Continental Water Balance Research Articles

Analysis of planetary boundary layer feedbacks on the multiple equilibria of the continental water cycle

A simple soil-atmosphere interaction model is developed to study the bimodal distribution of soil moisture in regions where there is feedback between precipitation and soil moisture. Our study includes planetary boundary layer (PBL) feedback in the coupled analytical soil-atmosphere interaction model with a varying PBL height and entrainment from the free atmosphere. This study aims to reveal the dominant PBL feedbacks that impact the occurrence of soil moisture bimodality by comparing the evolution of prognostic variables for a detailed PBL scheme (with varying PBL height and entrainment of heat and moisture from the free atmosphere) with the simple PBL scheme (with fixed PBL height and no entrainment). This study has shown the existence of multiple equilibria in the continental water balance for both detailed PBL and simple PBL schemes when the model is integrated for different initial soil moisture up to equilibria. Although both with and without PBL feedback, the results are in qualitative agreement; the soil moisture initialization to realize multiple equilibria of a detailed PBL scheme is larger than the simple PBL scheme. Although the direct quantitative comparison with observation is clearly limited in the simple model, it also demonstrates how the non-linear dynamics of the coupled soil-atmosphere system may be expected to occur in observations.

Satellite soil moisture data assimilation for improved operational continental water balance prediction

Abstract. A simple and effective two-step data assimilation framework was developed to improve soil moisture representation in an operational large-scale water balance model. The first step is a Kalman-filter-type sequential state updating process that exploits temporal covariance statistics between modelled and satellite-derived soil moisture to produce analysed estimates. The second step is to use analysed surface moisture estimates to impart mass conservation constraints (mass redistribution) on related states and fluxes of the model using tangent linear modelling theory in a post-analysis adjustment after the state updating at each time step. In this study, we assimilate satellite soil moisture retrievals from both Soil Moisture Active Passive (SMAP) and Soil Moisture and Ocean Salinity (SMOS) missions simultaneously into the Australian Water Resources Assessment Landscape model (AWRA-L) using the proposed framework and evaluate its impact on the model's accuracy against in situ observations across water balance components. We show that the correlation between simulated surface soil moisture and in situ observation increases from 0.54 (open loop) to 0.77 (data assimilation). Furthermore, indirect verification of root-zone soil moisture using remotely sensed Enhanced Vegetation Index (EVI) time series across cropland areas results in significant improvements from 0.52 to 0.64 in correlation. The improvements gained from data assimilation can persist for more than 1 week in surface soil moisture estimates and 1 month in root-zone soil moisture estimates, thus demonstrating the efficacy of this data assimilation framework.

Variability of Trends in Precipitation across the Amazon River Basin Determined from the CHIRPS Precipitation Product and from Station Records

The Amazon River Basin is the largest rainforest in the world. Long-term changes in precipitation trends in the basin can affect the continental water balance and the world’s climate. The precipitation trends in the basin are not spatially uniform; estimating these trends only at locations where station data are available has an inherent bias. In the present research, the spatially distributed annual precipitation trends were studied in the Amazon River Basin from the year 1981 to 2017 using the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) product. The precipitation trends were also cross-validated at locations where station data were available. The research also identifies clusters within the basin where trends showed a larger increase (nine clusters) or decrease in precipitation (10 clusters). The overall precipitation trend in the Amazon River Basin over 37 years showed a 2.8 mm/year increase, with a maximum of 45.1 mm/year and minimum of −37.9 mm/year. The highest positive cluster was in Cuzco in the Ucayali River basin, and the lowest negative was in Santa Cruz de la Sierra, in the upstream Madeira River basin. The total volume of the incoming precipitation was 340,885.1 km3, with a withdrawal of −244,337.1 km3. Cross-validation was performed using 98 in situ stations with more than 20 years of recorded data, obtaining an R2 of 0.981, a slope of 1.027, and a root mean square error (RMSE) of 363.6 mm/year. The homogeneous, standardized, and continuous long-term time series provided by CHIRPS is a valuable product for basins with a low-density network of stations such as the Amazon Basin.

