Climate, soil, and vegetation controls on the temporal variability of vadose zone transport C. J. Harman, 1 P. S. C. Rao, 2 N. B. Basu, 3 G. S. McGrath, 4 P. Kumar, 1 and M. Sivapalan 1,5,6 Received 1 November 2010 ; revised 15 August 2011 ; accepted 20 August 2011 ; published 30 September 2011. [1] Temporal patterns of solute transport and transformation through the vadose zone are driven by the stochastic variability of water fluxes. This is determined by the hydrologic filtering of precipitation variability into infiltration, storage, drainage, and evapotranspiration. In this work we develop a framework for examining the role of the hydrologic filtering and, in particular, the effect of evapotranspiration in determining the travel time and delivery of sorbing, reacting solutes transported through the vadose zone by stochastic rainfall events. We describe a 1-D vertical model in which solute pulses are tracked as point loads transported to depth by a series of discrete infiltration events. Numerical solutions of this model compare well to the Richards equation–based HYDRUS model for some typical cases. We then utilize existing theory of the stochastic dynamics of soil water to derive analytical and semianalytical expressions for the probability density functions (pdf’s) of solute travel time and delivery. The moments of these pdf’s directly relate the mean and variance of expected travel times to the water balance and show how evapotranspiration tends to reduce (and make more uncertain) the mass of a degrading solute delivered to the base of the vadose zone. The framework suggests a classification of different modes hydrologic filtering depending on hydroclimatic and landscape controls. Results suggest that variability in travel times decreases with soil depth in wet climates but increases with soil depth in dry climates. In dry climates, rare large storms can be an important mechanism for leaching to groundwater. Citation: Harman, C. J., P. S. C. Rao, N. B. Basu, G. S. McGrath, P. Kumar, and M. Sivapalan (2011), Climate, soil, and vegetation controls on the temporal variability of vadose zone transport, Water Resour. Res., 47, W00J13, doi:10.1029/2010WR010194. 1. Introduction [2] The complex, transient, nonlinear transport and trans- formation processes operating in the vadose zone determine the timing and magnitude of the delivery of surface-applied solutes to groundwater [Raats, 1981; Rao et al., 1985a; Wang et al., 2009]. Through these processes, the vadose zone acts as a hydrologic filter that transforms the variability of the climatic signals, coupled with a biogeochemical filter to retard and attenuate solute inputs [ Basu et al., 2011]. Understanding this filtering is important for assessing the risks associated with groundwater contamination of surface- applied solutes, such as pesticides [Rao and Davidson, 1980; Rao et al., 1985b; Gustafson, 1989; van Der Werf, 1996; Arias-este et al., 2008]. An important control on this temporal filtering is the reduction of soil water content by vegetation through root water uptake and the replenishing of this water content by the stochastic inputs of infiltration [Struthers et al., 2006, 2007; McGrath et al., 2007]. A full understanding of the temporal patterns of solute delivery through the vadose zone therefore depends not only on the processes of solute transport and degradation but also on the way these processes are controlled by the stochastic vari- ability of the climate and the hydrologic filtering of this var- iability in the vadose zone. [3] In previous work, several authors have examined solute transport through the root zone by assuming for simplicity that the flow is steady, thus neglecting the epi- sodic (stochastic) transport processes [Rao et al., 1985b]. In this work we examine the role of evapotranspiration in modifying the temporal variability of the transport of sol- utes carried through the vadose zone by the propagation of wetting fronts, using a low-dimensional model of this trans- port that lends itself ultimately to a semianalytical solution. [4] Surface-applied solutes can migrate very rapidly to groundwater through preferential flow pathways, such as mac- ropores, or more slowly through the bulk of the unsaturated zone [Flury, 1996]. While preferential flow can be important 1 Department of Civil and Environmental Engineering, University of Illi- nois at Urbana-Champaign, Urbana, Illinois, USA. 2 School of Civil Engineering and Department of Agronomy, Purdue University, West Lafayette, Indiana, USA. 3 Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa, USA. 4 School of Earth and Environment, University of Western Australia, Crawley, Western Australia, Australia. 5 Department of Geography, University of Illinois at Urbana-Cham- paign, Urbana, Illinois, USA. 6 Water Resources Section, Faculty of Civil Engineering and Geoscien- ces, Delft University of Technology, Delft, Netherlands. Copyright 2011 by the American Geophysical Union. 0043-1397/11/2010WR010194 W00J13 1 of 21 WATER RESOURCES RESEARCH, VOL. 47, W00J13, doi:10.1029/2010WR010194, 2011