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Maxwell R.M.,Integrated Groundwater Modeling Center | Condon L.E.,Integrated Groundwater Modeling Center | Kollet S.J.,Jülich Research Center
Geoscientific Model Development | Year: 2015

Interactions between surface and groundwater systems are well-established theoretically and observationally. While numerical models that solve both surface and subsurface flow equations in a single framework (matrix) are increasingly being applied, computational limitations have restricted their use to local and regional studies. Regional or watershed-scale simulations have been effective tools for understanding hydrologic processes; however, there are still many questions, such as the adaptation of water resources to anthropogenic stressors and climate variability, that can only be answered across large spatial extents at high resolution. In response to this grand challenge in hydrology, we present the results of a parallel, integrated hydrologic model simulating surface and subsurface flow at high spatial resolution (1 km) over much of continental North America (∼6.3 M km2). These simulations provide integrated predictions of hydrologic states and fluxes, namely, water table depth and streamflow, at very large scale and high resolution. The physics-based modeling approach used here requires limited parameterizations and relies only on more fundamental inputs such as topography, hydrogeologic properties and climate forcing. Results are compared to observations and provide mechanistic insight into hydrologic process interaction. This study demonstrates both the feasibility of continental-scale integrated models and their utility for improving our understanding of large-scale hydrologic systems; the combination of high resolution and large spatial extent facilitates analysis of scaling relationships using model outputs. © Author(s) 2015.

Siirila-Woodburn E.R.,Integrated Groundwater Modeling Center | Siirila-Woodburn E.R.,Polytechnic University of Catalonia | Maxwell R.M.,Integrated Groundwater Modeling Center
Advances in Water Resources | Year: 2015

Aquifer heterogeneity is known to affect solute characteristics such as spatial spreading, mixing, and residence time, and is often modeled geostatistically to address aquifer uncertainties. While parameter uncertainty is often considered, the model uncertainty of the heterogeneity structure is frequently ignored. In this high-resolution heterogeneity model comparison, we perform a stochastic analysis utilizing spatial moment and breakthrough curve (BTC) metrics on Gaussian (G), truncated Gaussian (TG), and non-Gaussian, or "facies" (F) heterogeneous domains. Three-dimensional plume behavior is rigorously assessed with meter (horizontal) and cm (vertical) scale discretization over a ten-kilometer aquifer. Model differences are quantified as a function of statistical anisotropy, ε, by varying the x-direction integral scale of hydraulic conductivity, K, from 15 to 960 (m). We demonstrate that the model is important only for certain metrics within a range of ε. For example, spreading is insensitive to the model selection at low ε, but not at high ε. In contrast, center of mass is sensitive to the model selection at low ε, and not at high ε. A conceptual model to explain these trends is proposed and validated with BTC metrics. Simulations show that G model effective K, and 1st and 2nd spatial moments are much greater than that of TG and F models. A comparison of G and TG models (which only differ in K-distribution tails) reveal drastically different behavior, exemplifying how accurate characterization of the K-distribution may be important in modeling efforts, especially in aquifers where extreme K values are often not measured, or inadvertently overlooked. © 2014 Elsevier Ltd.

Condon L.E.,Colorado School of Mines | Condon L.E.,Integrated Groundwater Modeling Center | Maxwell R.M.,Colorado School of Mines | Maxwell R.M.,Integrated Groundwater Modeling Center
Environmental Research Letters | Year: 2014

Regional scale water management analysis increasingly relies on integrated modeling tools. Much recent work has focused on groundwater-surface water interactions and feedbacks. However, to our knowledge, no study has explicitly considered impacts of management operations on the temporal dynamics of the natural system. Here, we simulate twenty years of hourly moisture dependent, groundwater-fed irrigation using a three-dimensional, fully integrated, hydrologic model (ParFlow-CLM). Results highlight interconnections between irrigation demand, groundwater oscillation frequency and latent heat flux variability not previously demonstrated. Additionally, the three-dimensional model used allows for novel consideration of spatial patterns in temporal dynamics. Latent heat flux and water table depth both display spatial organization in temporal scaling, an important finding given the spatial homogeneity and weak scaling observed in atmospheric forcings. Pumping and irrigation amplify high frequency (sub-annual) variability while attenuating low frequency (inter-annual) variability. Irrigation also intensifies scaling within irrigated areas, essentially increasing temporal memory in both the surface and the subsurface. These findings demonstrate management impacts that extend beyond traditional water balance considerations to the fundamental behavior of the system itself. This is an important step to better understanding groundwater's role as a buffer for natural variability and the impact that water management has on this capacity. © 2014 IOP Publishing Ltd.