Climate change impacts on South American water balance from a continental-scale hydrological model driven by CMIP5 projections

South America contributes to roughly 30% of global runoff to the oceans. Because the regional economy and biodiversity depend significantly on its water resources, assessing potential climate change impacts on the continental water balance is crucial to support water management planning. Here we evaluate the mean alterations of water balance variables and river discharge in South America by the end of this century using two different GHG scenarios (RCP4.5 and RCP8.5). An ensemble comprising 25 global climate models (GCM) from CMIP5 is used to force a continental-scale hydrologic-hydrodynamic model developed for that region. A negative signal with respect to changes in precipitation, evapotranspiration, and runoff is observed on most of the continent. Major decreases in the annual mean discharge are expected for the Orinoco, Tocantins, and Amazon basins, which would be around 8–14% at least (statistically significant – RCP4.5 and RCP8.5, respectively). Only the Uruguay Basin presents a positive trend for the mean discharge.

Impact of climate forcing uncertainty and human water use on global and continental water balance components

Abstract. The assessment of water balance components using global hydrological models is subject to climate forcing uncertainty as well as to an increasing intensity of human water use within the 20th century. The uncertainty of five state-of-the-art climate forcings and the resulting range of cell runoff that is simulated by the global hydrological model WaterGAP is presented. On the global land surface, about 62 % of precipitation evapotranspires, whereas 38 % discharges into oceans and inland sinks. During 1971–2000, evapotranspiration due to human water use amounted to almost 1 % of precipitation, while this anthropogenic water flow increased by a factor of approximately 5 between 1901 and 2010. Deviation of estimated global discharge from the ensemble mean due to climate forcing uncertainty is approximately 4 %. Precipitation uncertainty is the most important reason for the uncertainty of discharge and evapotranspiration, followed by shortwave downward radiation. At continental levels, deviations of water balance components due to uncertain climate forcing are higher, with the highest discharge deviations occurring for river discharge in Africa (−6 to 11 % from the ensemble mean). Uncertain climate forcings also affect the estimation of irrigation water use and thus the estimated human impact of river discharge. The uncertainty range of global irrigation water consumption amounts to approximately 50 % of the global sum of water consumption in the other water use sector.

Variations of global and continental water balance components as impacted by climate forcing uncertainty and human water use

Abstract. When assessing global water resources with hydrological models, it is essential to know about methodological uncertainties. The values of simulated water balance components may vary due to different spatial and temporal aggregations, reference periods, and applied climate forcings, as well as due to the consideration of human water use, or the lack thereof. We analyzed these variations over the period 1901–2010 by forcing the global hydrological model WaterGAP 2.2 (ISIMIP2a) with five state-of-the-art climate data sets, including a homogenized version of the concatenated WFD/WFDEI data set. Absolute values and temporal variations of global water balance components are strongly affected by the uncertainty in the climate forcing, and no temporal trends of the global water balance components are detected for the four homogeneous climate forcings considered (except for human water abstractions). The calibration of WaterGAP against observed long-term average river discharge Q significantly reduces the impact of climate forcing uncertainty on estimated Q and renewable water resources. For the homogeneous forcings, Q of the calibrated and non-calibrated regions of the globe varies by 1.6 and 18.5 %, respectively, for 1971–2000. On the continental scale, most differences for long-term average precipitation P and Q estimates occur in Africa and, due to snow undercatch of rain gauges, also in the data-rich continents Europe and North America. Variations of Q at the grid-cell scale are large, except in a few grid cells upstream and downstream of calibration stations, with an average variation of 37 and 74 % among the four homogeneous forcings in calibrated and non-calibrated regions, respectively. Considering only the forcings GSWP3 and WFDEI_hom, i.e., excluding the forcing without undercatch correction (PGFv2.1) and the one with a much lower shortwave downward radiation SWD than the others (WFD), Q variations are reduced to 16 and 31 % in calibrated and non-calibrated regions, respectively. These simulation results support the need for extended Q measurements and data sharing for better constraining global water balance assessments. Over the 20th century, the human footprint on natural water resources has become larger. For 11–18% of the global land area, the change of Q between 1941–1970 and 1971–2000 was driven more strongly by change of human water use including dam construction than by change in precipitation, while this was true for only 9–13 % of the land area from 1911–1940 to 1941–1970.

