Global Water Budget Research Articles

Hydroclimatology: Perspectives and Applications (review)

Reviewed by: Hydroclimatology: Perspectives and Applications Patrick Kahn (bio) Hydroclimatology: Perspectives and Applications. Marlyn Shelton. Cambridge University Press, 2009 A growing consensus of opinions in the scientific community indicates a warming trend in the climate record, which can be partially attributed to human influences. With the increasing relevance and concern for global warming, understanding the atmosphere-terrestrial dynamic has taken a prominent role in scientific investigations as attempts are made to understand future impacts on society. Hydroclimatology: Perspectives and Applications, by Marlyn Shelton, provides a concise introduction to an interdisciplinary science combining climatology and hydrology. This book provides a substantive analysis of relevant literature from prominent researchers in each field. Through discussion of the fundamental components of the hydroclimate system, data-gathering techniques, and spatial and temporal aspects, the textbook provides the foundation for further in-depth scientific analysis. While this book can be used as a standalone guide to hydroclimatology for those with little previous understanding, background knowledge of climatology and hydrology concepts would aide in easier comprehension. The book’s ten chapters can be broken down into five sections. Chapters one and two provide a framework for hydroclimatology; chapters three through five address methods for measuring and acquiring atmospheric and terrestrial components of the hydroclimate system, including a chapter on the use of remote sensing for data collection. Chapter six introduces fundamental concepts of hydrology and runoff and measurement methods. The fourth section includes chapters seven and eight, which introduce spatial and temporal aspects, and chapters nine and ten discuss two hydroclimate extremes: floods and drought. Within each chapter, suggestions are made [End Page 249] for further detailed reading on specific concepts, solidifying this book’s role as a fundamental course textbook. The first section discusses water as the unifying factor between hydrology and climatology. In the first two chapters, the reader is introduced to the hydrologic cycle, the radiation budget and its role as the engine that drives the hydroclimate system, and the global water budget as primary connections between the atmosphere and terrestrial subsystems. Shelton also offers a brief history of water’s importance throughout human civilization and the emergence of some common measurement methods. Ample diagrams illustrating important processes and notable mathematical expressions are included to further clarify important concepts. The second section, consisting of chapters three, four, and five, presents common methods and instruments for gathering in situ atmospheric and hydrologic data. The functionality of each instrument is defined and augmented with photos and equations where necessary, and each variable’s relevance to the greater hydroclimate system is discussed. Shelton also examines common mathematical models for estimating variables for instances where in situ measurements are otherwise difficult. Chapter five presents essential remote-sensing theory and explains common satellites for data collection. Technological advances in remote sensing have allowed for easier high-resolution data collection, and many relevant studies are referenced throughout the chapter. While a background in remote sensing would be desirable, the author successfully presents important concepts for less-familiar readers. Chapter six, a standalone section, explores hydrologic concepts as they pertain to runoff. Here the reader is presented with the various types and forms of runoff in order to grasp the inherent difficulty in accurately measuring runoff due to its widespread nature. Human reliance on terrestrial water is discussed, thus providing a greater understanding of atmospheric linkages. Section four addresses the spatial and temporal variations in global hydroclimatic processes. Through the liberal use of maps, the reader becomes familiar with the spatial and temporal distributions of notable hydroclimate variables and explanations for their variances. Shelton also discusses various meso-scale atmospheric processes, such as El Niño and the Pacific Decadal Oscillation, and their influence on temporal variations in the hydroclimate. [End Page 250] The final section examines the two hydroclimate extremes: floods and drought. As global temperatures continue to rise, rising global populations may observe an accompanying increase in these potentially catastrophic events. The atmospheric underpinnings of floods and their geographic distributions are discussed in chapter nine, and then case studies of notable past flood events are presented, accompanied by visuals of site-specific atmospheric conditions. In chapter ten, common drought indices are reviewed and two explanations for their causes are...

