Fersch, B.

Kunstmann, H.

Mauder, M.

Wagner, S.

English

KITopen

KIT – University of the State of Baden-Wuerttemberg andNational Research Center of the Helmholtz AssociationCoupled Modeling of Groundwater-Soil-Atmosphere Interaction in the TERENO Pre-Alpine RegionBenjamin Fersch (fersch@kit.edu), Sven Wagner, Matthias Mauder, and Harald Kunstmann• The interaction between groundwater and soil-moisture and its implications for the exchange of water and energy with the atmosphere has recently gained increasing attention, especially when groundwater levels are shallow.• Most of the current land-surface-models (LSMs) like the Noah-LSM of the WRF-ARW model neglect the process of interaction between the groundwater and the unsaturated soil.• Being the lower boundary for regional atmospheric simulations, the state of the upper soil layers directly connects to the water and energy exchange intensity at the land surface.• Under certain conditions, in comparison with free-drainage LSMs, groundwater induced patterns of soil moisture distribution can lead to different atmospheric states and dynamics in regional simulation models.• This study investigates the impact of groundwater soil-moisture coupling on the soil moisture characteristics and the  land-surface water budgets of the Noah-LSM. Observations from TERENO are used for model driving and evaluation. Cross-Compartment Coupled Regional Water Balance ModelingGroundwater (Phreatic Zone) Enabled Noah-Land-Surface-Model• This method is based on the work of Zeng et al. (2009), and De Rooij (2010) and it uses the soil moisture based Richard's equation.• The new bottom boundary condition is calculated by specifying the hydraulic head at the bottom of the LSM assuming hydrostatic equilibrium in the deep unsaturated zone.• The approach applies the method developed by Bogaart et al. (2008).• The net flux (up or down) between groundwater and the lowest soil layer of the LSM is calculated with the Darcy equation.• The equations are parametrized for the soil moisture content and the distance to the groundwater level.2) Darcy flow approximation for lower boundary fluxes1) Richard's  model with fixed-head lower boundary conditionChanges in simulated soil moisture content between uncoupled and coupled model configuration. Due to a calibration problem, the range of the moisture observations is shifted and increased. For com- parison the simulation results are therefore scaled by a factor of 2.1.Application Study for TERENO Pre-Alpine Site Graswang• The moisture interaction between phreatic and vadose zone is realized by an additional model layer that holds the properties of an imaginary deep unsaturated zone. • The intensity and direction of the flux between the lowest model soil layer and the groundwater depends on the moisture state of the vadose zone, the distance to the hydraulic head and differs for different soil types. • In this study, two different approaches are examined for the description of saturated-unsaturated zone interaction:Changes in simulated evapotranspiration between coupled and uncoupled simulations.Overview of the major processes that can be captured in a fully coupled model approach.Model input from climate station (TERENO) and nearby groundwater well (Wasserwirtschaftsamt Weilheim)TERENO climate/covariance station Graswang with soil moisture sensor array placed below• Wind speed• Wind direction• Temperature• Humidity• Pressure• Shortwave radiation• Longwave radiation• PrecipitationNoah-LSM 1Dwith GWcoupling-  it  liObservations for Model DrivingBogaart, P. W., Teuling, A. J., & Troch, P. A. (2008). A state-dependent parameterization of saturated-unsaturated zone interaction. Water Resources Research, 44(11), W11423.De Rooij, G. H. (2010). Comments on``Improving the Numerical Simulation of Soil Moisture-Based Richards Equation for Land Models with a Deep or Shallow Water Table''. Journal of Hydrometeorology, 11, 1044-1050.Zeng, X., & Decker, M. (2009). Improving the numerical solution of soil moisture-based Richards equation for land models with a deep or shallow water table. Journal of Hydrometeorology, 10(1), 308-319.Integration Into Distributed Regional Atmospheric Model System WRF• The coupled LSM has been implemented into the Weather and Research Forecasting Model WRF in combination with a 2D distributed groundwater model.• Currently, feedback studies for the distributed model system are evaluated out in a meso-scale application at China's Poyang Lake region (WRF-HMS) and in a catchment-scale application at  TERENO Alpine/Pre-Alpine in combination with the WRF-Hydro modeling system (http://www.ral.ucar.edu/projects/wrf_hydro/). 

Coupled modeling of groundwater-soil-atmosphere interaction in the TERENO Pre-Alpine region

