Sensitivity of a GCM simulation to subgrid infiltration and surface runoff

A subgrid parameterization of infiltration and surface runoff was evaluated using a land surface model coupled to an atmospheric general circulation model. Averaged over 5 year simulations, the subgrid parameterization resulted in significantly less infiltration of water into the soil compared to a simulation without subgrid hydrologic processes. As a result, the soils were drier, latent heat flux decreased, and surface air temperature increased. These results are consistent with other studies of subgrid hydrologic parameterizations, which also resulted in drier soils, decreased latent heat flux, and warmer surface temperatures. Several river basins were studied in detail. In the Amazon and Lena basins, the subgrid parameterization resulted in better annual runoff compared to observed annual river flow; surface air temperature was unchanged in the Amazon and better, compared to observations, in the Lena. In the Ob, Yenisey, and Amur basins, the subgrid parameterization resulted in too much annual runoff; July surface air temperature was unchanged or worse (Amur). Annual runoff for the Mississippi basin was better with the subgrid parameterization, but July surface air temperature was worse. These results suggest the utility of subgrid hydrologic parameterizations vary among river basins depending on the relative importance of Horton and Dunne runoff and the geologic factors affecting runoff generation.     

Global validation of the ISBA sub-grid hydrology

Over recent years, many numerical studies have suggested that the land surface hydrology contributes to atmospheric variability and predictability on a wide range of scales. Conversely, land surface models (LSMs) have been also used to study the hydrological impacts of seasonal climate anomalies and of global warming. Validating these models at the global scale is therefore a crucial task, which requires off-line simulations driven by realistic atmospheric fluxes to avoid the systematic biases commonly found in the atmospheric models. The present study is aimed at validating a new land surface hydrology within the ISBA LSM. Global simulations are conducted at a 1° by 1° horizontal resolution using 3-hourly atmospheric forcings provided by the Global Soil Wetness Project. Compared to the original scheme, the new hydrology includes a comprehensive and consistent set of sub-grid parametrizations in order to account for spatial heterogeneities of topography, vegetation, and precipitation within each grid cell. The simulated runoff is converted into river discharge using the total runoff integrating pathways (TRIP) river routing model (RRM), and compared with available monthly observations at 80 gauging stations distributed over the world’s largest river basins. The simulated discharges are also compared with parallel global simulations from five alternative LSMs. Globally, the new sub-grid hydrology performs better than the original ISBA scheme. Nevertheless, the improvement is not so clear in the high-latitude river basins (i.e. Ob, MacKenzie), which can be explained by a too late snow melt in the ISBA model. Over specific basins (i.e. Parana, Niger), the quality of the simulated discharge is also limited by the TRIP RRM, which does not account for the occurrence of seasonal floodplains and for their significant impact on the basin-scale water budget.     

Sensitivity of the hydrological cycle to the parametrization of soil hydrology in a GCM

 The sensitivity of the hydrological cycle to soil hydrology is investigated with the LMD GCM. The reference simulation includes the land-surface scheme SECHIBA, with a two-reservoir scheme for soil water storage and runoff at saturation. We studied a non-linear drainage parametrization, and a distributed surface runoff parametrization, accounting for the subgrid scale variability (SSV) of soil moisture capacity, through a distribution where the shape parameter was b. GCM results show that the drainage parametrization induces significant reductions in soil moisture and evaporation rate compared to the reference simulation. They are related to changes in moisture convergence in the tropics, and to a precipitation decrease in the extratropics. When drainage is implemented, the effect of the SSV parametrization (b=0.2) is also to reduce soil moisture and evaporation rates compared to the simulation with drainage only. These changes are much smaller than the former, but the sensitivity of the hydrological cycle to the SSV parametrization is shown to be larger in dry periods, and to be enhanced by an increase of the shape parameter b. The comparison of simulated total runoffs with observed data shows that the soil hydrological parametrizations does not reduce the GCM systematic errors in the annual water balance, but that they can improve the representation of the total runoff’s annual cycle.     

