Showing papers on "Water cycle published in 2013"

Ground water and climate change

Abstract: As the world's largest distributed store of fresh water, ground water plays a central part in sustaining ecosystems and enabling human adaptation to climate variability and change. The strategic importance of ground water for global water and food security will probably intensify under climate change as more frequent and intense climate extremes (droughts and floods) increase variability in precipitation, soil moisture and surface water. Here we critically review recent research assessing the impacts of climate on ground water through natural and human-induced processes as well as through groundwater-driven feedbacks on the climate system. Furthermore, we examine the possible opportunities and challenges of using and sustaining groundwater resources in climate adaptation strategies, and highlight the lack of groundwater observations, which, at present, limits our understanding of the dynamic relationship between ground water and climate.

Groundwater depletion in the Middle East from GRACE with implications for transboundary water management in the Tigris-Euphrates-Western Iran region

Abstract: In this study, we use observations from the Gravity Recovery and Climate Experiment (GRACE) satellite mission to evaluate freshwater storage trends in the north-central Middle East, including portions of the Tigris and Euphrates River Basins and western Iran, from January 2003 to December 2009. GRACE data show an alarming rate of decrease in total water storage of approximately -27.2 plus or minus 0.6 millimeters per year equivalent water height, equal to a volume of 143.6 cubic kimometers during the course of the study period. Additional remote-sensing information and output from land surface models were used to identify that groundwater losses are the major source of this trend. The approach used in this study provides an example of ''best current capabilities'' in regions like the Middle East, where data access can be severely limited. Results indicate that the region lost 17.3 plus or minus 2.1 millimeters per year equivalent water height of groundwater during the study period, or 91.3 plus or minus 10.9 cubic kilometers in volume. Furthermore, results raise important issues regarding water use in transboundary river basins and aquifers, including the necessity of international water use treaties and resolving discrepancies in international water law, while amplifying the need for increased monitoring for core components of the water budget.

The impact of global land-cover change on the terrestrial water cycle

Abstract: Human impacts on the terrestrial water cycle have the potential to influence hazards such as flooding and drought, so understanding the extent of our influence is an important research goal. A study utilizing estimates of evapotranspiration for different types of land cover and a database of changes in use now shows that the extent of land-cover change caused by people is already an important factor affecting the terrestrial water cycle.

The impact of global land-cover change on the terrestrial water cycle

Abstract: Human impacts on the terrestrial water cycle have the potential to influence hazards such as flooding and drought, so understanding the extent of our influence is an important research goal. A study utilizing estimates of evapotranspiration for different types of land cover and a database of changes in use now shows that the extent of land-cover change caused by people is already an important factor affecting the terrestrial water cycle.

Forest hydrology : an introduction to water and forests

Abstract: Introduction Water Spectrum Forest Spectrum Issues and Perspectives Functions of Water Biological Functions Chemical Functions Physical Functions Socioeconomic Functions Mechanical Functions Political Functions Military Functions Science of Water Water in History Hydrosciences Properties of Water Physical Properties Hydraulic Properties Chemical Properties Biological Properties Water Distribution Globe United States Hydrologic Cycle Water Resource Problems Water Demand and Supply Water Quantity Water Quality Water Rights Characteristic Forests A Natural Resource Environmental Functions Functional Forests Threats to Forests Forests and Climate Change Forests and Precipitation Precipitation Processes Forest Interception Snow Accumulation and Snowmelt Do Forests Increase Precipitation? Forests and Vaporization Vaporization Processes Sources of Energy Evapotranspiration Forested versus Nonforested Forests and Streamflow Quantity Runoff Generation Watershed Discharges Deforestation Forest Fires Afforestation Forests and Streamflow Quality Water Pollutants Sources of Water Pollution Water Quality Determination Forest Practices Forests and Stream Sediment Soil Erosion Processes Watershed Gross Erosion Estimation of Sediment Yield Vegetation Effects Trends in Sediment Loads of the World's Rivers Forests and Stream Habitat Stream Habitat Forest Impacts Forest Fires Forests and Flooding Folklore and Fallacies Flood Occurrences Flood Measures Forest Impacts Watershed Management Planning and Implementation Watershed Programs/Projects Watershed Inventory Watershed Analysis Watershed Management Strategies Watershed Management Plans Watershed Assessment Research in Forest Hydrology Research Issues Principles of Field Studies Research Methods The Wagon Wheel Gap Study