Williams J.L.,Colorado School of Mines | Maxwell R.M.,Colorado School of Mines | Maxwell R.M.,Integrated Groundwater Modeling Center
Journal of Hydrometeorology | Year: 2011

Feedbacks between the land surface and the atmosphere, manifested as mass and energy fluxes, are strongly correlated with soil moisture, making soil moisture an important factor in land-atmosphere interactions. It is shown that a reduction of the uncertainty in subsurface properties such as hydraulic conductivity (K) propagates into the atmosphere, resulting in a reduction in uncertainty in land-atmosphere feedbacks that yields more accurate atmospheric predictions. Using the fully coupled groundwater-to-atmosphere model ParFlow-WRF, which couples the hydrologic model ParFlow with the Weather Research and Forecasting (WRF) atmospheric model, responses in land-atmosphere feedbacks and wind patterns due to subsurface heterogeneity are simulated. Ensembles are generated by varying the spatial location of subsurface properties while maintaining the global statistics and correlation structure. This approach is common to the hydrologic sciences but uncommon in atmospheric simulations where ensemble forecasts are commonly generated with perturbed initial conditions or multiple model parameterizations. It is clearly shown that different realizations of K produce variation in soil moisture, latent heat flux, and wind for both point and domain-averaged quantities. Using a single random field to represent a control case, varying amounts of K data are sampled and subsurface data are incorporated into conditional Monte Carlo ensembles to show that the difference between the ensemble mean prediction and the control saturation, latent heat flux, and wind speed are reduced significantly via conditioning of K. By reducing uncertainty associated with land-atmosphere feedback mechanisms, uncertainty is also reduced in both spatially distributed and domain-averaged wind speed magnitudes, thus improving the ability to make more accurate forecasts, which is important for many applications such as wind energy. © 2011 American Meteorological Society.

Condon L.E.,Colorado School of Mines | Condon L.E.,Integrated Groundwater Modeling Center | Maxwell R.M.,Colorado School of Mines | Maxwell R.M.,Integrated Groundwater Modeling Center
Water Resources Research | Year: 2014

Groundwater-fed irrigation has been shown to deplete groundwater storage, decrease surface water runoff, and increase evapotranspiration. Here we simulate soil moisture-dependent groundwater-fed irrigation with an integrated hydrologic model. This allows for direct consideration of feedbacks between irrigation demand and groundwater depth. Special attention is paid to system dynamics in order to characterized spatial variability in irrigation demand and response to increased irrigation stress. A total of 80 years of simulation are completed for the Little Washita Basin in Southwestern Oklahoma, USA spanning a range of agricultural development scenarios and management practices. Results show regionally aggregated irrigation impacts consistent with other studies. However, here a spectral analysis reveals that groundwater-fed irrigation also amplifies the annual streamflow cycle while dampening longer-term cyclical behavior with increased irrigation during climatological dry periods. Feedbacks between the managed and natural system are clearly observed with respect to both irrigation demand and utilization when water table depths are within a critical range. Although the model domain is heterogeneous with respect to both surface and subsurface parameters, relationships between irrigation demand, water table depth, and irrigation utilization are consistent across space and between scenarios. Still, significant local heterogeneities are observed both with respect to transient behavior and response to stress. Spatial analysis of transient behavior shows that farms with groundwater depths within a critical depth range are most sensitive to management changes. Differences in behavior highlight the importance of groundwater's role in system dynamics in addition to water availability. © 2014. American Geophysical Union. All Rights Reserved.

Bearup L.A.,Integrated Groundwater Modeling Center | Maxwell R.M.,Integrated Groundwater Modeling Center | Mccray J.E.,Colorado School of Mines
Ecohydrology | Year: 2016

Current understanding of streamflow composition in mountain watersheds is often limited by inherent uncertainties and collection limitations in field data and assumptions associated with modelling techniques. Additional complexity arises in catchments experiencing land-cover change. Here, a hillslope model with fully integrated processes from the subsurface through the canopy is combined with Lagrangian particle tracking through the surface and subsurface domains to understand changes in flow paths and source waters with insect-induced tree death. This approach explicitly simulates end-member mixing by tracking parcels of water tagged as rain, snow, and pre-simulation ('old') groundwater and provides a method of separating outflows from these sources. Model simulations identify increases in subsurface water availability resulting from transpiration loss and altered canopy processes that increase throughfall and land-surface energy. Combined with changes in snowmelt timing, the shallower depth to saturation associated with tree death results in increased old groundwater contributions to streamflow. This shift in the source of outflow is consistent with prior field analysis of changing streamflow contributions with tree mortality from widespread insect infestation in the Rocky Mountains of North America. Model results also highlight mixing of old water and new precipitation within the groundwater end-member. Mixed hillslope outflows indicate that combinations of topography and precipitation can drive a range of signatures in groundwater inputs over meaningful time periods. Ultimately, this work and analysis of field observations provide insight into hillslope hydrologic processes and can serve as a platform for more complex simulations of land-cover perturbations to streamflow source partitioning. © 2016 John Wiley & Sons, Ltd.

