H24D-01 INVITED
The Representation of Groundwater in Land Surface Models
The representation of groundwater in land surface models is receiving greatly increased attention in recent years. In this presentation, the evolution of its representation is discussed. The differing perspectives of what groundwater means to an atmospheric scientist versus a hydrologist are reviewed, as are concepts for its parameterization. The importance of groundwater to land-atmosphere interactions is assessed from the viewpoint of both observations and simulations. The case is made for continued enhancement of groundwater representation in several key areas, including realistic pumping rates, improved definition of aquifer boundaries, groundwater model calibration, and better ties to emerging regional and global groundwater monitoring networks.
H24D-02 INVITED
Coupled groundwater-atmosphere modeling: effects on atmospheric boundary layer development
Newly-developed coupled land-atmosphere models which incorporate both subsurface and atmospheric moisture dynamics have the potential to change our understanding of the hydrologic cycle. This presentation describes the effects of coupled groundwater-atmosphere modeling on simulations of the atmospheric boundary layer. Both field observations and simulations indicate strong sensitivity of atmospheric dynamics to land-surface conditions, in particular surface soil moisture. Simulations of atmospheric flow in Owens Valley (California) and in the Riviera Valley (Switzerland) show strong sensitivity to land-surface conditions, thus motivating the need for more accurate representations of soil moisture. In addition to influences from weather and seasonal changes, soil moisture dynamics respond to diurnal heat fluxes on the land surface. Using our new fully-coupled groundwater-atmosphere model, we have demonstrated correlations of soil moisture and land-surface heat fluxes with groundwater fluctuations on short, diurnal time scales. By explicitly calculating groundwater dynamics for our domain of interest, we are able to produce realistic time- and space-varying soil moisture distributions that naturally correspond to variations in topography and surface evaporation. Simulations in idealized and real watersheds are shown to illustrate these effects. The observed variations in surface moisture distribution have large impacts on the moisture and temperature structure in the atmosphere, leading to changes in boundary layer depth and convective motions as compared to standard soil moisture representations. Our coupled model framework will allow detailed investigation of the complex cycle of land-atmosphere processes affecting moisture distributions in the subsurface and the atmosphere.
H24D-03 INVITED
The Role of the Groundwater Reservoir in the Global Water Cycle
Through its vertical and local interaction with soil-vegetation, and through its lateral and regional redistribution of near-surface surplus, the groundwater links the various terrestrial reservoirs and influences them at its own spatial and temporal scales. Here we present evidence (theoretical, observational, and modeling) that groundwater storage and fluxes control soil moisture, river flow, and wetland distribution over a continent. We note that without the explicit representation of the water table dynamics in our climate models, continental drainage is disconnected with the sea level, making it difficult to understand the coupling and feedbacks among the various components of the earth's climate system.
H24D-04
Representing groundwater dynamics in climate models and its impacts on climate modeling
We will present an efficient approach to representing groundwater dynamics for climate modeling studies and its impacts on climate modeling on both global and regional scales. We developed a simple groundwater model by representing recharge and discharge processes of the water storage in an unconfined aquifer, which is added as a single integration element below the soil of a land surface model. We evaluated the model against the Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage change data. The modeled total water storage (including unsaturated soil water and groundwater) change agrees fairly well with GRACE estimates in most river basins where the water storage is not affected by snow water or frozen soil. Together with other changes in the model terrestrial hydrology, including a groundwater component in a GCM increases evapotranspiration (ET) and precipitation in some arid-to-wet transition zones. In addition, through root uptakes of water in deep soil, it improves the seasonal prediction of precipitation in the Central United States.
