Ref. posted
17 April 2013
1 note

Terrestrial water fluxes dominated by transpiration

Jasechko,S., Z.D. Sharp, J.J. Gibson, S.J. Birks, Y. Yi, and P.J. Fawcett, 2013: “Terrestrial water fluxes dominated by transpiration.” Nature, v. 496, pp. 347-350, doi: 10.1038/nature11983.

Renewable fresh water over continents has input from precipitation and losses to the atmosphere through evaporation and transpiration. Global-scale estimates of transpiration from climate models are poorly constrained owing to large uncertainties in stomatal conductance and the lack of catchment-scale measurements required for model calibration, resulting in a range of predictions spanning 20 to 65 per cent of total terrestrial evapotranspiration (14,000 to 41,000 km3 per year). Here we use the distinct isotope effects of transpiration and evaporation to show that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration. On the basis of our analysis of a global data set of large lakes and rivers, we conclude that transpiration recycles 62,000 ± 8,000 km3 of water per year to the atmosphere, using half of all solar energy absorbed by land surfaces in the process. We also calculate CO2 uptake by terrestrial vegetation by connecting transpiration losses to carbon assimilation using water-use efficiency ratios of plants, and show the global gross primary productivity to be 129 ± 32 gigatonnes of carbon per year, which agrees, within the uncertainty, with previous estimates. The dominance of transpiration water fluxes in continental evapotranspiration suggests that, from the point of view of water resource forecasting, climate model development should prioritize improvements in simulations of biological fluxes rather than physical (evaporation) fluxes.

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Tags: water cycle forests evaporation transpiration precipitation soil moisture moisture transport carbon cycle water resources hydroclimatology climate modeling

Ref. posted
08 March 2013
1 note

Irrigation in California’s Central Valley strengthens the southwestern U.S. water cycle

Lo, M.-H., and J.S. Famiglietti, 2013: “Irrigation in California’s Central Valley strengthens the southwestern U.S. water cycle.” Geophysical Research Letters, v. 40, pp. 301–306, doi: 10.1002/grl.50108.

Characterizing climatological and hydrological responses to agricultural irrigation continues to be an important challenge to understanding the full impact of water management on the Earth’s environment and hydrological cycle. In this study, we use a global climate model, combined with realistic estimates of regional agricultural water use, to simulate the local and remote impacts of irrigation in California’s Central Valley. We demonstrate a clear mechanism that the resulting increase in evapotranspiration and water vapor export significantly impacts the atmospheric circulation in the southwestern United States, including strengthening the regional hydrological cycle. We also identify that irrigation in the Central Valley initiates a previously unknown, anthropogenic loop in the regional hydrological cycle, in which summer precipitation is increased by 15%, causing a corresponding increase in Colorado River streamflow of ~30%. Ultimately, some of this additional streamflow is returned to California via managed diversions through the Colorado River aqueduct and the All-American Canal.

Open Access

Tags: water cycle precipitation irrigation agriculture moisture transport soil moisture open access

Ref. posted
11 November 2012

Oceanic and terrestrial sources of continental precipitation

Gimeno, L., A. Stohl, R.M. Trigo, F. Dominguez, K. Yoshimura, L. Yu, A. Drumond, A.M. Durán-Quesada, and R. Nieto, 2012: “Oceanic and terrestrial sources of continental precipitation.” Reviews of Geophysics, v. 50, paper no. RG4003, doi: 10.1029/2012RG000389.

The most important sources of atmospheric moisture at the global scale are herein identified, both oceanic and terrestrial, and a characterization is made of how continental regions are influenced by water from different moisture source regions. The methods used to establish source-sink relationships of atmospheric water vapor are reviewed, and the advantages and caveats associated with each technique are discussed. The methods described include analytical and box models, numerical water vapor tracers, and physical water vapor tracers (isotopes). In particular, consideration is given to the wide range of recently developed Lagrangian techniques suitable both for evaluating the origin of water that falls during extreme precipitation events and for establishing climatologies of moisture source-sink relationships. As far as oceanic sources are concerned, the important role of the subtropical northern Atlantic Ocean provides moisture for precipitation to the largest continental area, extending from Mexico to parts of Eurasia, and even to the South American continent during the Northern Hemisphere winter. In contrast, the influence of the southern Indian Ocean and North Pacific Ocean sources extends only over smaller continental areas. The South Pacific and the Indian Ocean represent the principal source of moisture for both Australia and Indonesia. Some landmasses only receive moisture from the evaporation that occurs in the same hemisphere (e.g., northern Europe and eastern North America), while others receive moisture from both hemispheres with large seasonal variations (e.g., northern South America). The monsoonal regimes in India, tropical Africa, and North America are provided with moisture from a large number of regions, highlighting the complexities of the global patterns of precipitation. Some very important contributions are also seen from relatively small areas of ocean, such as the Mediterranean Basin (important for Europe and North Africa) and the Red Sea, which provides water for a large area between the Gulf of Guinea and Indochina (summer) and between the African Great Lakes and Asia (winter). The geographical regions of Eurasia, North and South America, and Africa, and also the internationally important basins of the Mississippi, Amazon, Congo, and Yangtze Rivers, are also considered, as is the importance of terrestrial sources in monsoonal regimes. The role of atmospheric rivers, and particularly their relationship with extreme events, is discussed. Droughts can be caused by the reduced supply of water vapor from oceanic moisture source regions. Some of the implications of climate change for the hydrological cycle are also reviewed, including changes in water vapor concentrations, precipitation, soil moisture, and aridity. It is important to achieve a combined diagnosis of moisture sources using all available information, including stable water isotope measurements. A summary is given of the major research questions that remain unanswered, including (1) the lack of a full understanding of how moisture sources influence precipitation isotopes; (2) the stationarity of moisture sources over long periods; (3) the way in which possible changes in intensity (where evaporation exceeds precipitation to a greater of lesser degree), and the locations of the sources, (could) affect the distribution of continental precipitation in a changing climate; and (4) the role played by the main modes of climate variability, such as the North Atlantic Oscillation or the El Niño–Southern Oscillation, in the variability of the moisture source regions, as well as a full evaluation of the moisture transported by low-level jets and atmospheric rivers.

