Matteo Camporese

Surface-subsurface flow modeling with path-based runoff routing, boundary condition-based coupling, and assimilation of multisource observation data

Received 21 October 2008; revised 2 September 2009; accepted 16 September 2009; published 13 February 2010. [1] A distributed physically based model incorporating novel approaches for the representation of surface-subsurface processes and interactions is presented. A path-based description of surface flow across the drainage basin is used, with several options for identifying flow directions, for separating channel cells from hillslope cells, and for representing stream channel hydraulic geometry. Lakes and other topographic depressions are identified and specially treated as part of the preprocessing procedures applied to the digital elevation data for the catchment. Threshold-based boundary condition switching is used to partition potential (atmospheric) fluxes into actual fluxes across the land surface and changes in surface storage, thus resolving the exchange fluxes, or coupling, between the surface and subsurface modules. Nested time stepping allows smaller steps to be taken for typically faster and explicitly solved surface runoff routing, while a mesh coarsening option allows larger grid elements to be used for typically slower and more compute-intensive subsurface flow. Sequential data assimilation schemes allow the model predictions to be updated with spatiotemporal observation data of surface and subsurface variables. These approaches are discussed in detail, and the physical and numerical behavior of the model is illustrated over catchment scales ranging from 0.0027 to 356 km 2 , addressing different hydrological processes and highlighting the importance of describing coupled surfacesubsurface flow.

Peat land oxidation enhances subsidence in the Venice watershed

The southernmost part of the Venice Lagoon catchment was progressively reclaimed from marshland starting from the end of the 19th century and finishing in the late 1930s (Figure 1). As a major result, the area was turned into a fertile farmland. At present, the area is kept dry by a distributed drainage system that collects the water from a capillary network of ditches, and pumps it into the lagoon or the sea. By its very origin, this area lies below sea level and progressively sinks mainly because of bio-oxidation of the histosols (soils with high organic content) that represent a large fraction of the outcropping soil in the area. The bio-oxidation process occurs in close connection with the agricultural practices and is currently responsible for a subsidence rate of between 1.5 and 2 cm/yr.

Coupled and uncoupled hydrogeophysical inversions using ensemble Kalman filter assimilation of ERT-monitored tracer test data

Recent advances in geophysical methods have been increasingly exploited as inverse modeling tools in groundwater hydrology. In particular, several attempts to constrain the hydrogeophysical inverse problem to reduce inversion errors have been made using time-lapse geophysical measurements through both coupled and uncoupled (also known as sequential) inversion approaches. Despite the appeal and popularity of coupled inversion approaches, their superiority over uncoupled methods has not been proved conclusively; the goal of this work is to provide an objective comparison between the two approaches within a specific inversion modeling framework based on the ensemble Kalman filter (EnKF). Using EnKF and a model of Lagrangian transport, we compare the performance of a fully coupled and uncoupled inversion method for the reconstruction of heterogeneous saturated hydraulic conductivity fields through the assimilation of ERT-monitored tracer test data. The two inversion approaches are tested in a number of different scenarios, including isotropic and anisotropic synthetic aquifers, where we change the geostatistical parameters used to generate the prior ensemble of hydraulic conductivity fields. Our results show that the coupled approach outperforms the uncoupled when the prior statistics are close to the ones used to generate the true field. Otherwise, the coupled approach is heavily affected by “filter inbreeding” (an undesired effect of variance underestimation typical of EnKF), while the uncoupled approach is more robust, being able to correct biased prior information, thanks to its capability of capturing the solute travel times even in presence of inversion artifacts such as the violation of mass balance. Furthermore, the coupled approach is more computationally intensive than the uncoupled, due to the much larger number of forward runs required by the electrical model. Overall, we conclude that the relative merit of the coupled versus the uncoupled approach cannot be assumed a priori and should be assessed case by case.

Catchment-scale Richards equation-based modeling of evapotranspiration via boundary condition switching and root water uptake schemes

In arid and semiarid climate catchments, where annual evapotranspiration (ET) and rainfall are typically comparable, modeling ET is important for proper assessment of water availability and sustainable land use management. The aim of the present study is to assess different parsimonious schemes for representing ET in a process-based model of coupled surface and subsurface flow. A simplified method for computing ET based on a switching procedure for the boundary conditions of the Richards equation at the soil surface is compared to a sink term approach that includes root water uptake, root distribution, root water compensation, and water and oxygen stress. The study site for the analysis is a small pasture catchment in southeastern Australia. A comprehensive sensitivity analysis carried out on the parameters of the sink term shows that the maximum root depth is the dominant control on catchment-scale ET and streamflow. Comparison with the boundary condition switching method demonstrates that this simpler scheme (only one parameter) can successfully reproduce ET when the vegetation root depth is shallow (not exceeding approximately 50 cm). For deeper rooting systems, the switching scheme fails to match the ET fluxes and is affected by numerical artifacts, generating physically unrealistic soil moisture dynamics. It is further shown that when transpiration is the dominant contribution to ET, the inclusion of oxygen stress and root water compensation in the model can have a considerable effect on the estimation of both ET and streamflow; this is mostly due to the water fluxes associated with the riparian zone.

