This paper discusses the results of numerical analysis of dispersion of passive solutes in twodimensional heterogeneous porous formations. Statistics of flow and transport variables, the accuracy and the role of approximations implicit in existing first-order theories, and the convergence of computational results are investigated. The results suggest that quite different rates of convergence with Monte Carlo runs hold for different spatial moments and that over 1000 realizations are required to stabilize second moments even for relatively mild heterogeneity (rr• < 1.6). This has implications for the extent of the spatial domain for single-realization numerical studies of the same type. A comparison of the variance of plumes with the results of linear theories (0.05 < rr•, < 1.6) shows an unexpectedly broad validity field for the theoretical solution obtained from a suitable linearization of flow and transport. Reformulation of the same problem linearizing in turn the flow or the transport Recent experimental evidence and theoretical results suggest that the transport of passive solutes in natural porous formations is dominated by the spatial variations in hydrau-!ic conductivity resulting in heterogeneous convection fields (see, for an exhaustive review, Dagan [1989]). A theory of flow and transport has been developing in recent years , 1990] which links the kinematics of the dispersion process to field measurable quantities, i.e., the spatial correlation structure and the variability of the log conductivity field Y(x) of porous formations viewed as a random space function. Fundamental experimental validations from accurate field analyses and critical elaborations of the data support the validity of linear theories at least for mildly heterogeneous aquifers [e.g.McLaughlin, 1991; Le Blanc et al., 1991, Garabedian et al., 1991; Rajaram and Gelhar, 1991]. Limits and validity of the theory of flow and transport in heterogeneous porous formations have recently been discussed IDagan, 1989]. It is accepted that the linear (denoted here as Dagan's) theory subsumes a number of previous results [Matheron and de Marsily, 1980; Dagan, 1984; Gelhar and Axness, 1983; Dagan, 1987] and captures the foremost features of the processes whenever the variance of log conductivity •r2r, a significant measure of heterogeneity, is small (cr• < 1) because a first-order perturbation is used as a consistent approximation involving such a parameter. Zhang, Y. K., and S. P. Neuman, A quasi-linear theory of non-Fickian and Fickian subsurface dispersion, 2, Application to anisotropic media and the Borden site, Water Resour. Res., 26(5), 903-913, 1990.

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[1] A sequential data assimilation procedure based on the ensemble Kalman filter (EnKF) is introduced and tested for a process-based numerical model of coupled surface and subsurface flow. The model is based on the three-dimensional Richards equation for variably saturated porous media and a diffusion wave approximation for overland and channel flow. A one-dimensional soil column experiment and a three-dimensional tilted v-catchment test case are presented. A preliminary analysis of the assimilation scheme is undertaken for the one-dimensional test case in order to validate the implementation by comparison with published results and to assess the influence of various factors on the filter's performance. The numerical results suggest robustness with respect to the ensemble size and provide useful information for the more complex tilted v-catchment test case. The assimilation frequency and the effects induced by data assimilation on the surface and/or subsurface system states are then evaluated for the v-catchment experiment using synthetic observations of pressure head and streamflow. The results suggest that streamflow prediction can be improved by assimilation of pressure head and streamflow, either individually or in tandem, whereas assimilation of streamflow data alone does not improve the subsurface system state. In terms of the global system state, i.e., surface and subsurface variables, frequent updates are especially beneficial when assimilating both pressure head and streamflow. Furthermore, it is shown that better evaluation of the subsurface volume resulting from assimilation of head data is crucial for improving subsequent surface response.

A distributed optical fibre sensing system is used to measure landslide-induced strains on an optical fibre buried in a large scale physical model of a slope. The fibre sensing cable is deployed at the predefined failure surface and interrogated by means of optical frequency domain reflectometry. The strain evolution is measured with centimetre spatial resolution until the occurrence of the slope failure. Standard legacy sensors measuring soil moisture and pore water pressure are installed at different depths and positions along the slope for comparison and validation. The evolution of the strain field is related to landslide dynamics with unprecedented resolution and insight. In fact, the results of the experiment clearly identify several phases within the evolution of the landslide and show that optical fibres can detect precursory signs of failure well before the collapse, paving the way for the development of more effective early warning systems.

[1] Peatlands respond to natural hydrologic cycles of precipitation and evapotranspiration with reversible deformations due to variations of water content in both the unsaturated and saturated zone. This phenomenon results in short-term vertical displacements of the soil surface that superimpose to the irreversible long-term subsidence naturally occurring in drained cropped peatlands because of bio-oxidation of the organic matter. These processes cause changes in the peat structure, in particular, soil density and void ratio. The consequential changes in the hydrological parameters need to be incorporated in water flow dynamical models. In this paper, we present a new constitutive relationship for the soil shrinkage characteristic (SSC) in peats by describing the variation of porosity with moisture content. This model, based on simple physical considerations, is valid for both anisotropic and isotropic three-dimensional peat deformations. The capability of the proposed SSC to accurately describe the deformation dynamics has been assessed by comparison against a set of laboratory experimental results recently published. The constitutive relationship has been implemented into a Richards' equation-based numerical code and applied for the simulation of the peat soil dynamics as observed in a peatland south of the Venice Lagoon, Italy, in an ad hoc field experiment where the relevant parameters are continuously measured. The modeling results match well a large set of field data encompassing a period of more than 50 days and demonstrate that the proposed approach allows for a reliable reproduction of the soil vertical displacement dynamics as well as the hydrological behavior in terms of, for example, water flow, moisture content, and suction.Citation: Camporese, M., S. Ferraris, M. Putti, P. Salandin, and P. Teatini (2006), Hydrological modeling in swelling/shrinking peat soils, Water Resour. Res., 42, W06420,