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Tags: precipitation hydrologic cycle water cycle hydroclimatology

Ref. posted
24 October 2012

Ocean salinities reveal strong global water cycle intensification during 1950 to 2000

Durack, P.J., S.E. Wijffels, and R.J. Matear, 2012: “Ocean salinities reveal strong global water cycle intensification during 1950 to 2000.” Science, v. 336, doi: 10.1126/science.1212222.

Fundamental thermodynamics and climate models suggest that dry regions will become drier and wet regions will become wetter in response to warming. Efforts to detect this long-term response in sparse surface observations of rainfall and evaporation remain ambiguous. We show that ocean salinity patterns express an identifiable fingerprint of an intensifying water cycle. Our 50-year observed global surface salinity changes, combined with changes from global climate models, present robust evidence of an intensified global water cycle at a rate of 8 +/- 5% per degree of surface warming. This rate is double the response projected by current-generation climate models and suggests that a substantial (16 to 24%) intensification of the global water cycle will occur in a future 2° to 3° warmer world.

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Tags: climate change water cycle hydrologic cycle hydroclimatology oceanography evaporation

Ref. posted
13 September 2012

Water security for a planet under pressure: interconnected challenges of a changing world call for sustainable solutions

Bogardi, J.J., D. Dudgeon, R. Lawford, E. Flinkerbusch, A. Meyn, C. Pahl-Wostl, K. Vielhauer, and C. Vörösmarty, 2012: “Water security for a planet under pressure: interconnected challenges of a changing world call for sustainable solutions.” Current Opinion in Environmental Sustainability, v. 4, pp. 35-43, doi: 10.1016/j.cosust.2011.12.002.

Sustainability, equitable allocation and protection of water resources must occur within the framework of integrated management and water governance, but its implementation is problematic. Ongoing global climate change, increasing population, urbanization, and aspirations for better living standards present a challenge to the planetary sustainability. While water use at global scale currently seems to be within its planetary boundary, shortages prevail in several water-scarce and overpopulated regions, and are projected to increase. Furthermore large-scale impoverishment of aquatic biodiversity, ecosystem degradation and reductions in water quality are unaddressed ‘side effects’ in areas where water can be secured for human and economic uses. As the world prepares for Rio+20, challenges to the sustainability of global water security should be scrutinized. Of particular concern is the likelihood that the water-related Millennium Development Goals (MDGs) targets may not be achievable due to lack of funding commitments, and a failure of delivery mechanisms including water governance. Constraints on water availability and reductions in water quality jeopardize secure access to this resource for all legitimate stakeholders, including aquatic and terrestrial ecosystems. Water connects several socio-ecological, economic and geophysical systems at multiple scales and hence constitutes a ‘global water system’. This should be considered both in technical interventions and in governance frameworks. Humans have been changing the global water system in globally significant ways since the industrial revolution, yet without adequate knowledge of the system and its response to change; and without sufficient understanding of how to govern the system at local and global scales. Water security in the 21st century will require better linkage of science and policy, as well as innovative and cross-sectoral initiatives, adaptive management and polycentric governance models that involve all stakeholders. Consensus solutions will need to be achieved by evidence-based mediation, rather than following untested ‘panaceas’, so as to ensure equitable and sustainable global water use.

Open Access

Tags: water resources water supply water cycle water security open access

Ref. posted
08 September 2012
1 note

Climate extremes and climate change: The Russian heat wave and other climate extremes of 2010

Trenberth, K.E., and J.T. Fasullo, 2012: “Climate extremes and climate change: The Russian heat wave and other climate extremes of 2010.” Journal of Geophysical Research, v. 117, paper no. D17103, doi: 10.1029/2012JD018020.

