Coupling water flow and solute transport into a physically-based surface–subsurface hydrological model

Abstract. Watershed-scale hydrological and biogeochemical models are usually discretized at resolutions coarser than where significant heterogeneities in topography, abiotic factors (e.g., soil properties), and biotic (e.g., vegetation) factors exist. Here we report on a method to use fine-scale (220 m grid cells) hydrological model predictions to build reduced-order models of the statistical properties of nearsurface soil moisture at coarse resolution (2 5 times coarser, ∼ 7 km). We applied a watershed-scale hydrological model (PAWS-CLM4) that has been previously tested in several watersheds. Using these simulations, we developed simple, relatively accurate (R 2 ∼ 0.7-0.8), reduced-order models for the relationship between mean and higher-order moments of near-surface soil moisture during the nonfrozen periods over five years. When applied to transient predictions, soil moisture variance and skewness were relatively accurately predicted (R 2 ∼ 0.7-0.8), while the kurtosis was less accurately predicted (R 2 ∼ 0.5). We also tested 16 system attributes hypothesized to explain the negative relationship between soil moisture mean and variance toward the wetter end of the distribution and found that, in the model, 59 % of the variance of this relationship can be explained by the elevation gradient convolved with mean evapotranspiration. We did not find significant relationships between the time rate of change of soil moisture variance and covariances between mean moisture and evapotranspiration, drainage, or soil properties, as has been reported in other modeling studies. As seen in previous observational studies, the predicted soil moisture skewness was predominantly positive and negative in drier and wetter regions, respectively. In individual coarse-resolution grid cells, the transition between positive and negative skewness occurred at a mean soil moisture of ∼ 0.25-0.3. The type of numerical modeling experiments presented here can improve understanding of the causes of soil moisture heterogeneity across scales, and inform the types of observations required to more accurately represent what is often unresolved spatial heterogeneity in regional and global hydrological models.

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Characterizing coarse-resolution watershed soil moisture heterogeneity using fine-scale simulations

Riley

2014

“…The simulations were conducted with the CATHY (CATchment HYdrology) model (Camporese et al, 2010;Weill et al, 2011), a physics-based numerical code that solves the 3-D Richards and advectiondispersion equations and includes coupling with surface routing equations. The availability of extensive observational data sets from detailed multidisciplinary experiments (recent examples in addition to LEO include the TERENO network of experimental catchments (Zacharias et al, 2011) and the Chicken Creek artificial catchment (Hofer et al, 2012)) can contribute vitally to testing and improving the current generation of integrated (surface-subsurface) hydrological models (Sebben et al, 2013;Maxwell et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

Multiresponse modeling of variably saturated flow and isotope tracer transport for a hillslope experiment at the Landscape Evolution Observatory

Scudeler

Pangle

Pasetto

et al. 2016

Abstract. This paper explores the challenges of model parameterization and process representation when simulating multiple hydrologic responses from a highly controlled unsaturated flow and transport experiment with a physically based model. The experiment, conducted at the Landscape Evolution Observatory (LEO), involved alternate injections of water and deuterium-enriched water into an initially very dry hillslope. The multivariate observations included point measures of water content and tracer concentration in the soil, total storage within the hillslope, and integrated fluxes of water and tracer through the seepage face. The simulations were performed with a three-dimensional finite element model that solves the Richards and advection-dispersion equations. Integrated flow, integrated transport, distributed flow, and distributed transport responses were successively analyzed, with parameterization choices at each step supported by standard model performance metrics. In the first steps of our analysis, where seepage face flow, water storage, and average concentration at the seepage face were the target responses, an adequate match between measured and simulated variables was obtained using a simple parameterization consistent with that from a prior flow-only experiment at LEO. When passing to the distributed responses, it was necessary to introduce complexity to additional soil hydraulic parameters to obtain an adequate match for the pointscale flow response. This also improved the match against point measures of tracer concentration, although model performance here was considerably poorer. This suggests that still greater complexity is needed in the model parameterization, or that there may be gaps in process representation for simulating solute transport phenomena in very dry soils.

“…Short (from minutes to hours) and medium (i.e., months) term simulations addressing the effects of the tidal fluctuations and ship wakes have been carried out by a flow and transport uncoupled approach using subsurface modules FLOW3D and TRAN3D of the finite element CATchment HYdrology Flow-Transport (CATHY_FT) model (Camporese et al, 2010;Weill et al, 2011). The mixed hybrid finite element-finite volume COUPHYB simulator (Mazzia and Putti, 2006) for the solution of density-dependent flow and transport has been used to perform long-time (i.e., decades) analyses of seawater leakage into the aquifer system below the lagoon bottom.…”

Section: Modelingmentioning

confidence: 99%

Hydrogeological effects of dredging navigable canals through lagoon shallows. A case study in Venice

Teatini

Isotton²,

Nardean

et al. 2017

Abstract. For the first time a comprehensive investigation has been carried out to quantify the possible effects of dredging a navigable canal on the hydrogeological system underlying a coastal lagoon. The study is focused on the Venice Lagoon, Italy, where the port authority is planning to open a new 10 m deep and 3 km long canal to connect the city passenger terminal to the central lagoon inlet, thus avoiding the passage of large cruise ships through the historic center of Venice. A modeling study has been developed to evaluate the short (minutes), medium (months), and long (decades) term processes of water and pollutant exchange between the shallow aquifer system and the lagoon, possibly enhanced by the canal excavation, and ship wakes. An in-depth characterization of the lagoon subsurface along the channel has supported the numerical modeling. Piezometer and sea level records, geophysical acquisitions, laboratory analyses of groundwater and sediment samples (chemical analyses and ecotoxicity testing), and the outcome of 3-D hydrodynamic and computational fluid dynamic (CFD) models have been used to set up and calibrate the subsurface multi-model approach. The numerical outcomes allow us to quantify the groundwater volume and estimate the mass of anthropogenic contaminants (As, Cd, Cu, Cr, Hg, Pb, Se) likely leaked from the nearby industrial area over the past decades, and released into the lagoon from the canal bed by the action of depression waves generated by ships. Moreover, the model outcomes help to understand the effect of the hydrogeological layering on the propagation of the tidal fluctuation and salt concentration into the shallow brackish aquifers underlying the lagoon bottom.