Numerical Simulation and Evaluation of a New Hydrological Model Coupled with GRAPES

Hydrological processes exert enormous influences on the land surface water and energy balance, and have a close relationship with human society. We have developed a new hydrological runoff parameterization called XXT to improve the performance of a coupled land surface-atmosphere modeling system. The XXT parameterization, which is based upon the Xinanjiang hydrological model and TOPMODEL, includes an optimized function of runoff calculation with a new soil moisture storage capacity distribution curve(SMSCC). We then couple XXT with the Global/Regional Assimilation Prediction System (GRAPES) and compare it to GRAPES coupled with a simple water balance model (SWB).For the model evaluation and comparison, we perform 72-h online simulations using GRAPES-XXT and GRAPES-SWB during two torrential events in August 2007 and July 2008, respectively. The results show that GRAPES can reproduce the rainfall distribution and intensity fairly well in both cases. Differences in the representation of feedback processes between surface hydrology and the atmosphere result in differences in the distributions and amounts of precipitation simulated by GRAPES-XXT and GRAPES-SWB. The runoff simulations are greatly improved by the use of XXT in place of SWB, particularly with respect to the distribution and amount of runoff. The average runoff depth is nearly doubled in the rainbelt area, and unreasonable runoff distributions simulated by GRAPES-SWB are made more realistic by the introduction of XXT. Differences in surface soil moisture between GRAPES-XXT and GRAPES-SWB show that the XXT model changes infiltration and increases surface runoff. We also evaluate river flood discharge in the Yishu River basin. The peak values of flood discharge calculated from the output of GRAPES-XXT agree more closely with observations than those calculated from the output of GRAPES-SWB.     

On the simulation of Laurentian Great Lakes water levels under projections of global climate change

A new method is proposed to estimate future net basin supplies and lake levels for the Laurentian Great Lakes based on GCM projections of global climate change. The method first dynamically downscales the GCM simulation with a regional climate model, and then bias—corrects the simulated net basin supply in order to be used directly in a river—routing/lake level scheme. This technique addresses two weaknesses in the traditional approach, whereby observed sequences of climate variables are perturbed with fixed ratios or differences derived directly from GCMs in order to run evaporation and runoff models. Specifically, (1) land surface—atmosphere feedback processes are represented, and (2) changes in variability can be analyzed with the new approach. The method is demonstrated with a single, high resolution simulation, where small changes in future mean lake levels for all the upper Great Lakes are found, and an increase in seasonal range—especially for Lake Superior—is indicated. Analysis of a small ensemble of eight lower resolution regional climate model simulations supports these findings. In addition, a direct comparison with the traditional approach based on the same GCM projections used as the driving simulations in this ensemble shows that the new method indicates smaller declines in level for all the upper Great Lakes than has been reported previously based on the traditional method, though median differences are only a few centimetres in each case.     

The impact of implementing the bare essentials of surface transfer land surface scheme into the BMRC GCM

This study describes the first order impacts of incorporating a complex land-surface scheme, the bare essentials of surface transfer (BEST), into the Australian Bureau of Meteorology Research Centre (BMRC) global atmospheric general circulation model (GCM). Land seasonal climatologies averaged over the last six years of integrations after equilibrium from the GCM with BEST and without BEST (the control) are compared. The modeled results are evaluated with comprehensive sources of data, including the layer-cloud climatologies from the international satellite cloud climatology project (ISCCP) data from 1983 to 1991 and the surface-observed global data of Warren et al., a five-year climatology of surface albedo estimated from earth radiation budget experiment (ERBE) top-of-the-atmosphere (TOA) radiatioe fluxes, global grid point datasets of precipitation, and the climatological analyses of surface evaporation and albedo. Emphasis is placed on the surface evaluation of simulations of landsurface conditions such as surface roughness, surface albedo and the surface wetness factor, and on their effects on surface evaporation, precipitation, layer-cloud and surface temperature. The improvements due to the inclusion of BEST are: a realistic geographical distribution of surface roughness, a decrease in surface albedo over areas with seasonal snow cover, and an increase in surface albedo over snow-free land. The simulated reduction in surface evaporation due, in part, to the biophysical control of vegetation, is also consistent with the previous studies. Since the control climate has a dry bias, the overall simulations from the GCM with BEST are degraded, except for significant improvements for the northern winter hemisphere because of the realistic vegetation-masking effects. The implications of our results for synergistic developments of other aspects of model parameterization schemes such as boundary layer dynamics, clouds, convection and rainfall are discussed.     

