Ian M. Ferguson

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.

Human impacts on terrestrial hydrology: climate change versus pumping and irrigation

Global climate change is altering terrestrial water and energy budgets, with subsequent impacts on surface and groundwater resources; recent studies have shown that local water management practices such as groundwater pumping and irrigation similarly alter terrestrial water and energy budgets over many agricultural regions, with potential feedbacks on weather and climate. Here we use a fully-integrated hydrologic model to directly compare effects of climate change and water management on terrestrial water and energy budgets of a representative agricultural watershed in the semi-arid Southern Great Plains, USA. At local scales, we find that the impacts of pumping and irrigation on latent heat flux, potential recharge and water table depth are similar in magnitude to the impacts of changing temperature and precipitation; however, the spatial distributions of climate and management impacts are substantially different. At the basin scale, the impacts on stream discharge and groundwater storage are remarkably similar. Notably, for the watershed and scenarios studied here, the changes in groundwater storage and stream discharge in response to a 2.5??C temperature increase are nearly equivalent to those from groundwater-fed irrigation. Our results imply that many semi-arid basins worldwide that practice groundwater pumping and irrigation may already be experiencing similar impacts on surface water and groundwater resources to a warming climate. These results demonstrate that accurate assessment of climate change impacts and development of effective adaptation and mitigation strategies must account for local water management practices.

Hydrologic and land–energy feedbacks of agricultural water management practices

Recent studies demonstrate strong interdependence between groundwater dynamics, land surface water and energy fluxes over some regions, including significant negative correlation between latent heat flux and groundwater depth. Other studies show that irrigation increases latent heat flux and decreases the Bowen ratio (ratio of sensible to latent heat flux), with subsequent feedbacks on local and regional climate. We use an integrated hydrologic model to evaluate impacts of groundwater pumping, irrigation, and combined pumping and irrigation on groundwater storage, land surface fluxes, and stream discharge over the Little Washita River watershed in the Southern Great Plains of North America. Pumping and irrigation are shown to impact simulated water and energy fluxes at local and watershed scales, with the magnitude of impacts governed by local water table depth. When pumping and irrigation are combined, irrigation has a dominant impact on spatially distributed surface energy processes while pumping has a dominant impact on basin-integrated hydrologic conditions.

Influence of SST Forcing on Stochastic Characteristics of Simulated Precipitation and Drought

Abstract Recent studies demonstrate that ocean–atmosphere forcing by persistent sea surface temperature (SST) anomalies is a primary driver of seasonal-to-interannual hydroclimatic variability, including drought events. Other studies, however, conclude that although SST anomalies influence the timing of drought events, their duration and magnitude over continental regions is largely governed by land–atmosphere feedbacks. Here the authors evaluate the direct influence of SST anomalies on the stochastic characteristics of precipitation and drought in two ensembles of AGCM simulations forced with observed (interannually varying) monthly SST and their climatological annual cycle, respectively. Results demonstrate that ocean–atmosphere forcing contributes to the magnitude and persistence of simulated seasonal precipitation anomalies throughout the tropics but over few mid- and high-latitude regions. Significant autocorrelation of simulated seasonal anomalies over oceans is directly forced by persistent SST ano...

Catalyzing frontiers in water-climate-society research: A view from early career scientists and junior faculty

AMEriCAN METEOrOlOGiCAl SOCiETY | 477 AffiliAtions: McNeeley, TesseNdorf, aNd lazrus—NCAR, Boulder, Colorado; lazrus—University of Oklahoma, Norman, Oklahoma; Heikkila—University of Colorado, Denver, Colorado; fergusoN—Colorado School of Mines, Golden, Colorado; arrigo—East Carolina University, Greenville, North Carolina; aTTari—Columbia University, New York, New York; ciaNfraNi— Hampshire College, Amherst, Massachusetts; dilliNg aNd kircHoff—University of Colorado, Boulder, Colorado; gurdak— San Francisco State University, San Francisco, California; kaMpf—Colorado State University, Fort Collins, Colorado; kauNeckis—University of Nevada, Reno, Nevada; lee—San Jose State University, San Jose, California; liNTNer—Rutgers, The State University of New Jersey, New Brunswick, New Jersey; MaHoNey—UCAR, Boulder, Colorado; opiTz-sTapleToN— Institute for Social and Environmental Transition, Boulder, Colorado; ray—University of Hawaii at Manoa, Honolulu, Hawaii; souTH—University of Exeter, Exeter, United Kingdom; sTubblefield—Humboldt State University, Arcata, California; brugger—University of California—Davis, Davis, California CoRREsPonDinG AUtHoR: Shannon M. McNeeley, Advanced Study Program, Research Applications Laboratory/Integrated Science Program, NCAR, P.O. Box 3000, Boulder, CO 80307 E-mail: smcneele@ucar.edu