Siirila E.R.,Colorado School of Mines | Maxwell R.M.,Colorado School of Mines | Maxwell R.M.,Integrated Groundwater Modeling Center
Water Resources Research | Year: 2012

The interplay between regions of high and low hydraulic conductivity, degree of aquifer stratification, and rate-dependent geochemical reactions in heterogeneous flow fields is investigated, focusing on impacts of kinetic sorption and local dispersion on plume retardation and channeling. Human health risk is used as an endpoint for comparison via a nested Monte Carlo scheme, explicitly considering joint uncertainty and variability. Kinetic sorption is simulated with finely resolved, large-scale domains to identify hydrogeologic conditions where reactions are either rate limited (nonreactive), in equilibrium (linear equilibrium assumption is appropriate), or are sensitive to time-dependent kinetic reactions. By utilizing stochastic ensembles, effective equilibrium conditions are examined, in addition to parameter interplay. In particular, the effects of preferential flow pathways and solute mixing at the field-scale (marcrodispersion) and subgrid (local dispersion, LD) are examined for varying degrees of stratification and regional groundwater velocities (v). Results show effective reaction rates of kinetic ensembles with the inclusion of LD yield disequilibrium transport, even for averaged (or global) Damkholer numbers associated with equilibrium transport. Solute behavior includes an additive tailing effect, a retarded peak time, and results in an increased cancer risk. The inclusion of LD for nonreactive solutes in highly anisotropic media results in either induced solute retardation or acceleration, a new finding given that LD has previously been shown to affect only the concentration variance. The distribution, magnitude, and associated uncertainty of cancer risk are controlled by the up scaling of these small-scale processes, but are strongly dependent on v and the source term.

Maxwell R.M.,Integrated Groundwater Modeling Center
Advances in Water Resources | Year: 2013

A terrain-following grid formulation (TFG) is presented for simulation of coupled variably-saturated subsurface and surface water flow. The TFG is introduced into the integrated hydrologic model, ParFlow, which uses an implicit, Newton Krylov solution technique. The analytical Jacobian is also formulated and presented and both the diagonal and non-symmetric terms are used to precondition the Krylov linear system. The new formulation is verified against an orthogonal stencil and is shown to provide increased accuracy at lower lateral spatial discretization for hillslope simulations. Using TFG, efficient scaling to a large number of processors (16,384) and a large domain size (8.1 Billion unknowns) is shown. This demonstrates the applicability of this formulation to high-resolution, large-spatial extent hydrology applications where topographic effects are important. Furthermore, cases where the analytical Jacobian is used for the Newton iteration and as a non-symmetric preconditioner for the linear system are shown to have faster computation times and better scaling. This demonstrates the importance of solver efficiency in parallel scaling through the use of an appropriate preconditioner. © 2012 Elsevier Ltd.

Siirila E.R.,Colorado School of Mines | Maxwell R.M.,Colorado School of Mines | Maxwell R.M.,Integrated Groundwater Modeling Center
Science of the Total Environment | Year: 2012

We present a new Time Dependent Risk Assessment (TDRA) that stochastically considers how joint uncertainty and inter-individual variability (JUV) associated with human health risk change as a function of time. In contrast to traditional, time independent assessments of risk, this new formulation relays information on when the risk occurs, how long the duration of risk is, and how risk changes with time. Because the true exposure duration (. ED) is often uncertain in a risk assessment, we also investigate how varying the magnitude of fixed size durations (ranging between 5 and 70. years) of this parameter affects the distribution of risk in both the time independent and dependent methodologies. To illustrate this new formulation and to investigate these mechanisms for sensitivity, an example of arsenic contaminated groundwater is used in conjunction with two scenarios of different environmental concentration signals resulting from rate dependencies in geochemical reactions. Cancer risk is computed and compared using environmental concentration ensembles modeled with sorption as 1) a linear equilibrium assumption (LEA) and 2) first order kinetics (Kin). Results show that the information attained in the new time dependent methodology reveals how the uncertainty in other time-dependent processes in the risk assessment may influence the uncertainty in risk. We also show that individual susceptibility also affects how risk changes in time, information that would otherwise be lost in the traditional, time independent methodology. These results are especially pertinent for forecasting risk in time, and for risk managers who are assessing the uncertainty of risk. © 2012 Elsevier B.V..

Bearup L.A.,Colorado School of Mines | Bearup L.A.,Integrated Groundwater Modeling Center | Maxwell R.M.,Colorado School of Mines | Maxwell R.M.,Integrated Groundwater Modeling Center | And 2 more authors.
Nature Climate Change | Year: 2014

The recent climate-exacerbated mountain pine beetle infestation in the Rocky Mountains of North America has resulted in tree death that is unprecedented in recorded history. The spatial and temporal heterogeneity inherent in insect infestation creates a complex and often unpredictable watershed response, influencing the primary storage and flow components of the hydrologic cycle. Despite the increased vulnerability of forested ecosystems under changing climate, watershed-scale implications of interception, ground evaporation, and transpiration changes remain relatively unknown, with conflicting reports of streamflow perturbations across regions. Here, contributions to streamflow are analysed through time and space to investigate the potential for increased groundwater inputs resulting from hydrologic change after infestation. Results demonstrate that fractional late-summer groundwater contributions from impacted watersheds are 30 ± 15% greater after infestation and when compared with a neighbouring watershed that experienced earlier and less-severe attack, albeit uncertainty propagations through time and space are considerable. Water budget analysis confirms that transpiration loss resulting from beetle kill can account for the relative increase in groundwater contributions to streams, often considered the sustainable flow fraction and critical to mountain water supplies and ecosystems. © 2014 Macmillan Publishers Limited.

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