Stefan Kollet

Hyperresolution global land surface modeling: Meeting a grand challenge for monitoring Earth's terrestrial water

Monitoring Earths terrestrial water conditions is critically important to many hydrological applications such as global food production; assessing water resources sustainability; and flood, drought, and climate change prediction. These needs have motivated the development of pilot monitoring and prediction systems for terrestrial hydrologic and vegetative states, but to date only at the rather coarse spatial resolutions (∼10–100 km) over continental to global domains. Adequately addressing critical water cycle science questions and applications requires systems that are implemented globally at much higher resolutions, on the order of 1 km, resolutions referred to as hyperresolution in the context of global land surface models. This opinion paper sets forth the needs and benefits for a system that would monitor and predict the Earths terrestrial water, energy, and biogeochemical cycles. We discuss six major challenges in developing a system: improved representation of surface-subsurface interactions due to fine-scale topography and vegetation; improved representation of land-atmospheric interactions and resulting spatial information on soil moisture and evapotranspiration; inclusion of water quality as part of the biogeochemical cycle; representation of human impacts from water management; utilizing massively parallel computer systems and recent computational advances in solving hyperresolution models that will have up to 109 unknowns; and developing the required in situ and remote sensing global data sets. We deem the development of a global hyperresolution model for monitoring the terrestrial water, energy, and biogeochemical cycles a “grand challenge” to the community, and we call upon the international hydrologic community and the hydrological science support infrastructure to endorse the effort.

Capturing the influence of groundwater dynamics on land surface processes using an integrated, distributed watershed model

[1] The influence of groundwater dynamics on the energy balance at the land surface is studied using an integrated, distributed watershed modeling platform. This model includes the mass and energy balance at the land surface; three-dimensional variably saturated subsurface flow; explicit representation of the water table; and overland flow. The model is applied to the Little Washita watershed in Central Oklahoma, USA and compared to runoff, soil moisture and energy flux observations. The connection between groundwater dynamics and the land surface energy balance is studied using a variety of conventional and spatial statistical measures. For a number of energy variables a strong interconnection is demonstrated with water table depth. This connection varies seasonally and spatially depending on the spatial composition of water table depth. A theoretical critical water table depth range is presented where a strong sensitivity between groundwater and land-surface processes may be observed. For this particular watershed, a critical depth range is established between 1 and 5 m in which the land surface energy budget is most sensitive to groundwater storage. Finally, concrete recommendations are put forth to characterize this interconnection in the field.

Changing structure of European precipitation: Longer wet periods leading to more abundant rainfalls

[1] Analysis of the duration of wet spells (consequent days with significant precipitation) in Europe and associated precipitation is performed over the period 1950–2008 using daily rain gauge data. During the last 60 years wet periods have become longer over most of Europe by about 15– 20%. The lengthening of wet periods was not caused by an increase of the total number of wet days. Becoming longer, wet periods in Europe are now characterized by more abundant precipitation. Heavy precipitation events during the last two decades have become much more frequently associated with longer wet spells and intensified in com‐ parison with 1950s and 1960s. The changes in the distri‐ bution of temporal characteristics of precipitation towards longer events and higher intensities should have a significant impact on the terrestrial hydrologic cycle including sub‐ surface hydrodynamics, surface runoff and European flooding. Citation: Zolina, O., C. Simmer, S. K. Gulev, and S. Kollet (2010), Changing structure of European precipitation: Longer wet periods leading to more abundant rainfalls, Geophys. Res. Lett., 37, L06704, doi:10.1029/2010GL042468.

Surface‐subsurface model intercomparison: A first set of benchmark results to diagnose integrated hydrology and feedbacks