Water in the Balance

Abstract: Earth's climate is changing, and so is its hydrologic cycle. Recent decades have witnessed rising rates of global precipitation, evaporation, and freshwater discharge ( 1 ). Extreme flooding is occurring with greater intensity and frequency in some regions; in others, extreme drought is becoming more common ( 2 ). Most climate models indicate that by the end of this century, the dry regions of the world will become drier, whereas the wet areas will become wetter ( 3 ). Meanwhile, groundwater reserves, the traditional backup for water supplies during extended periods of drought, are in decline globally ( 4 – 6 ). GRACE (the Gravity Recovery and Climate Experiment, a joint U.S.-German satellite mission) monitors these variations on monthly to decadal time scales, providing detailed data on the water cycle that are an essential prerequisite for contemporary water management.

Climate change impact on available water resources obtained using multiple global climate and hydrology models

Abstract: . Climate change is expected to alter the hydrological cycle resulting in large-scale impacts on water availability. However, future climate change impact assessments are highly uncertain. For the first time, multiple global climate (three) and hydrological models (eight) were used to systematically assess the hydrological response to climate change and project the future state of global water resources. This multi-model ensemble allows us to investigate how the hydrology models contribute to the uncertainty in projected hydrological changes compared to the climate models. Due to their systematic biases, GCM outputs cannot be used directly in hydrological impact studies, so a statistical bias correction has been applied. The results show a large spread in projected changes in water resources within the climate–hydrology modelling chain for some regions. They clearly demonstrate that climate models are not the only source of uncertainty for hydrological change, and that the spread resulting from the choice of the hydrology model is larger than the spread originating from the climate models over many areas. But there are also areas showing a robust change signal, such as at high latitudes and in some midlatitude regions, where the models agree on the sign of projected hydrological changes, indicative of higher confidence in this ensemble mean signal. In many catchments an increase of available water resources is expected but there are some severe decreases in Central and Southern Europe, the Middle East, the Mississippi River basin, southern Africa, southern China and south-eastern Australia.

Intensification of the Amazon hydrological cycle over the last two decades

Abstract: The Amazon basin hosts half the planet's remaining moist tropical forests, but they may be threatened in a warming world. Nevertheless, climate model predictions vary from rapid drying to modest wetting. Here we report that the catchment of the world's largest river is experiencing a substantial wetting trend since approximately 1990. This intensification of the hydrological cycle is concentrated overwhelmingly in the wet season driving progressively greater differences in Amazon peak and minimum flows. The onset of the trend coincides with the onset of an upward trend in tropical Atlantic sea surface temperatures (SST). This positive longer-term correlation contrasts with the short-term, negative response of basin-wide precipitation to positive anomalies in tropical North Atlantic SST, which are driven by temporary shifts in the intertropical convergence zone position. We propose that the Amazon precipitation changes since 1990 are instead related to increasing atmospheric water vapor import from the warming tropical Atlantic.

3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability

Abstract: The inner edge of the classical habitable zone is often defined by the critical flux needed to trigger the runaway greenhouse instability. This 1D notion of a critical flux, however, may not be all that relevant for inhomogeneously irradiated planets, or when the water content is limited (land planets). Based on results from our 3D global climate model, we present general features of the climate and large-scale circulation on close-in terrestrial planets. We find that the circulation pattern can shift from super-rotation to stellar/anti stellar circulation when the equatorial Rossby deformation radius significantly exceeds the planetary radius, changing the redistribution properties of the atmosphere. Using analytical and numerical arguments, we also demonstrate the presence of systematic biases among mean surface temperatures and among temperature profiles predicted from either 1D or 3D simulations. After including a complete modeling of the water cycle, we further demonstrate that two stable climate regimes can exist for land planets closer than the inner edge of the classical habitable zone. One is the classical runaway state where all the water is vaporized, and the other is a collapsed state where water is captured in permanent cold traps. We identify this “moist” bistability as the result of a competition between the greenhouse effect of water vapor and its condensation on the night side or near the poles, highlighting the dynamical nature of the runaway greenhouse effect. We also present synthetic spectra showing the observable signature of these two states. Taking the example of two prototype planets in this regime, namely Gl 581 c and HD 85512 b, we argue that depending on the rate of water delivery and atmospheric escape during the life of these planets, they could accumulate a significant amount of water ice at their surface. If such a thick ice cap is present, various physical mechanisms observed on Earth (e.g., gravity driven ice flows, geothermal flux) should come into play to produce long-lived liquid water at the edge and/or bottom of the ice cap. Consequently, the habitability of planets at smaller orbital distance than the inner edge of the classical habitable zone cannot be ruled out. Transiting planets in this regime represent promising targets for upcoming exoplanet characterization observatories, such as EChO and JWST.