지표수문해석모형을 활용한 동아시아 유출해석 및 평가

본 연구에서는 지표수문해석모형을 활용한 동아시아 지역의 유출해석을 수행하고 그 적용성을 평가하였다. 이에 전지구자료를 수집한 후 모형의 입력자료로 재구성하였으며, 모의 결과의 검증을 위해 GRDC에서 제공하는 국외 34개 지점의 관측유량자료를 수집하였다. 쾨펜의 기후대 구분을 통한 매개변수 전이 방법을 이용하여 미계측지역의 유출 매개변수를 결정하였으며, 동아시아지역에 유출해석을 수행하였다. 그 결과 미계측지역으로 가정한 17개 유역에서 모의치가 관측치와 유사하게 거동하는 것으로 나타나 결과의 신뢰성이 높음을 확인하였다. 최종 결정된 매개변수로부터 동아시아 전역에 수문성분을 산정하였으며, 대체로 동아시아 지역의 국가들의 수문성분 거동은 계절별로 유사한 것으로 확인되었다. 또한, 중국의 남부지역, 일본 및 대만은 동아시아 내에서도 유출의 발생이 가장 높은 반면, 몽골 및 중국 북부지역에서는 매우 낮은 것으로 분석되었다. The objective of this study is to evaluate the applicability of Land Surface Model (LSM) for estimating the runoff on East Asia. Global geographical and weather data are used as input for the model and for the model verification, the simulated runoff results are compared with observed data from 34 global observation stations provided by Global Runoff Data Center (GRDC). K<TEX>$\ddot{o}$</TEX>ppen's climate zone is used to calculate the model parameter for ungaged basins. As a result, the simulated runoff shows good performance comparing with observed data in 17 basins assumed as ungaged basins. The Hydrologic components on East Asia area are estimated from the model and the continental water balance components are seasonally similar to each country. Also, it reveals that runoffs from southern China, Japan and Taiwan are much higher than those from mongolian and northern China.

Actual evapotranspiration estimation by ground and remote sensing methods: the Australian experience

AbstractOn average, Australia is a dry continent with many competing uses for water. Hence, there is an urgent need to know actual evapotranspiration (ETa) patterns across wide areas of agricultural and natural ecosystems, as opposed to just point measurements of ETa. The Australian Government has tasked the science agencies with operationally developing monthly and annual estimates of ETa and other hydrological variables, and with forecasting water availability over periods of days to decades, as part of its national water assessment programme. To meet these needs, Australian researchers have become leaders in developing large‐area methods for estimating ETa at regional and continental scales. Ground methods include meteorological models, eddy covariance towers, sap flow sensors and catchment water balance models. Remote sensing methods use thermal infrared, mid infrared and/or vegetation indices usually combined with meteorological data to estimate ETa. Ground and remote sensing ETa estimates are assimilated into the Australian Water Resource Assessment, which issues annual estimates of the state of the continental water balance for policy and planning purposes. The best ETa models are estimated to have an error or uncertainty of 10% to 20% in Australia. Developments in Australian ETa research over the past 20 years are reviewed, and sources of error and uncertainty in current methods and models are discussed. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Outlier identification and correction for GRACE aggregated data

Gravity measurements within the Gravity Recovery and Climate Experiment (GRACE) provide a direct measure of monthly changes in mass over the Earth’s land masses. As such changes in mass mainly correspond to water storage changes, these measurements allow to close the continental water balance on large spatial scales and on a monthly time scale within the respective error bounds. When quantifying uncertainties, positive and negative peaks are detected in GRACE aggregated monthly time series (from different data providers) that do not correspond to hydrological or hydro-meteorological signals. These peaks must be interpreted as outliers, which carry the danger of signal degradation. In this paper an algorithm is developed to identify outliers and replace them with hydrologically plausible values. The algorithm is based on a statistical approach in which hydrological and hydro-meteorological signals are used to control the algorithm. The procedure of outlier detection is verified by evaluating catchment based aggregated GRACE monthly signals with ground truth from hydrology and hydro-meteorological signals. The results show improvement in the correlation of GRACE versus hydrometeorological and hydrological signals in most catchments. Also, the noise level is significantly reduced over 255 largest catchments.

