Résultat de la recherche
Infiltration under snow cover: Modeling approaches and predictive uncertainty
Groundwater recharge from snowmelt represents a temporal redistribution of precipitation. This is extremely important because the rate and timing of snowpack drainage has substantial consequences to aquifer recharge patterns, which in turn affect groundwater availability throughout the rest of the year. The modeling methods developed to estimate drainage from a snowpack, which typically rely on temporally-dense point-measurements or temporally-limited spatially-dispersed calibration data, range in complexity from the simple degree-day method to more complex and physically-based energy balance approaches. While the gamut of snowmelt models are routinely used to aid in water resource management, a comparison of snowmelt models’ predictive uncertainties had previously not been done. Therefore, we established a snowmelt model calibration dataset that is both temporally dense and represents the integrated snowmelt infiltration signal for the Vers Chez le Brandt research catchment, which functions as a rather unique natural lysimeter. We then evaluated the uncertainty associated with the degree-day, a modified degree-day and energy balance snowmelt model predictions using the null-space Monte Carlo approach. All three melt models underestimate total snowpack drainage, underestimate the rate of early and midwinter drainage and overestimate spring snowmelt rates. The actual rate of snowpack water loss is more constant over the course of the entire winter season than the snowmelt models would imply, indicating that mid-winter melt can contribute as significantly as springtime snowmelt to groundwater recharge in low alpine settings. Further, actual groundwater recharge could be between 2 and 31% greater than snowmelt models suggest, over the total winter season. This study shows that snowmelt model predictions can have considerable uncertainty, which may be reduced by the inclusion of more data that allows for the use of more complex approaches such as the energy balance method. Further, our study demonstrated that an uncertainty analysis of model predictions is easily accomplished due to the low computational demand of the models and efficient calibration software and is absolutely worth the additional investment. Lastly, development of a systematic instrumentation that evaluates the distributed, temporal evolution of snowpack drainage is vital for optimal understanding and management of cold-climate hydrologic systems.
Tutorials as aflexible alternative to GUIs: An example for advanced model calibration using Pilot Points
Environmental modelling software with graphical user interfaces (GUIs) are user friendly and help tofocus on the aspects of modelling rather than on the technicality of the underlying code. This is afundamental advantage compared to codes without a GUI, as the absence of a GUI can prohibit thewidespread application of a software tool. However, there is a downside to GUIs, too. They commonly lagbehind the newest software development. We argue that tutorials are aflexible but undervaluedalternative to GUIs, and are convinced that tutorials can help to make complex software accessible to anincreased number of users. As an example to demonstrate our point, we have written a tutorial thatillustrates the use of Pilot-Point based calibration made available through PEST in the HydroGeoSpheremodelling environment. We hope that this example will encourage the modelling community to developtutorials and make them available to the wider public.
The influence of model structure on groundwater recharge rates in climate-change impact studies
Numerous modeling approaches are available to provide insight into the relationship between climate change and groundwater recharge. However, several aspects of how hydrological model choice and structure affect recharge predictions have not been fully explored, unlike the well-established variability of climate model chains—combination of global climate models (GCM) and regional climate models (RCM). Furthermore, the influence on predictions related to subsoil parameterization and the variability of observation data employed during calibration remain unclear. This paper compares and quantifies these different sources of uncertainty in a systematic way. The described numerical experiment is based on a heterogeneous two-dimensional reference model. Four simpler models were calibrated against the output of the reference model, and recharge predictions of both reference and simpler models were compared to evaluate the effect of model structure on climate-change impact studies. The results highlight that model simplification leads to different recharge rates under climate change, especially under extreme conditions, although the different models performed similarly under historical climate conditions. Extreme weather conditions lead to model bias in the predictions and therefore must be considered. Consequently, the chosen calibration strategy is important and, if possible, the calibration data set should include climatic extremes in order to minimise model bias introduced by the calibration. The results strongly suggest that ensembles of climate projections should be coupled with ensembles of hydrogeological models to produce credible predictions of future recharge and with the associated uncertainties.
Tutorials as a flexible alternative to GUIs: An example for advanced model calibration using Pilot Points
Environmental modelling software with graphical user interfaces (GUIs) are user friendly and help to focus on the aspects of modelling rather than on the technicality of the underlying code. This is a fundamental advantage compared to codes without a GUI, as the absence of a GUI can prohibit the widespread application of a software tool. However, there is a downside to GUIs, too. They commonly lag behind the newest software development. We argue that tutorials are a flexible but undervalued alternative to GUIs, and are convinced that tutorials can help to make complex software accessible to an increased number of users. As an example to demonstrate our point, we have written a tutorial that illustrates the use of Pilot-Point based calibration made available through PEST in the HydroGeoSphere modelling environment. We hope that this example will encourage the modelling community to develop tutorials and make them available to the wider public.
Assessing groundwater quality trends in pumping wells using spatially
When implementing remediation programs to mitigate diffuse-source contamination of aquifers, tools are required to anticipate if the measures are sufficient to meet groundwater quality objectives and, if so, in what time frame. Transfer function methods are an attractive approach, as they are easier to implement than numerical groundwater models. However, transfer function approaches as commonly applied in environmental tracer studies are limited to a homogenous input of solute across the catchment area and a unique transfer compartment. The objective of this study was to develop and test an original approach suitable for the transfer of spatially varying inputs across multiple compartments (e.g. unsaturated and saturated zone). The method makes use of a double convolution equation accounting for transfer across two compartments separately. The modified transfer function approach was applied to the Wohlenschwil aquifer (Switzerland), using a formulation of the exponential model of solute transfer for application to subareas of aquifer catchments. A minimum of information was required: (1) delimitation of the capture zone of the outlet of interest; (2) spatial distribution of historical and future pollution input within the capture zone; (3) contribution of each subarea of the recharge zone to the flow at the outlet; (4) transfer functions of the pollutant in the aquifer. A good fit to historical nitrate concentrations at the pumping well was obtained. This suggests that the modified transfer function approach is suitable to explore the effect of environmental projects on groundwater concentration trends, especially at an early screening stage.