Anthropogenic impact on Earth’s hydrological cycle

Abstract: The impact of climate change on the global hydrological cycle is unclear, with land precipitation and river discharges not increasing as expected. This discrepancy is investigated and tropospheric aerosols are found to have weakened the hydrological cycle between the 1950s and 1980s. The increase in greenhouse gases since the 1980s strengthened the cycle, indicating a further increase in precipitation if the current trend continues. The global hydrological cycle is a key component of Earth’s climate system. A significant amount of the energy the Earth receives from the Sun is redistributed around the world by the hydrological cycle in the form of latent heat flux1. Changes in the hydrological cycle have a direct impact on droughts, floods, water resources and ecosystem services. Observed land precipitation2,3,4 and global river discharges5 do not show an increasing trend as might be expected in a warming world6,7,8,9,10,11. Here we show that this apparent discrepancy can be resolved when the effects of tropospheric aerosols are considered. Analysing state-of-the-art climate model simulations, we find for the first time that there was a detectable weakening of the hydrological cycle between the 1950s and the 1980s, attributable to increased anthropogenic aerosols, after which the hydrological cycle recovered as a result of increasing greenhouse gas concentrations. The net result of these two counter-acting effects is an insignificant trend in the global hydrological cycle, but the individual influence of each is substantial. Reductions in air pollution have already shown an intensification in the past two decades12,13,14 and a further rapid increase in precipitation could be expected if the current trend continues.

The hydrological impact of geoengineering in the Geoengineering Model Intercomparison Project (GeoMIP)

Abstract: The hydrological impact of enhancing Earth's albedo by solar radiation management is investigated using simulations from 12 Earth System models contributing to the Geoengineering Model Intercomparison Project (GeoMIP). We contrast an idealized experiment, G1, where the global mean radiative forcing is kept at preindustrial conditions by reducing insolation while the CO 2 concentration is quadrupled to a 4×CO2 experiment. The reduction of evapotranspiration over land with instantaneously increasing CO2 concentrations in both experiments largely contributes to an initial reduction in evaporation. A warming surface associated with the transient adjustment in 4×CO2 generates an increase of global precipitation by around 6.9% with large zonal and regional changes in both directions, including a precipitation increase of 10% over Asia and a reduction of 7% for the North American summer monsoon. Reduced global evaporation persists in G1 with temperatures close to preindustrial conditions. Global precipitation is reduced by around 4.5%, and significant reductions occur over monsoonal land regions: East Asia (6%), South Africa (5%), North America (7%), and South America (6%). The general precipitation performance in models is discussed in comparison to observations. In contrast to the 4×CO2 experiment, where the frequency of months with heavy precipitation intensity is increased by over 50% in comparison to the control, a reduction of up to 20% is simulated in G1. These changes in precipitation in both total amount and frequency of extremes point to a considerable weakening of the hydrological cycle in a geoengineered world.

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

Abstract: 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.

What do we know about past changes in the water cycle of Central Asian headwaters? A review

