetation stress. A comparison between the two models (the original version and the modified one) is carried out via numerical simulations. The limited infiltration capacity influences the soil moisture PDF by reducing its mean and variance. Major changes in the PDFs are found for climates characterized by storms of short duration and high rainfall intensity, as well as in humid climates and in cases where soils have moderate permeability (e.g. loam and clay soils). In the case of limited infiltration capacity, modifications to the dynamics of soil moisture generally lead to higher amounts of vegetation water stress. An investigation of the role of soil texture on vegetation water stress demonstrates that loam soil provides the most favorable condition for plant-growth under arid and semi-arid conditions, while vegetation may benefit from the presence of more permeable soils (e.g. loamy sand) in humid climates. Copyright  2009 John Wiley & Sons, Ltd. KEY WORDS soil moisture; ecohydrology; vegetation water stress; infiltration Received 24 February 2009; Accepted 24 June 2009 INTRODUCTION Soil moisture is a key variable for ecohydrological model- ing (e.g. Eagleson, 1982; Neilson, 1995; Rodriguez-Iturbe et al., 2000). Its evolution in time and space is driven by different processes acting over a variety of scales (e.g. Albertson and Montaldo, 2003; Rodr´ ıguez-Iturbe et al., 2006; Manfreda et al., 2007). The severity and persis- tence of water stress in plants, the outcomes of ecological competition, and the sustainability of vegetation com- munities are examples of important ecological research questions in which soil moisture dynamic plays a domi- nant role (e.g. Scholes and Archer, 1997; Porporato et al., 2001; Rodriguez-Iturbe et al., 2001; Sofo et al., 2008). In particular, vegetation water stress is intimately related to relative soil moisture and the length of time that the soil moisture is below a given threshold. The crossing proper- ties of the soil moisture levels are controlled by the drying process and the infiltration inputs into the soil matrix. * Correspondence to: Salvatore Manfreda, Dipartimento di Ingegneria e Fisica dell’Ambiente, Universit` a degli Studi della Basilicata, Potenza I-85100, Italy. E-mail: salvatore.manfreda@unibas.it This last property varies from soil to soil according to the texture and the permeability. Recent research has achieved significant progress in the description of soil moisture dynamics through the development of a steady-state probability density function of soil moisture within the growing season (Rodriguez- Iturbe et al., 1999; Laio et al., 2001). This approach is based on the steady-state solution of the stochastic dif- ferential equation for the soil water balance in which the rainfall represents the stochastic forcing. Although this model necessarily contains assumptions to sim- plify the mathematical form of the stochastic differen- tial equation used to derive the soil moisture PDF, it represents the most innovative and general method to describe, within a physically-based approach, the soil moisture dynamics. More specifically, this theory is based on the assumption that the infiltration is an additive noise in the soil water balance and it is equal to the rain- fall pulse unless the rainfall produces soil saturation. In this case, runoff is produced for saturation excess that is linearly related to the rainfall depth. A different infil- tration scheme would introduce a non-linear relationship between the rainfall forcing and the infiltration, which Copyright  2009 John Wiley & Sons, Ltd.