A global perspective is developed on a number of high impact climate extremes in 2010 through diagnostic studies of the anomalies, diabatic heating, and global energy and water cycles that demonstrate relationships among variables and across events. Natural variability, especially ENSO, and global warming from human influences together resulted in very high sea surface temperatures (SSTs) in several places that played a vital role in subsequent developments. Record high SSTs in the Northern Indian Ocean in May 2010, the Gulf of Mexico in August 2010, the Caribbean in September 2010, and north of Australia in December 2010 provided a source of unusually abundant atmospheric moisture for nearby monsoon rains and flooding in Pakistan, Colombia, and Queensland. The resulting anomalous diabatic heating in the northern Indian and tropical Atlantic Oceans altered the atmospheric circulation by forcing quasi-stationary Rossby waves and altering monsoons. The anomalous monsoonal circulations had direct links to higher latitudes: from Southeast Asia to southern Russia, and from Colombia to Brazil. Strong convection in the tropical Atlantic in northern summer 2010 was associated with a Rossby wave train that extended into Europe creating anomalous cyclonic conditions over the Mediterranean area while normal anticyclonic conditions shifted downstream where they likely interacted with an anomalously strong monsoon circulation, helping to support the persistent atmospheric anticyclonic regime over Russia. This set the stage for the “blocking” anticyclone and associated Russian heat wave and wild fires. Attribution is limited by shortcomings in models in replicating monsoons, teleconnections and blocking.

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Tags: climate change climatology droughts floods monsoons SST water cycle

Ref. posted
06 September 2012
2 notes

Observations of increased tropical rainfall preceded by air passage over forests

Spracklen, D.V., S.R. Arnold, and C.M. Taylor, 2012: “Observations of increased tropical rainfall preceded by air passage over forests.” Nature, doi: 10.1038/nature11390.

Vegetation affects precipitation patterns by mediating moisture, energy and trace-gas fluxes between the surface and atmosphere. When forests are replaced by pasture or crops, evapotranspiration of moisture from soil and vegetation is often diminished, leading to reduced atmospheric humidity and potentially suppressing precipitation. Climate models predict that large-scale tropical deforestation causes reduced regional precipitation, although the magnitude of the effect is model and resolution dependent. In contrast, observational studies have linked deforestation to increased precipitation locally but have been unable to explore the impact of large-scale deforestation. Here we use satellite remote-sensing data of tropical precipitation and vegetation, combined with simulated atmospheric transport patterns, to assess the pan-tropical effect of forests on tropical rainfall. We find that for more than 60 per cent of the tropical land surface (latitudes 30 degrees south to 30 degrees north), air that has passed over extensive vegetation in the preceding few days produces at least twice as much rain as air that has passed over little vegetation. We demonstrate that this empirical correlation is consistent with evapotranspiration maintaining atmospheric moisture in air that passes over extensive vegetation. We combine these empirical relationships with current trends of Amazonian deforestation to estimate reductions of 12 and 21 per cent in wet-season and dry-season precipitation respectively across the Amazon basin by 2050, due to less-efficient moisture recycling. Our observation-based results complement similar estimates from climate models, in which the physical mechanisms and feedbacks at work could be explored in more detail.

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Tags: precipitation forests water cycle deforestation land-atmosphere feedbacks

Ref. posted
14 June 2012

The Hydrological Cycle in Three State-of-the-art Reanalyses: Intercomparison and Performance Analysis

Lorenz, C., and H. Kunstmann, 2012: “The Hydrological Cycle in Three State-of-the-art Reanalyses: Intercomparison and Performance Analysis.” Journal of Hydrometeorology, doi: 10.1175/JHM-D-11-088.1.

The three state-of-the-art global atmospheric reanalysis models, namely, ERA-Interim (ECMWF), MERRA (NASA), and CFSR (NCEP) are analyzed and compared with independent observations in the period between 1989 and 2006. Comparison of precipitation and temperature estimates from the three models with gridded observations reveals large differences of the reanalyses and also of the observation data sets. A major source of uncertainty in the observations is the spatial distribution and change of the number of gauges over time. In South America, active measuring stations were reduced from 4267 to 390. The quality of precipitation estimates from the reanalyses strongly depends on the geographic location, as there are significant differences especially in tropical regions. The closure of the water cycle in the three reanalyses is analyzed by estimating long-term mean values for precipitation, evapotranspiration, surface runoff, and moisture flux divergence. Major shortcomings in the moisture budgets of the data sets are mainly due to inconsistencies of the net precipitation minus evaporation estimates over the land masses and oceans. This imbalance largely originates from the assimilation of radiance sounding data from the NOAA-15 satellite, which results in an unrealistic increase of oceanic P-E in the MERRA and CFSR budgets. Overall, ERA-Interim shows both a comparatively reasonable closure of the terrestrial and atmospheric water balance and a reasonable agreement with the observation data sets. The limited performance of the three state-of-the-art reanalyses in reproducing the hydrological cycle, however, puts the use of these models for climate trend analyses and long-term water budget studies into question.

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Tags: water cycle weather reanalysis climate precipitation