First look at changes in flood hazard in the Inter-Sectoral Impact Model Intercomparison Project ensemble

Dankers, R., N.W. Arnell, D.B. Clark, P.D. Falloon, B.M. Fekete, S.N. Gosling, J. Heinke, H. Kim, Y. Masaki, Y. Satoh, T. Stacke, Y. Wada, and D. Wisser, 2013: “First look at changes in flood hazard in the Inter-Sectoral Impact Model Intercomparison Project ensemble.” Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1302078110.

Climate change due to anthropogenic greenhouse gas emissions is expected to increase the frequency and intensity of precipitation events, which is likely to affect the probability of flooding into the future. In this paper we use river flow simulations from nine global hydrology and land surface models to explore uncertainties in the potential impacts of climate change on flood hazard at global scale. As an indicator of flood hazard we looked at changes in the 30-y return level of 5-d average peak flows under representative concentration pathway RCP8.5 at the end of this century. Not everywhere does climate change result in an increase in flood hazard: decreases in the magnitude and frequency of the 30-y return level of river flow occur at roughly one-third (20–45%) of the global land grid points, particularly in areas where the hydrograph is dominated by the snowmelt flood peak in spring. In most model experiments, however, an increase in flooding frequency was found in more than half of the grid points. The current 30-y flood peak is projected to occur in more than 1 in 5 y across 5–30% of land grid points. The large-scale patterns of change are remarkably consistent among impact models and even the driving climate models, but at local scale and in individual river basins there can be disagreement even on the sign of change, indicating large modeling uncertainty which needs to be taken into account in local adaptation studies.

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Developing predictive insight into changing water systems: Use-inspired hydrologic science for the Anthropocene

Thompson, S.E., M. Sivapalan, C.J. Harman, V. Srinivasan, M.R. Hipsey, P. Reed, A. Montanari, and G. Bloschl, 2013: “Developing predictive insight into changing water systems: Use-inspired hydrologic science for the Anthropocene.” Hydrology and Earth System Sciences, v. 17, pp. 5013-5039, doi: 10.5194/hess-17-5013-2013.

Globally, many different kinds of water resources management issues call for policy- and infrastructure-based responses. Yet responsible decision-making about water resources management raises a fundamental challenge for hydrologists: making predictions about water resources on decadal- to century-long timescales. Obtaining insight into hydrologic futures over 100 yr timescales forces researchers to address internal and exogenous changes in the properties of hydrologic systems. To do this, new hydrologic research must identify, describe and model feedbacks between water and other changing, coupled environmental subsystems. These models must be constrained to yield useful insights, despite the many likely sources of uncertainty in their predictions. Chief among these uncertainties are the impacts of the increasing role of human intervention in the global water cycle – a defining challenge for hydrology in the Anthropocene. Here we present a research agenda that proposes a suite of strategies to address these challenges from the perspectives of hydrologic science research. The research agenda focuses on the development of co-evolutionary hydrologic modeling to explore coupling across systems, and to address the implications of this coupling on the long-time behavior of the coupled systems. Three research directions support the development of these models: hydrologic reconstruction, comparative hydrology and model-data learning. These strategies focus on understanding hydrologic processes and feedbacks over long timescales, across many locations, and through strategic coupling of observational and model data in specific systems. We highlight the value of use-inspired and team-based science that is motivated by real-world hydrologic problems but targets improvements in fundamental understanding to support decision-making and management. Fully realizing the potential of this approach will ultimately require detailed integration of social science and physical science understanding of water systems, and is a priority for the developing field of sociohydrology.

Open Access

Potential evaporation estimation through an unstressed surface-energy balance and its sensitivity to climate change

Barella-Ortiz, A., J. Polcher, A. Tuzet, and K. Laval, 2013: “Potential evaporation estimation through an unstressed surface-energy balance and its sensitivity to climate change.” Hydrology and Earth System Sciences, v. 17, pp. 4625-4639, doi: 10.5194/hess-17-4625-2013.

Potential evaporation (ETp) is a basic input for many hydrological and agronomic models, as well as a key variable in most actual evaporation estimations. It has been approached through several diffusive and energy balance methods, out of which the Penman–Monteith equation is recommended as the standard one. In order to deal with the diffusive approach, ETp must be estimated at a sub-diurnal frequency, as currently done in land surface models (LSMs). This study presents an improved method, developed in the ORCHIDEE LSM, which consists of estimating ETp through an unstressed surface-energy balance (USEB method). The results confirm the quality of the estimation which is currently implemented in the model (Milly, 1992). The ETp underlying the reference evaporation proposed by the Food and Agriculture Organization, FAO, (computed at a daily time step) has also been analysed and compared.