Abstract: We have reviewed about 100 studies on past changes in climate, snow cover, glaciers and runoff in Central Asian headwater catchments, which have been published in the past 20 years. We included studies published by Central Asian researchers in Russian language, which are usually not easily accessible to international researchers. Most studies agreed on general warming trends in Central Asia with acceleration since the 1970s, but varied with regard to seasonal changes and the magnitude of the warming. Most studies also confirmed that glaciers in the Tien Shan and the Pamir continue to retreat and to shrink, though only little is known about mass and volume changes. Only few studies investigated changes in seasonal snow cover, and they suggested a decrease in maximum snow depth and a reduction in snow cover duration. The studies on runoff trends in the high mountain areas of Central Asia indicated a complex response of catchments to changes in climate. It appears that catchments with a higher fraction of glacierized area showed mainly increasing runoff trends in the past, while river basins with less or no glacierization exhibited large variations in the observed runoff changes. We conclude that our knowledge is still incomplete in particular with regard to the magnitude and the spatio-temporal patterns of changes in the water cycle of Central Asian headwater catchments. The limitations in our knowledge are due to (1) the scarcity of reliable and appropriate data sets especially for the glacio-nival zone; (2) methodological limitations of trend analysis; (3) the heterogeneity in both spatial and temporal extent of the available analyses, hampering the synthesis to a regional picture; and (4) the insufficiently understood interactions between changes in highly-variable climate parameters, the cryosphere, and the hydrological response of individual headwater catchments. Finally, there is a need for sound attribution studies linking the observed hydrological changes in individual catchments to particular processes triggered by climatic and cryospheric changes. This research gap needs urgently to be closed as projections of future hydrological changes are of vital importance for water management in Central Asia.

Southern Hemisphere Westerly Wind Changes during the Last Glacial

Abstract: 13 Abstract 14 Changes in the strength and position of Southern Hemisphere westerly winds during the 15 last glacial cycle have been invoked to explain both millennial and glacial-interglacial climate 16 fluctuations. However, neither paleo models nor paleo data agree on the magnitude, or even the 17 sign, of the change in wind strength and latitude during the most studied glacial period, the Last 18 Glacial Maximum (LGM), compared to the recent past. This paper synthesizes paleo- 19 environmental data that have been used to infer changes in LGM winds. Data compilations are 20 provided for changes in terrestrial moisture, dust deposition, sea surface temperatures and ocean 21 fronts, and ocean productivity, and existing data on Southern Hemisphere ocean circulation 22 changes during the LGM are summarized. We find that any hypothesis of LGM wind and 23 climate change needs to provide a plausible explanation for increased moisture on the west coast 24 of continents, cooler temperatures and higher productivity in the Subantarctic Zone, and 25 reductions in Agulhas leakage around southern Africa. Our comparison suggests that an overall 26 strengthening, an equatorward displacement, or no change at all in winds could all be interpreted 27 as consistent with observations. If a single cause related to the southern westerlies is sought for 28 all the evidence presented, then an equatorward displacement or strengthening of the winds 29 would be consistent with the largest proportion of the observations. However, other processes, 30 such as weakening or poleward shifts in winds, a weakened hydrological cycle, extended sea-ice 31 cover, and changed buoyancy fluxes, cannot be ruled out as potential explanations of observed 32 changes in moisture, surface temperature, and productivity. We contend that resolving the 33 position and strength of westerly winds during the LGM remains elusive based on data 34 reconstructions alone. However, we believe that these data reconstructions of environmental 35 conditions can be used in conjunction with model simulations to identify which processes best 36 represent westerly wind conditions during the LGM. 37 38

Identifying external influences on global precipitation

Abstract: Changes in global (ocean and land) precipitation are among the most important and least well-understood consequences of climate change. Increasing greenhouse gas concentrations are thought to affect the zonal-mean distribution of precipitation through two basic mechanisms. First, increasing temperatures will lead to an intensification of the hydrological cycle (“thermodynamic” changes). Second, changes in atmospheric circulation patterns will lead to poleward displacement of the storm tracks and subtropical dry zones and to a widening of the tropical belt (“dynamic” changes). We demonstrate that both these changes are occurring simultaneously in global precipitation, that this behavior cannot be explained by internal variability alone, and that external influences are responsible for the observed precipitation changes. Whereas existing model experiments are not of sufficient length to differentiate between natural and anthropogenic forcing terms at the 95% confidence level, we present evidence that the observed trends result from human activities.

Life Cycle Water Use of Energy Production and Its Environmental Impacts in China

Abstract: The energy sector is a major user of fresh water resources in China. We investigate the life cycle water withdrawals, consumptive water use, and wastewater discharge of China's energy sectors and their water-consumption-related environmental impacts, using a mixed-unit multiregional input-output (MRIO) model and life cycle impact assessment method (LCIA) based on the Eco-indicator 99 framework. Energy production is responsible for 61.4 billion m(3) water withdrawals, 10.8 billion m(3) water consumption, and 5.0 billion m(3) wastewater discharges in China, which are equivalent to 12.3%, 4.1% and 8.3% of the national totals, respectively. The most important feature of the energy-water nexus in China is the significantly uneven spatial distribution of consumptive water use and its corresponding environmental impacts caused by the geological discrepancy among fossil fuel resources, fresh water resources, and energy demand. More than half of energy-related water withdrawals occur in the east and south coastal regions. However, the arid north and northwest regions have much larger water consumption than the water abundant south region, and bear almost all environmental damages caused by consumptive water use.