Estimates of the Global Water Budget and Its Annual Cycle Using Observational and Model Data

Abstract A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. Results are used to provide a new estimate of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979–2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental landmasses, zonal means over land, hemispheric land means, and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high-frequency information taken from atmospheric reanalyses. Results for E − P are contrasted with those from atmospheric moisture budgets based on 40-yr ECMWF Re-Analysis (ERA-40) data. The latter show physically unrealistic results, because evaporation often exceeds precipitation over land, especially in the Tropics and subtropics.

Flooding on California's Russian River: Role of atmospheric rivers

Experimental observations collected during meteorological field studies conducted by the National Oceanic and Atmospheric Administration near the Russian River of coastal northern California are combined with SSM/I satellite observations offshore to examine the role of landfalling atmospheric rivers in the creation of flooding. While recent studies have documented the characteristics and importance of narrow regions of strong meridional water vapor transport over the eastern Pacific Ocean (recently referred to as atmospheric rivers), this study describes their impact when they strike the U.S. West Coast. A detailed case study is presented, along with an assessment of all 7 floods on the Russian River since the experimental data were first available in October 1997. In all 7 floods, atmospheric river conditions were present and caused heavy rainfall through orographic precipitation. Not only do atmospheric rivers play a crucial role in the global water budget, they can also lead to heavy coastal rainfall and flooding, and thus represent a key phenomenon linking weather and climate.

Virtual water trade: an assessment of water use efficiency in the international food trade

Abstract. Amid an increasing water scarcity in many parts of the world, virtual water trade as both a policy instrument and practical means to balance the local, national and global water budget has received much attention in recent years. Building upon the knowledge of virtual water accounting in the literature, this study assesses the efficiency of water use embodied in the international food trade from the perspectives of exporting and importing countries and at the global and country levels. The investigation reveals that the virtual water flows primarily from countries of high crop water productivity to countries of low crop water productivity, generating a global saving in water use. Meanwhile, the total virtual water trade is dominated by green virtual water, which constitutes a low opportunity cost of water use as opposed to blue virtual water. A sensitivity analysis, however, suggests high uncertainties in the virtual water accounting and the estimation of the scale of water saving. The study also raises awareness of the limited effect of water scarcity on the global virtual water trade and the negative implications of the global water saving for the water use efficiency and food security in importing countries and the environment in exporting countries. The analysis shows the complexity in evaluating the efficiency gains in the international virtual water trade. The findings of the study, nevertheless, call for a greater emphasis on rainfed agriculture to improve the global food security and environmental sustainability.

Water saving through international trade of agricultural products

Abstract. Many nations save domestic water resources by importing water-intensive products and exporting commodities that are less water intensive. National water saving through the import of a product can imply saving water at a global level if the flow is from sites with high to sites with low water productivity. The paper analyses the consequences of international virtual water flows on the global and national water budgets. The assessment shows that the total amount of water that would have been required in the importing countries if all imported agricultural products would have been produced domestically is 1605 Gm3/yr. These products are however being produced with only 1253 Gm3/yr in the exporting countries, saving global water resources by 352 Gm3/yr. This saving is 28 per cent of the international virtual water flows related to the trade of agricultural products and 6 per cent of the global water use in agriculture. National policy makers are however not interested in global water savings but in the status of national water resources. Egypt imports wheat and in doing so saves 3.6 Gm3/yr of its national water resources. Water use for producing export commodities can be beneficial, as for instance in Cote d'Ivoire, Ghana and Brazil, where the use of green water resources (mainly through rain-fed agriculture) for the production of stimulant crops for export has a positive economic impact on the national economy. However, export of 28 Gm3/yr of national water from Thailand related to rice export is at the cost of additional pressure on its blue water resources. Importing a product which has a relatively high ratio of green to blue virtual water content saves global blue water resources that generally have a higher opportunity cost than green water.

Effect of gravitational consistency and mass conservation on seasonal surface mass loading models

Increasingly, models of surface mass loads are used either to correct geodetic time coordinates by removing seasonal and other “noise”, or for comparison with other geodetic parameters. However, models of surface loading obtained by simply combining the mass redistribution due to individual phenomena will not in general be self‐consistent, in that (i) the implied global water budget will not be mass‐conserving, and (ii) the modelled sea level will not be an equipotential surface of Earth's total gravity field. We force closure of the global water budget by allowing the “passive” ocean to change in mass. This medium‐term passive ocean response will not be a uniform change in non‐steric ocean surface height, but must necessarily be spatially variable to keep the “passive” ocean surface on an equipotential. Using existing load models, we demonstrate the effects of our consistency theory. Geocenter motion is amplified significantly, by up to 43%.