H24D-05
Coupling of Processes and Data in PennState Integrated Hydrologic Modeling (PIHM) System
Full physical coupling, "natural" numerical coupling and parsimonious but accurate data coupling is needed to comprehensively and accurately capture the interaction between different components of a hydrologic continuum. Here we present a physically based, spatially distributed hydrologic model that incorporates all the three coupling strategies. Physical coupling of interception, snow melt, transpiration, overland flow, subsurface flow, river flow, macropore based infiltration and stormflow, flow through and over hydraulic structures likes weirs and dams, and evaporation from interception, ground and overland flow is performed. All the physically coupled components are numerically coupled through semi-discrete form of ordinary differential equations, that define each hydrologic process, using Finite-Volume based approach. The fully implicit solution methodology using CVODE solver solves for all the state variables simultaneously at each adaptive time steps thus providing robustness, stability and accuracy. The accurate data coupling is aided by use of constrained unstructured meshes, flexible data model and use of PIHMgis. The spatial adaptivity of decomposed domain and temporal adaptivity of the numerical solver facilitates capture of varied spatio-temporal scales that are inherent in hydrologic process interactions. The implementation of the model has been performed on a meso-scale Little-Juniata Watershed. Model results are validated by comparison of streamflow at multiple locations. We discuss some of the interesting hydrologic interactions between surface, subsurface and atmosphere witnessed during the year long simulation such as a) inverse relationship between evaporation from interception storage and transpiration b) relative influence of forcing (precipitation, temperature and radiation) and source (soil moisture and overland flow) on evaporation c) influence of local topography on gaining, loosing or "flow-through" behavior of river-aquifer interactions d) role of macropores on base flow during wetting and drying conditions. In addition to its use as a potential predictive and exploratory science tool, we present a test case for the application of model in water management by mapping of water table decline index for the whole watershed. Also discussed will be the efficient parallelization strategy of the model for high spatio-temporal resolution simulations.
H24D-06
Soil water content and temperature induced spatial structure of heterotrophic respiration at field scale
Heterotrophic soil respiration is known to be highly variable in space and time. The spatial structure of this carbon dioxide flux strongly depends on the spatial pattern of soil water content and energy fluxes at the boundary layer between soil and atmosphere. The goal of this study was to determine the spatial variability of soil respiration, soil water content and soil temperature and their inter-relations at field scale. We simultaneously measured soil CO2 efflux, soil water content and soil temperature at 48 locations within a 14 by 14 m field plot under different soil and meteorological conditions. We measured an overall mean CO2 efflux of 2.4 g C m-2 d-1 for the bare soil, associated with a coefficient of variation of 32 %. The observed data was also analysed with geostatistical means including cross-semivariograms. For the scale under investigation we detected a mean correlation length of 2.5 m for soil respiration. Further we found a high structural semivariance of CO2 efflux for wet soil conditions, which is attributed to the effect of limited diffusion within the soil. The correlation length of the cross-semivariance between to soil water content and soil temperature is 5.9 and 3.3 m, respectively. From multiple regression we determined a higher influence of soil water content on the variance of soil respiration than for soil temperature, whereas the mean of soil respiration depended rather on soil temperature than on water content. Sequential Gaussian simulations were able to reproduce the spatial structure of the measured soil respiration.
H24D-07
Effects of water table dynamics on regional climate
Water table is an important component of hydrologic cycle and its anomaly will result in variations of soil moisture, water and energy balances between the land surface and atmosphere, which will ultimately influence climate. In this work, we investigate the effects of water table dynamics on regional climate by implementing a groundwater model into the regional climate model RegCM3. Three-month integrations with the coupled model (named RegCM3-GW) and the original climate model RegCM3 show that dynamical representation of water table in regional climate model changes the surface heat and moisture fluxes through modifying the vertical profile of soil moisture. The differences of simulated surface fluxes between RegCM3-GW and RegCM3 result in different structures of the boundary layer. For instance, the soil moisture simulated by the RegCM3-GW is wetter than that by the RegCM3 over northern China, which enhances the evapotranspiration and then increases cloud cover. Although the net shortwave absorbed by the soil decreases due to increased cloud cover, this effect is obscured by the decrease in net longwave emission. Thus the net radiative energy flux simulated by the RegCM3-GW is larger than that by the RegCM3, which provides more energy for the heating and moistening of the planetary boundary layer and builds a shallower boundary layer, and ultimately increases the potential for convective instability. In a short, RegCM3-GW produces more accurate monthly precipitation amount and geopotential height for the boundary atmosphere over semiarid areas, and the moisture tend to transport from the northeast of China to northern China as compared with RegCM3 because of the changes of circulation.