Contributions of the International Atomic Energy Agency to the development and practice of isotope hydrology

The International Atomic Energy Agency (IAEA) has played a crucial role in promoting and expanding the use of isotopes in hydrology over the last five decades to gain unique insights into the functioning of terrestrial and atmospheric water cycles. In this essay, an historical perspective of IAEA activities is provided, along with some of the achievements and recent initiatives. When the IAEA was formally established in 1957, radioactive fallout from atmospheric thermonuclear testing, particularly tritium, and its impact on human health and the environment posed serious societal concerns. This led the IAEA to initiate a global project to measure tritium concentrations in precipitation. Brian Payne, first Head of the Isotope Hydrology Section of IAEA, convened a scientific advisory committee in 1959 to define the scope of hydrological aspects of tritium monitoring in precipitation. The committee of three eminent scientists — Fritz Begemann, Erik Eriksson and Hans E. Suess — emphasized that, based on contemporary publications, tritium and stable isotopes of oxygen and hydrogen in precipitation would be useful for studying many watercycle processes, including water-residence time and evaporation losses from lakes and reservoirs, groundwater age and aquifer dynamics, continental water balance, age of hot spring water, and mixing time of the oceans (e.g. Friedman 1953; Epstein and Mayeda 1953; Dansgaard 1953; Urey et al. 1951; Libby 1953). The IAEA’s precipitation monitoring efforts began in 1960, and were later expanded to a global scale in 1961 by collaboration with the World Meteorological Organisation (WMO). This expanded network became known as the IAEA-WMO Global Network for Isotopes in Precipitation (GNIP). The number of operational GNIP stations has varied over the years. Originally 161 stations were in operation. By 1963–1964 there was a maximum of about 220 stations, and currently 185 stations in 53 countries contribute samples. In its 50 years of existence, the GNIP database has gained about 120,000 monthly records of stable oxygen and hydrogen isotopes and tritium from more than 900 meteorological stations worldwide. Dansgaard (1964) conducted a comprehensive analysis of stable isotope data from the GNIP and proposed key relationships between precipitation isotopes and climate. In particular, isotope correlations between surface air temperature and amount of precipitation were crucial to interpreting palaeoclimate records preserved in proxy archives such as ice cores, lake sediments, groundwater, tree rings and speleothems. Stable isotope data from the GNIP continue to be extensively utilized in climate research. Most general circulation models currently in use have been expanded to simulate precipitation isotopes (Joussaume et al. 1984; Jouzel et al. 1997; Hoffmann et al. 2000) and GNIP data provide a means to validate these models. A number of IAEA publications have reported on the statistical evaluations of precipitation isotope distributions (e.g., Yurtsever and Gat 1981; Rozanski et al. 1991, 1993; Aggarwal et al. 2010). Figure 1 shows all GNIP stations past and present, and a global map of interpolated stable oxygen isotope data (1961–2004) using active and inactive stations. The continuous operation of GNIP has also been critical for the use of tritium in hydrology, hydrogeology, and hydrometeorology. In addition to tritium in precipitation, tritium in rivers, lakes and groundwater has yielded invaluable information on the dynamic aspects of the water cycle such as the residence time of water in the different hydrologic reservoirs and hydrograph separation in stream flow. Similarly, stable isotope data in precipitation, surface water and groundwater were invaluable for characterizing the role of groundwater in streams, rivers and lakes, the geographic and temporal origin of groundwater recharge and its effect on the water balance of hydrologic systems, especially in wetlands and evaporation prone areas. Due to the very nature of the methodology, isotope hydrology’s emphasis has been Received: 4 March 2010 /Accepted: 28 August 2010 Published online: 24 September 2010

