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Michael T. Coe

Abstract

A model (SWAM) to predict surface waters (lakes and wetlands) on the scale of atmospheric general circulation models is developed. SWAM is based on a linear reservoir hydrologic model and is driven by runoff, precipitation, evaporation, topography, and water transport directions.

SWAM is applied to the modern climate using observed estimates of the hydrologic variables and a 5′ × 5′ digital terrain model to represent topography. It simulates the surface water area of northern Africa (about 1% of the land area) in reasonable agreement with observed estimates (0.65%). A middle Holocene (6000 yr BP) simulation using the results of the GENESIS atmospheric general circulation model (AGCM) illustrates the sensitivity of the simulated surface waters to climatic changes and the model’s utility as a diagnostic tool for AGCMs. SWAM and GENESIS capture the general pattern of climate change 6000 yr BP. There is an increase in the simulated surface water area from about 1% to about 3% of the land area, including an increase in the area of Lake Chad by about five times and extensive surface water throughout northern Mali, consistent with observed patterns of surface water change during the Holocene. Limitations in the modeling of surface waters appear to result from the relatively coarse resolution of global elevation data.

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Michael T. Coe

Abstract

Each continental grid box of the National Center for Atmospheric Research (NCAR) community climate model (CCM1) is assigned to a particular continental drainage basin based on river basin extent and river flow direction. Boundaries are similarly assigned for the Arctic, Atlantic, Indian, and Pacific Oceans. The hydrologic variables from general circulation model simulations representing modern (control) and interglacial-6000 yr before present (6 ka bp)—climates are then summarized for continental drainage basins and oceans in order to examine the regional response of the hydrologic cycle to these climatic extremes.

The NCAR CCM1 simulation of the modem climate reproduces the general features of the observed hydrologic cycle. The simulations for the individual oceans are generally in good agreement with the observational estimates; however, the magnitude of the precipitation, evaporation, and runoff for nearly all continental regions is 20% to 30% greater than the observed.

Comparisons of the CCM1 control and 6 ka bp simulations reveal an increase in the simulated monsoon precipitation, evaporation, and runoff of South Asia in the 6 ka bp experiment relative to the control. There is also an increase in the freshwater surplus of the Indian Ocean basin, a larger freshwater deficit for the Pacific Ocean basin, and an enhanced hydrologic cycle in the high latitudes in the 6 ka bp experiment. An order of magnitude calculation suggests that the increased runoff simulated for South Asia is capable of altering the salinity of the northern Bay of Bengal to values consistent with observational estimates for this time period.

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Michael T. Coe

Abstract

A global hydrological routing algorithm (HYDRA) that simulates seasonal river discharge and changes in surface water level on a spatial resolution of 5′ long × 5′ lat is presented. The model is based on previous work by M. T. Coe and incorporates major improvements from that work including 1) the ability to simulate monthly and seasonal variations in discharge and lake and wetland level, and 2) direct representation of man-made dams and reservoirs. HYDRA requires as input daily or monthly mean averages of runoff, precipitation, and evaporation either from GCM output or observations.

As an example of the utility of HYDRA in evaluating GCM simulations, the model is forced with monthly mean estimates of runoff from the National Centers for Environmental Prediction (NCEP) reanalysis dataset. The simulated river discharge clearly shows that although the NCEP runoff captures the large-scale features of the observed terrestrial hydrology, there are numerous differences in detail from observations. The simulated mean annual discharge is within ±20% at only 13 of 90 fluvial gauging stations compared. In general, the discharge is overestimated for most of the northern high latitudes, midcontinental North America, eastern Europe, central and eastern Asia, India, and northern Africa. Only in western Europe and eastern North America is the discharge consistently underestimated. Although there appears to be a need for improved simulation of land surface physics in the NCEP product and parameterization of flow velocities within HYDRA, the timing of the monthly mean discharge is in fair agreement with the observations.

Including lakes within HYDRA reduces the amplitude of the seasonal cycle of discharge and the magnitude of the annual mean discharge of the St. Lawrence River system, in qualitative agreement with the observations. In addition, including the wetlands of the Sudd reduces the magnitude of the simulated annual discharge of the Nile River to values in better agreement with observations.