Implementation of a large‐scale variable velocity river flow routing algorithm in the Canadian Regional Climate Model (CRCM)

Abstract

Implementation and validation of a flow routing scheme for the North American domain of the Canadian Regional Climate Model (CRCM) is described. A variable velocity flow routing algorithm is used to transport runoff from the land surface to the continental edges and provide freshwater flux forcing for the oceans. The flow routing scheme uses Manning's equation to estimate flow velocities for river channels whose cross‐sections are assumed to be rectangular. Discretization of major North American river basins and their flow directions are obtained at the polar stereographic resolution of the CRCM using 5‐minute global river flow direction data as a template. In the absence of observation‐based gridded estimates of runoff, model runoff estimates from a global simulation of the Variable Infiltration Capacity (VIC) hydrological model (forced with observationbased meteorological data) are used to validate the flow routing scheme. Model results show that the inclusion of flow routing improves the comparison with observation‐based streamflow estimates when compared to the unrouted runoff. Monthly comparison of simulated streamflow with observation‐based estimates, and basin‐wide averaged flow velocities, suggests that the flow routing scheme performs satisfactorily.     

Global and continental water balance in a GCM

Surface hydrology is recognised as an important component of general circulation climate models. The global and regional climates simulated by such models are demonstrably sensitive to the parameterization of terrestrial hydrologic processes. There exists, therefore, a clear requirement to evaluate different parameterization approaches in terms of the representation of the terrestrial phase of the hydrologic cycle. One potential means of meeting this requirement is by using available continental water-balance summaries. In this study three versions of a GCM, the National Center for Atmospheric Research (NCAR) Community Climate Model Version l (CCM1), differing mainly in spatial resolution and the representation of the surface hydrology, are compared against existing water-balance studies. Additional streamflow data are incorporated as a means of further validating both the water-balance approach and the GCM surface hydrologic parameterization in capturing the gross features of continental-scale hydrology.     

Introduction of a sub-grid hydrology in the ISBA land surface model

In atmospheric models, the partitioning of precipitation between infiltration and runoff has a major influence on the terrestrial water budget, and thereby on the simulated weather or climate. River routing models are now available to convert the simulated runoff into river discharge, offering a good opportunity to validate land surface models at the regional scale. However, given the low resolution of global atmospheric models, the quality of the hydrological simulations is much dependent on various processes occurring on unresolved spatial scales. This paper focuses on the parameterization of sub-grid hydrological processes within the ISBA land surface model. Five off-line simulations are performed over the French Rhône river basin, including various sets of parameterizations related to the sub-grid variability of topography, precipitation, maximum infiltration capacity and land surface properties. Parallel experiments are conducted at a high (8 km by 8 km) and low (1° by 1°) resolution, in order to test the robustness of the simulated water budget. Additional simulations are performed using the whole package of sub-grid parameterizations plus an exponential profile with depth of saturated hydraulic conductivity, in order to investigate the interaction between the vertical soil physics and the horizontal heterogeneities. All simulations are validated against a dense network of gauging measurements, after the simulated runoff is converted into discharge using the MODCOU river routing model. Generally speaking, the new version of ISBA, with both the sub-grid hydrology and the modified hydraulic conductivity, shows a better simulation of river discharge, as well as a weaker sensitivity to model resolution. The positive impact of each individual sub-grid parameterization on the simulated discharges is more obvious at the low resolution, whereas the high-resolution simulations are more sensitive to the exponential profile with depth of saturated hydraulic conductivity.     