There are a growing number of large-scale, complex hydrologic models that are capable of simulating integrated surface and subsurface flow. Many are coupled to land-surface energy balance models, biogeochemical and ecological process models, and atmospheric models. Although they are being increasingly applied for hydrologic prediction and environmental understanding, very little formal verification and/or benchmarking of these models has been performed. Here we present the results of an intercomparison study of seven coupled surface-subsurface models based on a series of benchmark problems. All the models simultaneously solve adapted forms of the Richards and shallow water equations, based on fully 3-D or mixed (1-D vadose zone and 2-D groundwater) formulations for subsurface flow and 1-D (rill flow) or 2-D (sheet flow) conceptualizations for surface routing. A range of approaches is used for the solution of the coupled equations, including global implicit, sequential iterative, and asynchronous linking, and various strategies are used to enforce flux and pressure continuity at the surface-subsurface interface. The simulation results show good agreement for the simpler test cases, while the more complicated test cases bring out some of the differences in physical process representations and numerical solution approaches between the models. Benchmarks with more traditional runoff generating mechanisms, such as excess infiltration and saturation, demonstrate more agreement between models, while benchmarks with heterogeneity and complex water table dynamics highlight differences in model formulation. In general, all the models demonstrate the same qualitative behavior, thus building confidence in their use for hydrologic applications.

Demonstrating fractal scaling of baseflow residence time distributions using a fully‐coupled groundwater and land surface model

[1] The influence of the vadose zone, land surface processes, and macrodispersion on the shape and scaling behavior of residence time distributions of baseflow is studied using a fully coupled watershed model in conjunction with a Lagrangian, particle-tracking approach. Numerical experiments are used to simulate groundwater flow paths from recharge locations along the hillslope to the streambed. These experiments are designed to isolate the influences of topography, vadose zone/land surface processes, and macrodispersion on subsurface transport of tagged parcels of water. The results of these simulations agree with previous observations that such distributions exhibit a power law form and fractal behavior, which can be identified from plots of the residence time distribution and the power spectra. It is shown that vadose zone/land surface processes significantly affect both the residence time distributions and their spectra.

Stream depletion predictions using pumping test data from a heterogeneous stream - aquifer system (a case study from the Great Plains, USA)

This uniquely designed study investigates a fundamental issue—the feasibility of predicting stream depletion rates using linear uniform two-dimensional models. Required input for these models includes the hydraulic parameter estimates of the aquifer and the stream – aquifer interface, which may be obtainable through pumping test data analysis. This study utilizes pumping test data collected near the naturally meandering Prairie Creek, Platte River watershed, Nebraska, USA. Drawdown data were obtained in eight piezometer clusters, located on both sides of the stream, each containing three piezometers screened at different aquifer depths. Parameter estimates and, thus, stream depletion predictions varied over a wide range. Large parameter variance and a low degree of goodness of fit between the calculated and measured data encountered during the analysis suggest deficiencies of the uniform aquifer models in describing significant physical processes. This was also shown by additional field experiments that indicate lateral and vertical aquifer heterogeneity. Hydrogeological and sedimentological considerations of the meandering stream architecture (point bar versus cut bank) and the application of a linear piecewisehomogeneous model yielded a higher degree of goodness of fit and higher confidence in stream depletion predictions. Aquifer heterogeneity appears to be the major reason for uncertainty in stream depletion predictions, though other possible sources of uncertainty should be considered. These include the model linearity, the Dupuit assumption, the simplified representation of the stream – aquifer interface, the approximation of the stream as a straight line or a strip, and the impact of regional groundwater flow. q 2003 Elsevier B.V. All rights reserved.

A Scale-Consistent Terrestrial Systems Modeling Platform Based on COSMO, CLM, and ParFlow

A highly modular and scale-consistent Terrestrial Systems Modeling Platform (TerrSysMP) is presented. The modeling platform consists of an atmospheric model (Consortium for Small-Scale Modeling; COSMO), a land surface model (the NCARCommunityLand Model,version3.5; CLM3.5), anda 3D variablysaturated groundwater flow model (ParFlow). An external coupler (Ocean Atmosphere Sea Ice Soil, version 3.0; OASIS3) with multiple executable approaches is employed to couple the three independently developed component models, which intrinsically allows for a separation of temporal‐spatial modeling scales and the coupling frequencies between the component models. IdealizedTerrSysMPsimulations arepresented,whichfocuson theinteractionofkey hydrologic processes, like runoff production (excess rainfall and saturation) at different hydrological modeling scales and the drawdown of the water table through groundwater pumping, with processes in the atmospheric boundary layer. The results show a strong linkage between integrated surface‐groundwater dynamics, biogeophysical processes, and boundary layer evolution. The use of the mosaic approach for the hydrological component model (to resolve subgrid-scale topography) impacts simulated runoff production, soil moisture redistribution, and boundary layer evolution, which demonstrates the importance of hydrological modeling scales and thus the advantages of the coupling approach used in this study. Real data simulations were carried out with TerrSysMP over the Rur catchment in Germany. The inclusion oftheintegratedsurface‐groundwaterflowmodelresultsin systematicpatternsin therootzonesoilmoisture, which influence exchange flux distributions and the ensuing atmospheric boundary layer development. In a first comparison to observations, the 3D model compared to the 1D model shows slightly improved predictions of surface fluxes and a strong sensitivity to the initial soil moisture content.