Water balance complexities in ephemeral catchments with different land uses: Insights from monitoring and distributed hydrologic modeling

Although ephemeral catchments are widespread in arid and semiarid climates, the relationship of their water balance with climate, geology, topography, and land cover is poorly known. Here we use 4 years (2011-2014) of rainfall, streamflow, and groundwater level measurements to estimate the water balance components in two adjacent ephemeral catchments in south-eastern Australia, with one catchment planted with young eucalypts and the other dedicated to grazing pasture. To corroborate the interpretation of the observations, the physically based hydrological model CATHY was calibrated and validated against the data in the two catchments. The estimated water balances showed that despite a significant decline in groundwater level and greater evapotranspiration in the eucalypt catchment (104-119% of rainfall) compared with the pasture catchment (95-104% of rainfall), streamflow consistently accounted for 1-4% of rainfall in both catchments for the entire study period. Streamflow in the two catchments was mostly driven by the rainfall regime, particularly rainfall frequency (i.e., the number of rain days per year), while the downslope orientation of the plantation furrows also promoted runoff. With minimum calibration, the model was able to adequately reproduce the periods of flow in both catchments in all years. Although streamflow and groundwater levels were better reproduced in the pasture than in the plantation, model-computed water balance terms confirmed the estimates from the observations in both catchments. Overall, the interplay of climate, topography, and geology seems to overshadow the effect of land use in the study catchments, indicating that the management of ephemeral catchments remains highly challenging.

Modeling peatland hydrology and related elastic deformation

Reversible surface displacements occur at time scales of hours in peatlands that are subject to drying and rewetting cycles. This phenomenon is related to the long term subsidence caused by biological oxidation of the organic matter under aerobic conditions that peat drainage for agricultural purposes induces in temperate regions. Modeling land subsidence of peatland surface requires, at a preliminary stage, the separation between the elastic deformation related to water table and soil moisture changes and the irreversible sinking of land due to peat oxidation. In this paper peat hydrology is modeled together with soil deformation in a one-dimensional column subject to soil moisture fluctuations due to precipitation, evapotranspiration, and drainage. The test case is representative of a field test bog located south of Venice (Italy), where hydrologic and displacement data have been continuously recorded since November 2001. A linear, one-dimensional pattern of elastic peat swelling/shrinkage relating the soil deformation to the gravimetric water content is implemented by coupling a variably saturated finite element flow model to a soil volume change equation. In this study the oxidation of peat is ignored, the time scale of water table fluctuations being much less than that of the long term subsidence process. The comparison of the numerical results with the large dataset available from the Venice monitoring site allows for the calibration of the hydraulic and mechanical parameters of the soil and assesses the validity of the proposed deformation model.

High density distributed strain sensing of landslide in large scale physical model

This paper describes the application of a commercial distributed optical fiber sensing system to a large scale physical model of landslide. An optical fiber cable, deployed inside the landslide body, is interrogated by means of optical frequency domain reflectometry with very high spatial density. A shallow landslide is triggered in the physical model by artificial rainfall and the evolution of the strain is measured up to the slope failure. Precursory signs of failure are detected well before the collapse, providing insights to the failure dynamic.

Electrical Resistivity Tomography Time-lapse Monitoring of Three-dimensional Synthetic Tracer Test Experiments

Time-lapse electrical resistivity tomography (ERT) represents a powerful tool for subsurface solute transport characterization since a full picture of the spatio-temporal evolution of the process can be obtained. However, the quantitative interpretation of tracer tests is difficult because of the uncertainty related to the geo-electrical inversion and the constitutive models linking geophysical and hydrological quantities. Here a new approach based on the Lagrangian formulation of transport and the ensemble Kalman filter (EnKF) data assimilation technique is applied to assess the spatial distribution of hydraulic conductivity K by incorporating time-lapse cross-hole ERT data. Under the assumption that the solute spreads as a passive tracer, for high Peclet numbers the spatial moments of the evolving plume are dominated by the spatial distribution of the hydraulic conductivity. The assimilation of the electrical conductivity 4D images allows updating of the hydrological state as well as the spatial distribution of K. Thus, delineation of the tracer plume and estimation of the local aquifer heterogeneity can be achieved at the same time by means of this interpretation of time-lapse electrical images from tracer tests.