An approach to using snow areal depletion curves inferred from MODIS and its application to land surface modelling in Alaska

Snowcover areal depletion curves inferred from the moderate resolution imaging spectroradiometer (MODIS) are validated and then applied in NASA’s catchment-based land surface model (CLSM) for numerical simulations of hydrometeorological processes in the Kuparuk River basin (KRB) of Alaska. The results demonstrate that the MODIS snowcover fraction f derived from a simple relationship in terms of the normalized difference snow index compares well with Landsat values over the range 20 � f � 100%. For f< 20%, however, MODIS 500 m subpixel data underestimate the amount of snow by up to 13% compared with Landsat at spatial resolutions of 30 m binned to equivalent 500 m pixels. After a bias correction, MODIS snow areal depletion curves during the spring transition period of 2002 for the KRB exhibit similar features to those derived from surface-based observations. These results are applied in the CLSM subgrid-scale snow parameterization that includes a deep and a shallow snowcover fraction. Simulations of the evolution of the snowpack and of freshwater discharge rates for the KRB over a period of 11 years are then analysed with the inclusion of this feature. It is shown that persistent snowdrifts on the arctic landscape, associated with a secondary plateau in the snow areal depletion curves, are hydrologically important. An automated method is developed to generate the shallow and deep snowcover fractions from MODIS snow areal depletion curves. This provides the means to apply the CLSM subgrid-scale snow parameterization in all watersheds subject to seasonal snowcovers. Improved simulations and predictions of the global surface energy and water budgets are expected with the incorporation of the MODIS snow data into the CLSM. Copyright  2005 John Wiley & Sons, Ltd.

Correction of the High-Latitude Rain Day Anomaly in the NCEP–NCAR Reanalysis for Land Surface Hydrological Modeling

A spurious wavelike pattern in the monthly rain day statistics exists within the National Centers for Environmental Prediction‐National Center for Atmospheric Research (NCEP‐NCAR) reanalysis precipitation product. The anomaly, which is an artifact of the parameterization of moisture diffusion, occurs during the winter months in the Northern and Southern Hemisphere high latitudes. The anomaly is corrected by using monthly statistics from three different global precipitation products from 1) the University of Washington (UW), 2) the Global Precipitation Climate Project (GPCP), and 3) the Climatic Research Unit (CRU), resulting in three slightly different corrected precipitation products. The correction methodology, however, compromises spatial consistency (e.g., storm tracking) on a daily time scale. The effect that the precipitation correction has on the reanalysisderived global land surface water budgets is investigated by forcing the Variable Infiltration Capacity (VIC) land surface model with all four datasets (i.e., the original reanalysis product and the three corrected datasets). The main components of the land surface water budget cycle are not affected substantially; however, the increased spatial variability in precipitation is reflected in the evaporation and runoff components but reduced in the case of soil moisture. Furthermore, the partitioning of precipitation into canopy evaporation and throughfall is sensitive to the rain day statistics of the correcting dataset, especially in the Tropics, and this has implications for the required accuracy of the correcting dataset. The output fields from these long-term land surface simulations provide a global, consistent dataset of water and energy states and fluxes that can be used for model intercomparisons, studies of annual and seasonal climate variability, and comparisons with current versions of numerical weather prediction models.