Estimation of Evapotranspiration Across the Conterminous United States Using a Regression with Climate and Land-Cover Data

Abstract: Sanford, Ward E. and David L. Selnick, 2012. Estimation of Evapotranspiration Across the Conterminous United States Using a Regression with Climate and Land-Cover Data. Journal of the American Water Resources Association (JAWRA) 1-14. DOI: 10.1111/jawr.12010 Abstract: Evapotranspiration (ET) is an important quantity for water resource managers to know because it often represents the largest sink for precipitation (P) arriving at the land surface. In order to estimate actual ET across the conterminous United States (U.S.) in this study, a water-balance method was combined with a climate and land-cover regression equation. Precipitation and streamflow records were compiled for 838 watersheds for 1971-2000 across the U.S. to obtain long-term estimates of actual ET. A regression equation was developed that related the ratio ET/P to climate and land-cover variables within those watersheds. Precipitation and temperatures were used from the PRISM climate dataset, and land-cover data were used from the USGS National Land Cover Dataset. Results indicate that ET can be predicted relatively well at a watershed or county scale with readily available climate variables alone, and that land-cover data can also improve those predictions. Using the climate and land-cover data at an 800-m scale and then averaging to the county scale, maps were produced showing estimates of ET and ET/P for the entire conterminous U.S. Using the regression equation, such maps could also be made for more detailed state coverages, or for other areas of the world where climate and land-cover data are plentiful.

Australia's unique influence on global sea level in 2010-2011

Abstract: [1] In 2011, a significant drop in global sea level occurred that was unprecedented in the altimeter era and concurrent with an exceptionally strong La Nina. This analysis examines multiple data sets in exploring the physical basis for the drop's exceptional intensity and persistence. Australia's hydrologic surface mass anomaly is shown to have been a dominant contributor to the 2011 global total, and associated precipitation anomalies were among the highest on record. The persistence of Australia's mass anomaly is attributed to the continent's unique surface hydrology, which includes expansive arheic and endorheic basins that impede runoff to ocean. Based on Australia's key role, attribution of sea level variability is addressed. The modulating influences of the Indian Ocean Dipole and Southern Annular Mode on La Nina teleconnections are found to be key drivers of anomalous precipitation in the continent's interior and the associated surface mass and sea level responses.

Water Vapor Tracers as Diagnostics of the Regional Hydrologic Cycle

Abstract: Numerous studies suggest that local feedback of evaporation on precipitation, or recycling, is a significant source of water for precipitation. Quantitative results on the exact amount of recycling have been difficult to obtain in view of the inherent limitations of diagnostic recycling calculations. The current study describes a calculation of the amount of local and remote sources of water for precipitation, based on the implementation of passive constituent tracers of water vapor (termed water vapor tracers, WVT) in a general circulation model. In this case, the major limitation on the accuracy of the recycling estimates is the veracity of the numerically simulated hydrological cycle, though we note that this approach can also be implemented within the context of a data assimilation system. In this approach, each WVT is associated with an evaporative source region, and tracks the water until it precipitates from the atmosphere. By assuming that the regional water is well mixed with water from other sources, the physical processes that act on the WVT are determined in proportion to those that act on the model's prognostic water vapor. In this way, the local and remote sources of water for precipitation can be computed within the model simulation, and can be validated against the model's prognostic water vapor. Furthermore, estimates of precipitation recycling can be compared with bulk diagnostic approaches. As a demonstration of the method, the regional hydrologic cycles for North America and India are evaluated for six summers (June, July and August) of model simulation. More than 50% of the precipitation in the Midwestern United States came from continental regional tracers, and the local source was the largest of the regional tracers (14%). The Gulf of Mexico and Atlantic 2 regions contributed 18% of the water for Midwestern precipitation, but further analysis suggests that the greater region of the Tropical Atlantic Ocean may also contribute significantly. In general, most North American land regions showed a positive correlation between evaporation and recycling ratio (except the Southeast United States) and negative correlations of recycling ratio with precipitation and moisture transport (except the Southwestern United States). The Midwestern local source is positively correlated with local evaporation, but it is not correlated with water vapor transport. This is contrary to bulk diagnostic estimates of precipitation recycling. In India, the local source of precipitation is a small percentage of the precipitation owing to the dominance of the atmospheric transport of oceanic water. The southern Indian Ocean provides a key source of water for both the Indian continent and the Sahelian region.