Irrigation increases moisture recycling and climate feedback

Existing research in soil moisture atmosphere feedbacks suggests that large irrigation areas are likely to have an impact on local precipitation, due to increased levels of soil moisture. However, some of the past research attempting to prove this has been inconclusive. Using a simplified analytical model, the influence of different land uses, climatic conditions and length scales of irrigation areas is investigated to identify when such effects have an impact on local precipitation. The results indicate a modest recycling ratio of about 10–20% from irrigated areas, but this is governed by the length scale of the irrigation area, the magnitude of the externally derived precipitation and the magnitude of the atmospheric moisture flux. This suggests irrigation areas only modestly intensify the continental water balance. In contrast, recycling coefficients of oceanic moisture are found to be much higher for Australia (around 95%) as lateral oceanic moisture fluxes are low at the continental scale, indicating much stronger feedback. However, this is governed by the scale of analysis. At the local scale, overall atmospheric moisture fluxes are higher, resulting in lower feedback. In addition, the moisture feedback varies between wet and dry years, with stronger effects in wetter years when external moisture supply is higher. This variation explains the lack of clearly observed increases in irrigation induced precipitation in field studies.

Hot and cool summers: Multiple equilibria of the continental water cycle

Large variations in soil water reserves and surface temperature over the continents are linked to a positive feedback between precipitation and soil moisture. This mechanism can generate bimodal distributions of soil moisture. Here, we show that bimodality results from the existence of multiple equilibria in the continental water balance, considering the coupled system including the upper soil layer and the atmospheric planetary boundary layer. This mechanism is described with an idealized box model, that includes convergence and divergence of moisture fluxes, convection, precipitation and evapotranspiration. The existence of two equilibria is associated with the variation of precipitation efficiency, which depends on convection intensity. The two regimes correspond to realistic values of climatic variables associated with mean wet or dry summers, and can persist for the whole summer season when forced by a stochastic moisture convergence flux. This suggests that a key role for midlatitude continental summer climate is played by the continental soil water content.

Spacebased observations of oceanic influence on the annual variation of South American water balance

The mass change of South America (SA) continent measured by the Gravity Recovery and Climate Experiment (GRACE) imposes a constraint on the uncertainties in estimating the annual variation of rainfall measured by Tropical Rain Measuring Mission (TRMM) and ocean moisture influx derived from QuikSCAT data. The approximate balance of the mass change rate with the moisture influx less climatological river discharge, in agreement with the conservation principle, bolsters not only the credibility of the spacebased measurements, but supports the characterization of ocean's influence on the annual variation of continental water balance. The annual variation of rainfall is found to be in phase with the mass change rate in the Amazon and the La Plata basins, and the moisture advection across relevant segments of the Pacific and Atlantic coasts agrees with the annual cycle of rainfall in the two basins and the Andes mountains.

Snow Mass over North America: Observations and Results from the Second Phase of the Atmospheric Model Intercomparison Project

AbstractEighteen global atmospheric general circulation models (AGCMs) participating in the second phase of the Atmospheric Model Intercomparison Project (AMIP-2) are evaluated for their ability to simulate the observed spatial and temporal variability in snow mass, or water equivalent (SWE), over North America during the AMIP-2 period (1979–95). The evaluation is based on a new gridded SWE dataset developed from objective analysis of daily snow depth observations from Canada and the United States with snow density estimated from a simple snowpack model. Most AMIP-2 models simulate the seasonal timing and the relative spatial patterns of continental-scale SWE fairly well. However, there is a tendency to overestimate the rate of ablation during spring, and significant between-model variability is found in every aspect of the simulations, and at every spatial scale analyzed. For example, on the continental scale, the peak monthly SWE integrated over the North American continent in AMIP-2 models varies between ±50% of the observed value of ∼1500 km3. The volume of water in the snowpack, and the magnitudes of model errors, are significant in comparison to major fluxes in the continental water balance. It also appears that the median result from the suite of models tends to do a better job of estimating climatological mean features than any individual model. Year-to-year variations in large-scale SWE are only weakly correlated to observed variations, indicating that sea surface temperatures (specified from observations as boundary conditions) do not drive interannual variations of SWE in these models. These results have implications for simulations of the large-scale hydrologic cycle and for climate change impact assessments.