First, a comparison for a reference period under current climate conditions shows that USEB and FAO’s ETp estimations differ, especially in arid areas. However, they produce similar values when the FAO’s assumption of neutral stability conditions is relaxed, by replacing FAO’s aerodynamic resistance by that of the model’s. Furthermore, if the vapour pressure deficit (VPD) estimated for the FAO’s equation, is substituted by ORCHIDEE’s VPD or its humidity gradient, the agreement between the daily mean estimates of ETp is further improved.

In a second step, ETp’s sensitivity to climate change is assessed by comparing trends in these formulations for the 21st century. It is found that the USEB method shows a higher sensitivity than the FAO’s. Both VPD and the model’s humidity gradient, as well as the aerodynamic resistance have been identified as key parameters in governing ETp trends. Finally, the sensitivity study is extended to two empirical approximations based on net radiation and mass transfer (Priestley–Taylor and Rohwer, respectively). The sensitivity of these ETp estimates is compared to the one provided by USEB to test if simplified equations are able to reproduce the impact of climate change on ETp.

Open Access

Consequences of climate change for biotic disturbances in North American forests

Weed, A.S., M.P. Ayres, and J. Hicke, 2013: “Consequences of climate change for biotic disturbances in North American forests.” Ecological Monographs, doi: 10.1890/13-0160.1.

About one third of North America is forested. These forests are of incalculable value to human society in terms of harvested resources and ecosystem services and are sensitive to disturbance regimes. Epidemics of forest insects and diseases are the dominant sources of disturbance to North American forests. Here we review current understanding of climatic effects on the abundance of forest insects and diseases in North America, and of the ecological and socioeconomic impacts of biotic disturbances. We identify 27 insects (6 nonindigenous) and 22 diseases (9 nonindigenous) that are notable agents of disturbance in North American forests. The distribution and abundance of forest insects and pathogens respond rapidly to climatic variation due to their physiological sensitivity to temperature, high mobility, short generation times, and high reproductive potential. Additionally, climate affects tree defenses, tree tolerance, and community interactions involving enemies, competitors, and mutualists of insects and diseases. Recent research affirms the importance of milder winters, warmer growing seasons, and changes in moisture availability to the occurrence of biotic disturbances. Predictions from the first US National Climate Assessment of expansions in forest disturbances from climate change have been upheld - in some cases more rapidly and dramatically than expected. Clear examples are offered by recent epidemics of spruce beetles in Alaska, mountain pine beetle in high-elevation five-needle pine forests of the Rocky Mountains, and southern pine beetle in the New Jersey Pinelands. Pathogens are less studied with respect to climate but some are facilitated by warmer and wetter summer conditions.

Changes in biotic disturbances have broad consequences for forest ecosystems and the services they provide to society. Climatic effects on forest insect and disease outbreaks may foster further changes in climate by influencing the exchange of carbon, water, and energy between forests and the atmosphere. Climate-induced changes in forest productivity and disturbance create opportunities as well as vulnerabilities (e.g., increases in productivity in many areas, and probably decreases in disturbance risks in some areas). There is a critical need to better understand and predict the interactions among climate, forest productivity, forest disturbance, and the socioeconomic relations between forests and people.

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Near-term acceleration of hydroclimatic change in the western U.S.

Ashfaq, M., S. Ghosh, S.-C. Kao, L. C. Bowling, P. Mote, D. Touma, S.A. Rauscher, and N.S. Diffenbaugh, 2013: “Near-term acceleration of hydroclimatic change in the western U.S.” Journal of Geophysical Research: Atmospheres, v. 118, doi: 10.1002/jgrd.50816.

Given its large population, vigorous and water-intensive agricultural industry, and important ecological resources, the western United States presents a valuable case study for examining potential near-term changes in regional hydroclimate. Using a high-resolution, hierarchical, five-member ensemble modeling experiment that includes a global climate model (Community Climate System Model), a regional climate model (RegCM), and a hydrological model (Variable Infiltration Capacity model), we find that increases in greenhouse forcing over the next three decades result in an acceleration of decreases in spring snowpack and a transition to a substantially more liquid-dominated water resources regime. These hydroclimatic changes are associated with increases in cold-season days above freezing and decreases in the cold-season snow-to-precipitation ratio. The changes in the temperature and precipitation regime in turn result in shifts toward earlier snowmelt, base flow, and runoff dates throughout the region, as well as reduced annual and warm-season snowmelt and runoff. The simulated hydrologic response is dominated by changes in temperature, with the ensemble members exhibiting varying trends in cold-season precipitation over the next three decades but consistent negative trends in cold-season freeze days, cold-season snow-to-precipitation ratio, and 1 April snow water equivalent. Given the observed impacts of recent trends in snowpack and snowmelt runoff, the projected acceleration of hydroclimatic change in the western U.S. has important implications for the availability of water for agriculture, hydropower, and human consumption, as well as for the risk of wildfire, forest die-off, and loss of riparian habitat.

Open Access

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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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

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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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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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