Global Climate Modeling of the Martian water cycle with improved microphysics and radiatively active water ice clouds

Abstract: Radiative effects of water ice clouds have noteworthy consequences on the Martian atmosphere, its thermal structure and circulation. Accordingly, the inclusion of such effects in the LMD Mars Global Climate Model (GCM) greatly modifies the simulated Martian water cycle. The intent of this paper is to address the impact of radiatively active clouds on atmospheric water vapor and ice in the GCM and improve its representation. We propose a new enhanced modeling of the water cycle, consisting of detailed cloud microphysics with dynamic condensation nuclei and a better implementation of perennial surface water ice. This physical modeling is based on tunable parameters. This new version of the GCM is compared to the Thermal Emission Spectrometer observations of the water cycle. Satisfying results are reached for both vapor and cloud opacities. However, simulations yield a lack of water vapor in the tropics after Ls=180° which is persistent in simulations compared to observations, as a consequence of aphelion cloud radiative effects strengthening the Hadley cell. Every year, our GCM simulations indicate that permanent surface water ice on the north polar cap increases at latitudes higher than 80°N and decreases at lower latitudes. Supersaturation above the hygropause is predicted in line with SPICAM observations. The model also shows for the first time that the scavenging of dust by water ice clouds alone fails to fully account for observed dust detached layers.

Solar irradiance reduction via climate engineering: Impact of different techniques on the energy balance and the hydrological cycle

Abstract: [1] Different techniques of solar radiation management (SRM) have been suggested to counteract global warming, among them the injection of sulfur into the stratosphere, mirrors in space, and marine cloud brightening through artificial emissions of sea salt. This study focuses on to what extent climate impacts of these three methods would be different. We present results from simulations with an Earth system model where the forcing from the increase of greenhouse gases in a transient scenario (RCP4.5) was balanced over 50 years by SRM. While global mean temperature increases slightly due to the inertia of the climate system and evolves similar with time for the different SRM methods, responses of global mean precipitation differ considerably among the methods. The hydrological sensitivity is decreased by SRM, most prominently for aerosol-based techniques, sea salt emissions, and injection of sulfate into the stratosphere. Reasons for these differences are discussed through an analysis of the surface energy budget. Furthermore, effects on large-scale tropical dynamics and on regional climate are discussed.

The rivers of Africa : witness of climate change and human impact on the environment

Abstract: In this paper, we study the impact of climate change on river regimes in several parts of Africa, and we look at the most probable causes of these changes either climatically or anthropogenically driven. We study time series of updated monthly and annual runoff of rivers of North Africa, West Africa (Sahelian and humid tropical regions) and Central Africa, including the largest river basins: Niger and Volta rivers in West Africa, and Congo and Ogooue rivers in Central Africa. The recent years are studied in the perspective of multi-decadal variability. In West Africa and in a part of Central Africa, the climate has changed since 1970, and rainfall has not returned to previous annual amounts, except in Equatorial Africa. The consequences of the long-lasting drought are, depending on the area concerned, the modification of seasonal regimes (Equatorial area), the groundwater table decrease (Tropical humid area) and the land cover degradation (Sahelian area). The increasing number of dams and of agricultural areas also plays a major role on the modification of river regimes. The population increase will continue to impact on the environment: land cover change, deforestation, agriculture and increasing number of dams will be associated with a reduction of water and sediment discharges to the sea, and major impacts on downstream ecosystems and coastal areas. It seems necessary to share with stakeholders a comprehensive approach of the water cycle from the basin to the sea, to prevent long-lasting damages to ecosystems and infrastructures. Copyright © 2013 John Wiley & Sons, Ltd.