Uncertainty in Regional Climate Model Mean Runoff Projections under Climate Change: Case Study of Labrador's Churchill River Basin

An ensemble of seven climate models from the North American Regional Climate Change Assessment Program (NARCCAP) was used to examine uncertainty in simulated runoff changes from a base period (1971–2000) to a future period (2041–2070) for the Churchill River basin, Labrador, Canada. Three approximations for mean annual runoff from each ensemble member were included in the analysis: (i) atmospheric moisture convergence, (ii) the balance between precipitation and evaporation, and (iii) instantaneous runoff output from respective land-surface schemes. Using data imputation (i.e., reconstruction) and variance decomposition it was found that choice of regional climate model (RCM) made the greatest contribution to uncertainty in the climate change signal, whereas the boundary forcing of a general circulation model (GCM) played a smaller, though non-negligible, role. It was also found that choice of runoff approximation made a substantial contribution to uncertainty, falling between the contribution from RCM and GCM choice. The NARCCAP output and imputed data were used to calculate mean and median annual changes and results were presented via probability distribution functions to facilitate decision making. Mean and median increases in runoff for the basin were found to be 11.2% and 8.9%, respectively.     

Towards closing the vertical water balance in Canadian atmospheric models: Coupling of the land surface scheme class with the distributed hydrological model watflood

Abstract

Second generation land surface schemes are the subject of much development activity among atmospheric modellers. This work is aimed at, among other things, improving the representation of the soil water balance in order to simulate, more properly, exchanges with the atmosphere and to permit the use of model output to generate streamflow for model validation. The Canadian development program is centred on CLASS, the Canadian Land Surface Scheme, developed at Environment Canada. This paper focuses on the improvement of hydrology in CLASS. This was accomplished by designing a two‐way interface to WATFLOOD, a distributed hydrologic model developed at the University of Waterloo. The two models share many features, which facilitated the coupling procedure.

The interface retains the three‐layer vertical moisture budget representation in CLASS but adds three horizontal runoff possibilities. Runoff from the surface water follows Manning's equation for overland flow. Interflow is generated from the near‐surface soil layer using a parametrization of Richard's equation and base flow is produced by Darcian flow from the bottom of layer 3. An approximation of the internal topography of grid elements is used to supply horizontal gradients for the runoff components.

Tests are in progress in four Canadian study areas. Initial results are presented for the summer of 1993 for the Saugeen River in southwestern Ontario. The new scheme produces realistic hydrographs, whereas the old scheme did not. Bare ground evaporation is reduced by about 17% as a consequence of reduced water availability in layer 1. Evapotranspiration is not affected because the rooting depth extends into layer 3, in which soil moisture does not change appreciably with the new scheme. These results suggest that the new scheme improves the representation of streamflow in WATFLOOD/CLASS and of the soil moisture budget in CLASS. Work is in progress to validate this result over basins, such as the BOREAS study watersheds, where both runoff and evapotranspiration measurements are available.     

Comparing surface energy, water and carbon cycle in dry and wet regions simulated by a land-surface model