V: SEA LEVEL: Benefits of GRACE and GOCE to sea level studies

The recently published Third Assessment Reports of the Intergovernmental Panel on Climate Change have underlined the scientific interest in, and practical importance of past and potential future sea level changes. Space gravity missions will provide major benefits to the understanding of the past, and, thereby, in the prediction of future, sea level changes in many ways. The proposal for the GOCE mission described well the improvements to be expected from improved gravity field and geoid models in oceanography (for example, in the measurement of the time-averaged, or ‘steady state’, ocean surface circulation and better estimation of ocean transports), in geophysics (in the improvement of geodynamic models for vertical land movements), in geodesy (in positioning of tide gauge data into the same reference frame as altimeter data, and in improvement of altimeter satellite orbits), and possibly in glaciology (in improved knowledge of bedrock topography and ice sheet mass fluxes). GRACE will make many important steps towards these ‘steady state’ aims. However, its main purpose is the provision of oceanographic (and hydrological and meteorological) temporally-varying gravity information, and should in effect function as a global ‘bottom pressure recorder’, providing further insight into the 3-D temporal variation of the ocean circulation, and of the global water budget in general. This paper summaries several of these issues, pointing the way towards improved accuracy of prediction of future sea level change.

CSE Water and Energy Budgets in the NCEP–DOE Reanalysis II

Abstract During the past several years, the Global Energy and Water Cycle Experiment (GEWEX) continental-scale experiments (CSEs) have started to develop regional hydroclimatological datasets and water and energy budget studies (WEBS). To provide some global background for these regional experiments, the authors describe vertically integrated global and regional water and energy budgets from the National Centers for Environmental Prediction (NCEP)–U.S. Department of Energy (DOE) Reanalysis II (NCEPRII). It is shown that maintaining the NCEPRII close to observations requires some nudging to the short-range model forecast, and this nudging is an important component of analysis budgets. Still, to first order one can discern important hydroclimatological mechanisms in the reanalysis. For example, during summer, atmospheric water vapor, precipitation, evaporation, and surface and atmospheric radiative heating all increase, while the dry static energy convergence decreases almost everywhere over the land region...

Estimate of Precipitation from the Dual-Beam Airborne Radars in TOGA COARE. Part II: Precipitation Efficiency in the 9 February 1993 MCS

Dual-beam airborne Doppler radars are commonly used in convection experiments for their ability to describe the dynamical structure of weather systems. However, instrumental limitations impose the use of wavelengths such as X-band, which are largely attenuated through heavy rain. This paper is the second of a series of two, which aim at developing schemes for attenuation correction. The authors’ final objective is to improve the estimation of precipitation sampled from airborne radars. The first paper was dealing with the application of “differential algorithms” (“stereoradar” and “quad beam”) to the independent retrieval of the specific attenuation and nonattenuated reflectivity, which shed some light on the physics of the precipitation. This second paper develops a more extensive procedure based upon the hybridization of a “differential” and an “integral” algorithm. It is much more flexible than the methods proposed in part one and allows full rainfall-rate retrievals in single aircraft experiments. This procedure is applied to the 9 February mesoscale convective system (MCS) study case from Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE), and the impact of the reflectivity correction on the water budget at the cloud system scale is discussed. As expected, the production of water in the 9 February squall line is maximum below the freezing level and is located in the updraft resulting from the interaction between the warm inflow and rear-to-front cold flow. The authors’ analysis shows that the precipitation efficiency in the convective region of the system is 31%. Therefore, the large majority of water vapor condensed into cloud droplets and ice crystals does not immediately reach the surface as precipitation. It travels toward the rear of the system at the speed of the horizontal air motion, which suggests a large contribution of the stratiform area in the global water budget. The same calculation performed using raw attenuated data (without correcting scheme) gives an efficiency of only 19%. That result points out the importance of the correction for attenuation when measured reflectivities are used in rain-rate retrievals and water budgets.

Vertical structure of humidity and temperature budget residuals over the Mississippi River basin

Analyses have systematic tendencies or budget residuals that are not small. Unlike the budgets in general circulation model simulations, these budget residuals are an important component of analyses’ water and energy budgets. We can either get good budgets or good climates, but not both. To better understand the character of the residuals in the National Centers for Environmental Prediction's (NCEP) global analysis and reanalysis water and energy budgets, the vertical structure of the budget residuals in NCEP's global analyses pertinent to the Mississippi River basin water and energy budgets are described herein. The vertical structure of the residuals is obtained by using NCEP's reanalysis modeling system, which is initialized (started) from NCEP's operational Global Data Assimilation System analyses as well as from the new NCEP reanalyses; forecasts of 1 day duration, 4 times a day, are then made for several years. The budget residuals in the forecasts after the first 6 hours are similar to the original residuals in the analyses and reanalyses. The residuals appear to be related to a summertime “spin‐down” and wintertime “spin‐up” in cumulus convection, an adjustment in boundary layer diffusive transports near the surface and near the top of the boundary layer, and adjustments in large‐scale transports of moisture and temperature. These residuals may thus be providing clues as to what needs to be improved in analysis systems.