Soil hydrology: Recent methodological advances, challenges, and perspectives

Technological and methodological progress is essential to improve our understanding of fundamental processes in natural and engineering sciences. In this paper, we will address the potential of new technological and methodological advancements in soil hydrology to move forward our understanding of soil water related processes across a broad range of scales. We will focus on advancements made in quantifying root water uptake processes, subsurface lateral flow, and deep drainage at the field and catchment scale, respectively. We will elaborate on the value of establishing a science-driven network of hydrological observatories to test fundamental hypotheses, to study organizational principles of soil hydrologic processes at catchment scale, and to provide data for the development and validation of models. Finally, we discuss recent developments in data assimilation methods, which provide new opportunities to better integrate observations and models and to improve predictions of the short-term evolution of hydrological processes.

Monitoring and Modeling the Terrestrial System from Pores to Catchments: The Transregional Collaborative Research Center on Patterns in the Soil–Vegetation–Atmosphere System

AbstractMost activities of humankind take place in the transition zone between four compartments of the terrestrial system: the unconfined aquifer, including the unsaturated zone; surface water; vegetation; and atmosphere. The mass, momentum, and heat energy fluxes between these compartments drive their mutual state evolution. Improved understanding of the processes that drive these fluxes is important for climate projections, weather prediction, flood forecasting, water and soil resources management, agriculture, and water quality control. The different transport mechanisms and flow rates within the compartments result in complex patterns on different temporal and spatial scales that make predictions of the terrestrial system challenging for scientists and policy makers. The Transregional Collaborative Research Centre 32 (TR32) was formed in 2007 to integrate monitoring with modeling and data assimilation in order to develop a holistic view of the terrestrial system. TR32 is a long-term research program ...

Influence of soil heterogeneity on evapotranspiration under shallow water table conditions: transient, stochastic simulations

Ensembles of soil column numerical experiments were performed to study the influence of heterogeneity in the saturated hydraulic conductivity on the evapotranspiration or latent heat, LE, under varying water table conditions. In the numerical experiments, a variably saturated groundwater flow model coupled with a land surface model (ParFlow[CLM]) was used. The model was forced at the top with an atmospheric time series from Oklahoma, USA. The heterogeneity was simulated for two different soils (clay and sand) using uncorrelated Gaussian random fields with different variances. Soil heterogeneity has a strong influence on LE during the dry months of the year and negligible influence during months with sufficient moisture availability. The influence is stronger for unstructured soils, such as sand. An increase in shallow water table depth collapses the ensemble onto a single curve, i.e., heterogeneity plays a minor role and LE is limited by the increasing redistribution distance from the water table to the land surface. Accounting for correlations between the hydraulic conductivity and the shape factor in the pressure‐saturation function increases the variability in LE. In the case of laterally interconnected soil columns, 3D hydrodynamics homogenize LE fluxes by establishing additional flow paths toward the land surface. Comparison with geometric mean simulations shows good agreement over many time periods for weekly and monthly averaged LE fluxes. Instantaneous daily and hourly differences sometimes exhibit values on the order of 10 0 ‐10 1 Wm −2 (10 −2 ‐10 −1 mm d −1 ) and white noise behavior over extended time periods. In the simulations, perhaps the strongest limitation is the application of a constant atmospheric time series at the top of the simulation domain. In order to assess the impact of this limitation in simulations of subsurface‐land surface‐atmosphere interactions, it would be necessary to allow for two-way feedbacks with the lower atmosphere. This will require resolving the boundary layer immediately adjacent to the land surface, including the lateral exchange of mass, energy, and momentum.