In this study, we analyze results from 47-year (1954?C2000) offline simulations using an Australian land-surface model CSIRO Atmosphere Biosphere Land Exchange. We focus on exploring its surface mean climatology, interannual and decadal variations in Australia and Amazonia basin in South America which are distinguished by dry and wet climates respectively. Its skill is assessed by using observational datasets and four model products from the Global Land-surface Data Assimilation System. Surface evaporation and runoff climatologies are satisfactorily simulated, including surface energy and water partitions in dry and wet climates. In the Australian continent dominated by dry climate, slowly varying soil moisture processes are simulated in the southeast during austral winter. The model is skilful in reproducing the nonlinear relationship between rainfall and runoff variations in the southwestern part of the Australia. It shows that the significant downward trend of river inflow in the region is associated with enhanced surface evaporation which is caused by increased surface radiation and wind speed. In its carbon-cycle modeling, the model simulates an upward trend of NPP by about 0.69%/year over the Amazonia forest region in the 47-year period. By comparing two sets of the model results with/without CO₂ variations, it shows that 35% of such increases are caused by changes in climatic conditions, while 65% is due to the increase in atmospheric CO₂ concentration. Given the close linkage between climate, water and vegetation (carbon cycle), this work promotes an integrated modeling and evaluation approach for better representation of land-surface processes in Earth system studies.     

Systematic Hydrological Evaluation of the Noah-MP Land Surface Model over China

We evaluate water budget components—namely, soil moisture, runoff, evapotranspiration, and terrestrial water storage (TWS)—simulated by the Noah land surface model with multi-parameterization options (Noah-MP) in China, a large geographic domain challenging for hydrological modeling due to poor observational data and a lack of one single parameterization that can fit for complex hydrological processes. By comparing the model simulations with multi-source reference data, we show that Noah-MP can generally reproduce the overall spatiotemporal patterns of runoff and evapotranspiration over six major river basins, with the annual correlation coefficients generally greater than 0.8 and the Nash–Sutcliffe model efficiency coefficient exceeding 0.5. Among the six basins evaluated, the best model performance is seen over the Huaihe River basin. The temporal trend of the modeled TWS anomalies agrees well with GRACE (Gravity Recovery and Climate Experiment) observations, capturing major flood and drought events in different basins. Experiments with 12 selected physical parameterization options show that the runoff parameterization has a stronger impact on the simulated soil moisture–runoff–evapotranspiration relationships than the soil moisture factor for stomatal resistance schemes, a result consistent with previous studies. Overall, Noah-MP driven by GLDAS forcing simulates the hydrological variables well, except for the Songliao basin in northeastern China, likely because this is a transitional region with extensive freeze–thaw activity, while representations of human activities may also help improve the model performance.     

Validation and sensitivity of the global hydrologic budget in stand-alone simulations with the ISBA land-surface scheme

 Global soil moisture data of high quality and resolution are not available by direct observation, but are useful as boundary and initial conditions in comprehensive climate models. In the framework of the GSWP (Global Soil Wetness Project), the ISBA land-surface scheme of Météo-France has been forced with meteorological observations and analyses in order to study the feasibility of producing a global soil wetness climatology at a 1°×1° horizontal resolution. A control experiment has been performed from January 1987 to December 1988, using the ISLSCP Initiative I boundary conditions. The annual mean, the standard deviation and the normalised annual harmonic of the hydrologic fields have been computed from the 1987 monthly results. The global maps which are presented summarise the surface hydrologic budget and its annual cycle. The soil wetness index and snow cover distributions have been compared respectively to the results of the ECMWF reanalysis and to satellite and in situ observations. The simulated runoff has been validated against a river flow climatology, suggesting a possible underestimation over some large river basins. Besides the control run, other simulations have been performed in order to study the sensitivity of the hydrologic budget to changes in the surface parameters, the precipitation forcing and the runoff scheme. Such modifications have a significant impact on the partition of total precipitation into evaporation and runoff. The sensitivity of the results suggests that soil moisture remains one of the most difficult climatological parameters to model and that any computed soil wetness climatology must be considered with great caution. Received: 3 January 1997 / Accepted: 19 August 1987     