Intercomparison of Hydrologic Processes in AMIP GCMs

Abstract Results of an intercomparison study under the Atmospheric Model Intercomparison Project (AMIP) to assess the abilities of 29 global climate models (GCMS) in simulating various aspects of regional and hydrologic processes in response to observed sea surface temperature and sea ice boundary forcings are presented. The authors find that the models generally portray an earthlike climate to approximately 10%–20% of the global land surface temperature (=14.8°C) and global precipitation (=2.3 mm day−1). While a majority of the models have a reasonable global water budget, about a quarter of the models show significant errors in the total global water balance. While the model frequency distributions of heavy precipitation associated with deep convection are in reasonable agreement with observations, a systematic underestimate of the frequency of occurrence of light precipitation events (< 1 mm day−1) is present in almost all the AMIP models, especially over continental desert regions and over tropical an...

Estimation of global annual river runoff based on atmospheric water balance

It is very important to investigate the global water circulation and budget for the understanding and the prediction of the global changes. The distribution of precipitation minus evaporation over the globe is estimated by the atmospheric water balance, using the ECMWF global analysis data for the period 1985 to 1988. The calculated vapor flux convergence -∇H·Q shows good correspondence with former estimations of annual mean evaporation minus precipitation in the meridional profiles. The global annual runoff from land surface to ocean is estimated to be as much as 167 mm/year. -∇H·Q also shows good relationship with the annual runoff of large rivers, especially in the northern hemisphere.

Mean fluxes across sections in the Mediterranean Sea derived from GCM results

The Princeton OGCM is used to simulate the circulation in the Mediterranean Sea. A combination of prognostic and robust diagnostic techniques are applied to Levitus' (1982) data set in order to adjust the initial data to model physics and to restore to some extent the most general climatic features in salinity and temperature close to their climatic values. The simulated data are used to calculate water, salt and heat transport across sections in the Western Mediterranean Sea. Model estimates are compared with the estimates based on global water and salt budget analysis. It is shown that a strong correlation exists between vertical density anomaly and the baroclinic velocity when the two layer structure is well developed and in particular when the deep current direction is opposite to the direction of the surface current. For the Mediterranean Sea models, a formulation of open boundary conditions at the Straits is also discussed.

Oceanic freshwater transport during the Last Glacial Maximum

Annual mean freshwater transports during the last ice age are calculated using data from a global atmospheric general circulation model. The atmospheric model is used to simulate the climate of 18,000 years ago using Climate: Long‐Range Investigation, Mapping, and Prediction data for the model's seasonally varying sea surface temperatures, ice coverage, and surface topography. The water transports are calculated for each ocean basin using the model's evaporation, precipitation, and runoff. For the present climate, the model's glacial ice sheets and the global water budget are near equilibrium. For the ice age the model does not maintain the glacial ice sheets at steady state but causes them to melt; therefore we reduced the runoff from melting ice to bring the global water budget into equilibrium. Recent studies have shown that salinity anomalies in the North Atlantic caused by changes in the freshwater transport could change the thermohaline circulation. For the present climate the evaporation exceeds the sum of precipitation and runoff in the Atlantic Ocean which leads to a higher mean salinity in the Atlantic than in the Pacific Ocean. The results suggest that this water budget reversed in the Atlantic during the last ice age. This conclusion, however, cannot be stated firmly because the model's global water budget was not in equilibrium for the ice age. The model's freshwater transports for the last ice age are compared with those for the present climate, and implications on changes in thermohaline circulation are discussed.

Variations in the Global Water Budget

Variations in the Global Water Budget

Variations in the global water budget

Variations in the global water budget