A 53‐year forcing data set for land surface models

As most variables describing the state of the surface are not directly observable, we have to use land surface models in order to reconstruct an estimate of their evolution. These large‐scale land surface models often require high‐quality forcing data with a subdiurnal sampling. Building these data sets is a major challenge but an essential step for estimating the land surface water budget, which is a crucial part of climate change prediction. To study the interannual variability of surface conditions over the last half century, we have built a 53‐year forcing data set, named NCC. NCC has a 6‐hourly time step from 1948 to 2000 and a spatial resolution of 1° × 1°. It is based on the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis project and a number of independent in situ observations. In this study we show the adjustments which need to be applied to the reanalysis and how they impact the simulated continental water balance. The model outputs are validated with the observed discharges of the world's 10 largest rivers to estimate the combined errors of the forcing data and the land surface model. The seasonal and interannual variations of these discharges are used for this validation. Five numerical experiments have been carried out. They used the forcing data sets obtained after each step of data adjustment and the forcing of the Global Soil Wetness Project 2 as inputs for the Organizing Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) land surface model. The quality of forcing data is improved after each adjustment. The precipitation correction gives the most important improvement in the simulated river discharges, while the temperature correction has a significant effect only at high latitudes. The radiation correction also improves the forcing quality, especially in term of discharge amplitude. The NCC forcing data set can be used to study the water budget over many areas and catchment basins that have not been yet analyzed in this study. With its period of 53 years, NCC can also be used to evaluate the trends of terrestrial water storage in particular regions.

Contribution of climate-driven change in continental water storage to recent sea-level rise.

Using a global model of continental water balance, forced by interannual variations in precipitation and near-surface atmospheric temperature for the period 1981-1998, we estimate the sea-level changes associated with climate-driven changes in storage of water as snowpack, soil water, and ground water; storage in ice sheets and large lakes is not considered. The 1981-1998 trend is estimated to be 0.12 mm/yr, and substantial interannual fluctuations are inferred; for 1993-1998, the trend is 0.25 mm/yr. At the decadal time scale, the terrestrial contribution to eustatic (i.e., induced by mass exchange) sea-level rise is significantly smaller than the estimated steric (i.e., induced by density changes) trend for the same period, but is not negligibly small. In the model the sea-level rise is driven mainly by a downtrend in continental precipitation during the study period, which we believe was generated by natural variability in the climate system.

Oscillations of the water balance during the Holocene in interior Central Europe—features, dating and consequences

The continental water balance depends on precipitation and evaporation, with the latter due to temperature. Moreover, both the components of weather and climate are influenced by the annual seasonality. All these factors have changed during the last millennia, and periods of significant moisture have alternated with ones of low moisture. Evidence of these oscillations results from geostratigraphical subdivision of sequences of freshwater lime deposits and from lake level changes of inland lakes, but also from many archaeological observations concerning settlement distribution. All these observations reflect oscillations that can be correlated through archaeological and other dating. Alternation of drier and moister periods in the course of the Central European Holocene can be recognized.

Northern Hemisphere Ice-Sheet Influences on Global Climate Change

Large ice sheets actively interact with the rest of the climate system by amplifying, pacing, and potentially driving global climate change over several time scales. Direct and indirect influences of ice sheets on climate cause changes in ocean surface temperatures, ocean circulation, continental water balance, vegetation, and land-surface albedo, which in turn cause additional feedbacks in the climate system and help to synchronize global climate change. The effect of the underlying geological substrate on ice-sheet dynamics may be the missing link in understanding the ice sheet–climate interactions that are integral to the middle Pleistocene transition; the 100,000-year climate cycle; high-amplitude, millennial-scale climate variability; and low–aspect ratio ice sheets of the Last Glacial Maximum.

Atmospheric water vapor transport and continental hydrology over the Americas

The advective transport of atmospheric water vapor, its role in global hydrology, and the water balance of continental regions are discussed and explored. A data set consisting of 10 years of global wind and humidity observations that has been interpolated onto a regular grid is used to estimate atmospheric water vapor fluxes across the boundaries of selected continental regions. The flux of water vapor from the waters surrounding the Americas is computed and graphically represented as integrated normal flux across meridional and zonal segments. The total flux is decomposed into transport by mean motion and transport by correlated transient anomalies in the wind and humidity fields. Seasonal maps of these observations for the Americas reveal the underlying mechanisms and illustrate the characterizing features of the hydrologic cycle and continental water balance for this large land mass.

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.