A new snow parameterization for the Météo-France climate model

Both observational and numerical studies demonstrate the sensitivity of the atmosphere to variations in the extent and mass of snow cover. There is therefore a need for simple but realistic snow parameterizations in forecast and climate models. A new snow hydrology scheme has recently been developed at Météo-France for use in the ARPEGE climate model and has been successfully tested against local field measurements in stand-alone experiments. This study describes the global validation of the parameterization in a 3-year integration for the present-day climate within the T42L30 version of ARPEGE. Results are compared with those from a control simulation and with available observed climatologies, in order to assess the impact of the new snow parameterization on the simulated surface climate. The seasonal cycle of the Northern Hemisphere snow cover is clearly improved when using the new scheme. The snow pack is still slightly overestimated in winter, but its poleward retreat is better reproduced during the melting season. As a consequence, the modified GCM performs well in simulating the springtime continental heating, which may play a strong role in the simulation of the Asian summer monsoon.     

Using moisture conservation to evaluate oceanic surface freshwater fluxes in climate models

We apply a diagnostic based on moisture conservation in the atmosphere, integrated over planetary-scale ocean basins and drainage areas to evaluate freshwater fluxes over the ocean surface to three generations of the Hadley Centre climate model (HadCM3, HadGEM1 and HadGEM2-AO). The coherent inclusion of runoff by the diagnostic enables model surface freshwater fluxes to be compared directly with observational estimates of precipitation, evaporation and river discharge. We also introduce a normalised metric, based on model-observation RMS differences, to assess the representation of the fluxes by the model. This methodology could be a powerful tool for evaluating model performance during future model development and model intercomparison exercises. Using this diagnostic, and defining the drainage areas from the global river routing model TRIP, we obtain large-scale surface oceanic fluxes from ERA40 and NCAR-NCEP reanalysis data, which we compare with analogous budgets computed from a set of individual observational estimates of evaporation, precipitation and river discharge. The sum of errors in the Hadley Centre climate model in all ocean basins suggests a steady improvement over the three generations of the model. However, an analysis of sources and sinks of water vapour shows common errors in the models, like an excess of evaporation in the tropical-subtropical Atlantic, and a surplus of water vapour export from tropical-subtropical areas to the mid-latitude regions, making the oceanic surface fluxes too fresh at mid latitudes. Errors in the models are consistent with an excessively strong hydrological cycle.     

Validation of model improvements for the GISS GCM

The general circulation model of the NASA/Goddard Institute for Space Studies (GISS GCM) was designed primarily for global climate change and climate sensitivity applications. The modelling group at GISS has developed new and more physically appropriate parameterizations of meteorological/hydrological processes which are being validated in an effort to improve the performance of the Model II version of the GISS GCM. This study discusses some preliminary evaluations of this testing based on multiple-year simulations at 4° latitude by 5° longitude horizontal resolution. These runs individually incorporate new formulations of the planetary boundary layer (PBL), the moist cumulus convection scheme and the ground hydrology and compare results using B-grid and C-grid numerics. The new PBL produces a realistically stronger tropical surface circulation, while the new cumulus scheme generates more realistic distributions of tropical convection and moisture. The main impact of the more sophisticated ground hydrology model is to increase surface air temperatures. Improvements in modelled sea level pressure and rainfall features by the C-grid are somewhat offset by increases in speed excesses at the cores of the summer hemisphere westerly jets. Each modelling innovation targeted a different aspect of the climate not adequately represented by Model II. However, since the various modelling changes were tested individually, the present evaluation could not demonstrate many dramatic improvements in the simulated climates. This documentation of impacts should, however, serve as a benchmark for the validation of future simulations of the GISS GCM that combine all of the modelling improvements.     