Water Accounting Plus (WA+) - a water accounting procedure for complex river basins based on satellite measurements

Abstract: Coping with water scarcity and growing competition for water among different sectors requires proper water management strategies and decision processes. A pre-requisite is a clear understanding of the basin hydrological processes, manageable and unmanageable water flows, the interaction with land use and opportunities to mitigate the negative effects and increase the benefits of water depletion on society. Currently, water professionals do not have a common framework that links depletion to user groups of water and their benefits. The absence of a standard hydrological and water management summary is causing confusion and wrong decisions. The non-availability of water flow data is one of the underpinning reasons for not having operational water accounting systems for river basins in place. In this paper, we introduce Water Accounting Plus (WA+), which is a new framework designed to provide explicit spatial information on water depletion and net withdrawal processes in complex river basins. The influence of land use and landscape evapotranspiration on the water cycle is described explicitly by defining land use groups with common characteristics. WA+ presents four sheets including (i) a resource base sheet, (ii) an evapotranspiration sheet, (iii) a productivity sheet, and (iv) a withdrawal sheet. Every sheet encompasses a set of indicators that summarise the overall water resources situation. The impact of external (e.g., climate change) and internal influences (e.g., infrastructure building) can be estimated by studying the changes in these WA+ indicators. Satellite measurements can be used to acquire a vast amount of required data but is not a precondition for implementing WA+ framework. Data from hydrological models and water allocation models can also be used as inputs to WA+.

Simulating Clouds with Global Climate Models: A Comparison of CMIP5 Results with CMIP3 and Satellite Data

Abstract: Clouds are a key component of the climate system affecting radiative balances and the hydrological cycle. Previous studies from the Coupled Model Intercomparison Project phase 3 (CMIP3) showed quite large biases in the simulated cloud climatology affecting all GCMs as well as a remarkable degree of variation among the models that represented the state of the art circa 2005. Here the progress that has been made in recent years is measured by comparing mean cloud properties, interannual variability, and the climatological seasonal cycle from the CMIP5 models with satellite observations and with results from comparable CMIP3 experiments. The focus is on three climate-relevant cloud parameters: cloud amount, liquid water path, and cloud radiative forcing. The comparison shows that intermodel differences are still large in the Coupled Model Intercomparison Project phase 5 (CMIP5) simulations, and reveals some small improvements of particular cloud properties in some regions in the CMIP5 ensemble over C...

Global Changes of the Water Cycle Intensity

Abstract: In this study, we evaluate numerical simulations of the twentieth century climate, focusing on the changes in the intensity of the global water cycle. A new diagnostic of atmospheric water vapor cycling rate is developed and employed, that relies on constituent tracers predicted at the model time step. This diagnostic is compared to a simplified traditional calculation of cycling rate, based on monthly averages of precipitation and total water content. The mean sensitivity of both diagnostics to variations in climate forcing is comparable. However, the new diagnostic produces systematically larger values and more variability than the traditional average approach. Climate simulations were performed using SSTs of the early (1902-1921) and late (1979- 1998) twentieth century along with the appropriate C02 forcing. In general, the increase of global precipitation with the increases in SST that occurred between the early and late twentieth century is small. However, an increase of atmospheric temperature leads to a systematic increase in total precipitable water. As a result, the residence time of water in the atmosphere increased, indicating a reduction of the global cycling rate. This result was explored further using a number of 50-year climate simulations from different models forced with observed SST. The anomalies and trends in the cycling rate and hydrologic variables of different GCMs are remarkably similar. The global annual anomalies of precipitation show a significant upward trend related to the upward trend of surface temperature, during the latter half of the twentieth century. While this implies an increase in the hydrologic cycle intensity, a concomitant increase of total precipitable water again leads to a decrease in the calculated global cycling rate. An analysis of the land/sea differences shows that the simulated precipitation over land has a decreasing trend while the oceanic precipitation has an upward trend consistent with previous studies and the available observations. The decreasing continental trend in precipitation is located primarily over tropical land regions, with some other regions, such as North America experiencing an increasing trend. Precipitation trends are diagnosed further using the water tracers to delineate the precipitation that occurs because of continental evaporation, as opposed to oceanic evaporation. These diagnostics show that over global land areas, the recycling of continental moisture is decreasing in time. However, the recycling changes are not spatially uniform so that some regions, most notably over the United States, experience continental recycling of water that increases in time.