KIT – Universität des Landes Baden-Württemberg undnationales Forschungszentrum in der Helmholtz-GemeinschaftInstitute for Meteorology and Climate Research (IMK-IFU), Garmisch-Partenkirchen, Germanywww.kit.eduCoupled Simulation of Groundwater-Soil-Atmosphere Interaction for the TERENO Pre-Alpine Region (Topic 4)Benjamin Fersch, S. Wagner, M. Mauder, H. KunstmannReklim Workshop, Bad Honnef, 16.-18. April 2013MotivationClosed expression of the regional water balance in dynamic model systemRegional atmospheric modeling studies focus usually on the prediction of temperature and precipitationCoupling to hydrological models is typically realized in a one way mode and by using bias correction Such approaches violate the closure of the water balance of the full system from the subsurface via the surface to the atmosphere MotivationTowards fully coupled cross-compartment hydrological-atmospheric model systemsConsider effects of lateral redistribution of surface waterReinfiltrationRunoff routingAccount for subsurface processes of the vadose and the phreatic zonePercolation to the groundwaterCapillary rise to root layersHorizontal transportProvide feedback capability from the subsurface via the surface to the atmospheric model and vice versaCoupled Atmosphere-Hydrology Model SystemCoupled Atmosphere-Hydrology Model SystemsCoupled Atmosphere-Hydrology Model SystemsCommon Land-Surface-Description in Regional Atmospheric ModelsStandard column approach in LSMsOnly vertical transport of moisture and energy is consideredInfiltration excess and bottom drainage terms act as sink term with no possibility of returningLower model boundary is defined by a downward gravitation driven flux(Unified Noah / OSU Land Surface Model © NCAR) Common Land-Surface-Description in Regional Atmospheric ModelsStandard column approach in LSMsNo lateral redistribution of surface and subsurface water in the LSMNo interaction between unsaturated and saturated zoneMutual non-linear interaction of a closed water cycle cannot emerge(Unified Noah / OSU Land Surface Model © NCAR) Coupled atmosphere-hydrology model systemsLSM(Richard's eq.)GW-HeadVariable lower boundaryDescription of moisture transportGroundwater Coupling ApproachApproach 1: Richard's equation with fixed-head boundary condition (based on Zeng et al. (2009), De Rooij (2010)Free drainage boundary condition of the LSM is replaced by bottom boundary condition which assumes an equilibrium soil moisture distribution of the deep unsaturated zoneThe new bottom boundary condition is calculated by specifying the hydraulic head at the bottom of the LSM New boundary condition realized with additional layer below the bottom of the Noahl-LSMSoil moisture content of additional layer depends on the distance to the hydraulic head fully saturated, hydraulic head above or equal layerpartly saturated, hydraulic head within layerunsaturated, hydraulic head below layerLSMGroundwater Coupling ApproachApproach 2: Darcy flux boundary condition (based on Bogaart et al. 2008)Assumes a quasi steady-state moisture profile between groundwater head and the lowest soil layer of the LSMDarcy equation is used to describe flow through this transition zone depending on the relative saturation at the bottom of the LSM and on the thickness of the transition zoneParametrization that approximates net Darcy flux qdarcy for different thicknesses of the transition zone and different values of saturation for the lowest LSM soil layerApproximation available for the 12 soil classes of Clapp & Hornberger (1978)LSMProof of Concept – 1D Groundwater Coupling Study with the Noah-LSM and TERENO DATAThe validity of the coupling approaches can hardly be analyzed in a distributed (2D) application1D – stand-alone study is performed for a well measured site of the terrestrial environmental observatory (TERENO) pre-alpineLinder valley of the Ammer catchmentGrassland, influenced by temporarily shallow groundwater tableData at TERENO Climate Station GraswangNoah-LSM stand-alone model drivingTime-series of soil-moisture for evaluationTime-series of eddy-flux evapotranspiration for evaluation Data Source: TERENO Site GraswangData Source: Groundwater Head Time-Series from the Local Water Authority (WWA Weilheim)Data Source: Groundwater Head Time-Series from the Local Water Authority (WWA Weilheim)Configuration Noah-LSM for 1D StudyNoah-LSM 1D, simple-driver, V.3.3Simulation period: 2010-07-01 to 2011-12-31 (half year of spin-up)Simulation time-step: 10 minutesSoil-type 4, silt loamVegetation / land-use: grassland (USGS 7, modified)Monthly update of albedo, LAI, vegetation fraction, and roughness length4 soil layers, 0-10, 10-40, 40-100, 100-200 cm (standard setup)Modes: Uncoupled, fixed-head-boundary and darcy-flux coupling with time-variant and constant (-7m) groundwater headImpact of Coupling on Soil Moisture DynamicsImpact of GW-Coupling on Evapotranspiration~10% decrease in evapotranspirationDaily Bias of EvapotranspirationETa, Comparison Static vs. Variable GW - HeadImpact of GW-Coupling on Infiltration Excess~20% decrease in surface runoffDistributed Model Systems WRF-Hydro & WRF-HMSClimate and Long Term ApplicationProcess Study and Forecast ApplicationDistributed Application, HMS-WRF, Poyang Lake Region, ChinaDifference in potential evaporation for groundwater coupled HMS-WRF simulation.Summary & ConclusionLower boundary condition for LSM Richard's equation important for simulation of near-surface soil moisture fieldFor 1D column study with TERENO input data, improvement of soil moisture states of the upper LSM soil levelsGroundwater coupling impacts also evapotranspiration and surface runoff with considerable amountsOutlookAdditional comparison of 1D Study for other TERENO sites throughout Germany (but groundwater is not monitored for most of the observatories)Application of a groundwater enabled version of the model system WRF-Hydro for the Ammer catchment (incl. 2D groundwater model)Distributed application requires distributed hydrogeological information about saturated conductivity, porosity, and thickness for aquifers → approximation strategy Validation of coupled simulations with respect to soil moisture patterns (sensor networks, ACROSS, remote sensing products)

Coupled simulation of groundwater-soil-atmosphere interaction in the TERENO Pre-Alpine region

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Coupled simulation of groundwater-soil-atmosphere interaction in the TERENO Pre-Alpine region

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