Characteristics and spatio-temporal variability of the Amazon River Basin Water Budget

The spatio-temporal variations of the water budget components in the Amazon region are investigated by using a combination of hydrometeorological observations and moisture fluxes derived from the NCEP/NCAR reanalyses, for the period 1970–1999. The key new finding of this study identifies the major differences in the water balance characteristics and variability between the northern and southern parts of the basin. Our results show that there is a seasonality and interannual variability of the water balance that varies across the basin. At interannual time scales, anomalies in the water balance components in the northern Amazon region show relatively stronger links with tropical Pacific interannual variability. Over the entire region, precipitation exceeds evaporation and the basin acts as a sink of moisture (P>E). However, on some occasions the basin can act as a source for moisture (P<E) under extreme conditions, such as those related to deficient rainfall in northern Amazonia during the strong El Niño of 1983. Our estimates of the Amazon regions water balance do not show a closure of the budget, with an average imbalance of almost 50%, suggesting that some of the moisture that converges in the Amazon region is not accounted for. The imbalance is larger over the southern Amazon region than over the northern region, and it also exhibits interannual variability. Large uncertainties are detected in the evaporation and moisture-convergence fields derived from the reanalyses, and in the case of evaporation it can be as large as 10–20% when compared with the few field observations across the basin. Observed precipitation fields derived from station data and from grid-box products also show some discrepancies due to sampling problems and interpolation techniques. The streamflow observed at the mouth of the river is obtained after corrections on the series observed taken at a gauging site almost 200 km inland. However, variability in the evaporation, moisture convergence, and observed rainfall and runoff matches quite well.     

Evapotranspiration in the Don River basin and its variability

Issues concerning the use of earlier developed technique of estimating evapotranspiration from the river basins using observational results from the network of water-and soil-evaporation stations are considered. Some parameters of the calculations and the technique itself are specified. The dynamics of basic elements of the underlying surface of the Don basin participating in the process of evaporation and runoff formation is estimated. It is shown that evapotranspiration and its interannual variability depend on tendencies towards changes in evaporation from the water and land surface, on the one hand, and changes in landscape characteristics, on the other hand. The work continues similar studies started under the leadership of V.S. Golubev in the Volga River basin.     

Hydrologic Sensitivity of Global Rivers to Climate Change

Climate predictions from four state-of-the-art general circulation models (GCMs) were used to assess the hydrologic sensitivity to climate change of nine large, continental river basins (Amazon, Amur, Mackenzie, Mekong, Mississippi, Severnaya Dvina, Xi, Yellow, Yenisei). The four climate models (HCCPR-CM2, HCCPR-CM3, MPI-ECHAM4, and DOE-PCM3) all predicted transient climate response to changing greenhouse gas concentrations, and incorporated modern land surface parameterizations. Model-predicted monthly average precipitation and temperature changes were downscaled to the river basin level using model increments (transient minus control) to adjust for GCM bias. The variable infiltration capacity (VIC) macroscale hydrological model (MHM) was used to calculate the corresponding changes in hydrologic fluxes (especially streamflow and evapotranspiration) and moisture storages. Hydrologic model simulations were performed for decades centered on 2025 and 2045. In addition, a sensitivity study was performed in which temperature and precipitation were increased independently by 2 °C and 10%, respectively, during each of four seasons. All GCMs predict a warming for all nine basins, with the greatest warming predicted to occur during the winter months in the highest latitudes. Precipitation generally increases, but the monthly precipitation signal varies more between the models than does temperature. The largest changes in the hydrological cycle are predicted for the snow-dominated basins of mid to higher latitudes. This results in part from the greater amount of warming predicted for these regions, but more importantly, because of the important role of snow in the water balance. Because the snow pack integrates the effects of climate change over a period of months, the largest changes occur in early to mid spring when snow melt occurs. The climate change responses are somewhat different for the coldest snow dominated basins than for those with more transitional snow regimes. In the coldest basins, the response to warming is an increase of the spring streamflow peak, whereas for the transitional basins spring runoff decreases. Instead, the transitional basins have large increases in winter streamflows. The hydrological response of most tropical and mid-latitude basins to the warmer and somewhat wetter conditions predicted by the GCMs is a reduction in annual streamflow, although again, considerable disagreement exists among the different GCMs. In contrast, for the high-latitude basins increases in annual flow volume are predicted in most cases.