Finally, the impact of man-made dams and their reservoirs on the magnitude of monthly mean discharge can be explicitly included within HYDRA. As an example, including dams and reservoirs on the Parana River improves the agreement of the simulated mean monthly discharge with observations by reducing the amplitude of the seasonal cycle to values in good agreement with the observations.

The results of this study show that, although improvements can be obtained through better representations of flow velocities and more accurate digital elevation models, HYDRA can be a powerful tool for diagnosing simulated terrestrial hydrology and investigations of global climate change.

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Jeffrey Cardille, Michael T. Coe, and Julie A. Vano

Abstract

Lakes are a major geologic feature in humid regions, and multiple lake hydrologic types exist with varying physical and chemical characteristics, connections among lakes, and relationships to the landscape. The authors developed a model of water fluxes through major components of groundwater-dominated lake catchments in a region containing thousands of lakes, the Northern Highland Lake District (NHLD) of northern Wisconsin and the Upper Peninsula of Michigan. The model was calibrated with data from widely differing lakes using the same set of simple equations to represent the hydrologic type, water residence time, and amount and timing of stream and groundwater flows of representative lakes in today's climate. The authors investigated the sensitivity of the water balance of a set of three connected representative lakes and their catchments to systematic increases and decreases in the precipitation regime, and contrasted results using lake-specific morphometry to those for a lake having size and shape parameters typical of the region. Results indicate that a common set of equations can successfully represent major water balance characteristics of the three basic lake hydrologic types (hydraulically mounded, groundwater flowthrough, and drainage) in the NHLD. Sensitivity of modeled lakes varied by lake type, with drainage lakes more strongly buffered against substantial hydrologic changes in extreme climate scenarios. Catchment-scale water budgets differed substantially among lakes of different types, yet can be understood along a continuum of relative catchment size. These results suggest that a simple model of lake and catchment water balance can be extended to entire lake districts, where the detailed morphometry of most lakes is not well known.

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Diana C. Garcia-Montiel, Michael T. Coe, Meyr P. Cruz, Joice N. Ferreira, Euzebio M. da Silva, and Eric A. Davidson

Abstract

Water distributed in deep soil reservoirs is an important factor determining the ecosystem structure of water-limited environments, such as the seasonal tropical savannas of South America. In this study a two-dimensional (2D) geoelectrical profiling technique was employed to estimate seasonal dynamics of soil water content to 10-m depth along transects of 275 m in savanna vegetation during the period between 2002 and 2006. Methods were developed to convert resistivity values along these 2D resistivity profiles into volumetric water content (VWC) by soil depth. The 2D resistivity profiles revealed the following soil and aquifer structure characterizing the underground environment: 0–4 m of permanently unsaturated and seasonally droughty soil, less severely dry unsaturated soil at about 4–7 m, nearly permanently saturated soil between 7 and 10 m, mostly impermeable saprolite interspaced with fresh bedrock of parent material at about 10–30 m, and a region of highly conductive water-saturated material at 30 m and below. Considerable spatial variation of these relative depths is clearly demonstrated along the transects. Temporal dynamics in VWC indicate that the active zone of water uptake is predominantly at 0–7 m, and follows the seasonal cycles of precipitation and evapotranspiration. Uptake from below 7 m may have been critical for a short period near the beginning of the rainy season, although the seasonal variations in VWC in the 7–10-m layer are relatively small and lag the surface water recharge for about 6 months. Calculations using a simple 1-box water balance model indicate that average total runoff was 15–25 mm month−1 in the wet season and about 6–9 mm month−1 in the dry season. Modeled ET was about 75–85 mm month−1 in the wet season and 20–25 mm month−1 in the dry season. Variation in basal area and tree density along one transect was positively correlated with VWC of the 0–3-m and 0–7-m soil depths, respectively, during the wettest months. These multitemporal measurements demonstrate that the along-transect spatial differences in soil moisture are quasi-permanent and influence vegetation structure at the scale of tens to hundreds of meters.

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