Climate change impacts on NATURAL SYSTEMS AND PATTERNS 2.6 RIVERS |
Type of knowledge |
Results and interpretation |
Observation and analysis methods |
References |
| Reconstructions | Large alpine valleys and their piedmont to the Mediterranean Sea: Large-scale geomorphological phenomena have been recorded during the Little Ice Age and are related to the propagation downstream of sedimentary masses mobilised from the 14th century in high alpine watersheds. It leaded to a durable modification of the landscape and the hydromorphological behavior of streams. The behavior analysis of torrents near the catchment area heads reveals that they were subjected to perceptible fluctuations during the 19th century depending on climatic fluctuations. An extension of the active minor bed during the Little Ice Age has been identified on several sections of the Rhone river and its tributaries; some experienced a clear progradation of a sedimentary wave (bedload transport), accompanied or not with a river metamorphosis (from a meandering river to a braided river). These events, classified according to their increasing scale, began in the 14th century for the Rhone river in Lyon, which is under influence of the sedimentary contributions of the Ain river. Note that the Isère river metamorphosis is more premature downstream Grenoble (Drac influence) than upstream the city, which is less influenced by sedimentary flux of the Isère and Arc high watersheds due to greater distance. The phenomenon also concerns the Camargue at the beginning of the 18th century. It seems that the phenomenon reached its peak in the 18th century, although the 19th century is known for its high floods and alpine torrential events. The relative respite of the 19th century might be linked to the effect of the embankment or simply a progressive return to calmer period after the very active 18th century. This study demonstrates the strong vertical variability of the river beds when they are subjected to a repeated and quick alternation of weak and strong torrentiality phases. It also suggests that the Little Ice Age was heterogeneous concerning climate and geomorphological appearances recorded in the active torrential environments. |
The studies of river paleo-dynamics performed for about fifteen years on the basis of old maps and archives, have been analysed. |
Bravard 2000 - P |
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Observations |
Europe: In Europe, river flow has tended to increase where precipitation has increased and to decrease where precipitation has decreased in recent years. Flow variation from one year to the next appears to be more closely linked to changes in regimes of precipitation than to temperature changes. In a major part of Europe […], seasonal flow variation from spring to winter has been tied to not only a change in total precipitation but more precisely to an increase in temperature: the precipitation fell more often as rain than snow and so reached the rivers more quickly then previously |
IPCC 2001 - R | |
France:
In France, a statistical analysis of discharges at 140 gauging stations from 1975 to 1990 show a reduction of snow-melt regimes to the benefit of “transitional” regimes and to a marked irregularity in the seasonality of regimes. |
Krasovskaia et al., 2002 in Bravard 2006 - P | ||
Switzerland: Streamflow has been significantly increasing in the winter period, especially the winter annual maxima (about 60% of the stations). To a lesser degree this increase affects other quantiles as well, especially in the latter period of study. Looking at the whole period 1931-2000, we see that upward and downward trends are balanced in the low quantile range. Streamflow has been significantly decreasing in the summer period, in particular in the summer low streamflow quantile range. In this case, the whole period 1931-2000 shows the most substantial change in the streamflow regime. In the 70-years analysis, downward trends are dominant, while for the 30 and 40-years periods, upward trends are dominant. The spatial distribution of trend significance for the recent period 1971-2000 for winter maximum streamflow are shown. Spring and autumn act as transition periods between winter and summer. They show a change from the downward trends of 70-year period to predominating upward trends of the recent period. Spring trend results are most similar to the annual case. Autumn appears to be more influenced by summer in the 70-year period (almost only descending trends) and by winter (almost only increasing trends) in the other two periods. On an annual basis, the seasonal trends compensate each other, which results in more upward trends in lower quantiles and more downward trends in maximum and upper quantiles. The percentages of significant annual trends are decreasing with shorter record lengths. The trend results for streamflow in this study are in agreement with recent studies regarding precipitation trends in Switzerland conducted by Widmann & Schär (1997) and Frei & Schär (2001), where significant increases in winter precipitation were observed. However, it appears that changes in precipitation can only partly explain the observed trends in streamflow. |
The data used in this study are mean daily streamflow of 54 currently operating gauging stations in Switzerland, with continuous records which are not affected by major anthropogenic influences and can be considered independent in space (from base network of Pfaundler, 2001). The analysis was conducted for three periods of study: 1971-2000 for 54 sites (70 years), 1961-2000 for 35 sites (40 years), and 1931-2000 for 17 sites (30 years) of the base network. Mean daily streamflow data were analysed for trends with the Mann-Kendall nonparametric trend test. To find shifts in the distribution of daily streamflow, a range of quantiles on annual and seasonal bases were studied for three different observation periods. Two approaches in the trend analysis have been used: the trend analysis was conducted on the original data series; and series with positive lag-1 serial correlation were prewhitened by applying a first order autoregressive filter to the data prior to trend analysis. |
Bîrsan & al 2003 - P | |
Ebro - Rhone - Pô basins: |
Duband 2003 - P | ||
| Rhone
catchment:
Concerning river discharge, statistical tests applied to 8 gauging stations of the Rhone River downstream Geneva demonstrated that hydrology is stationary. However two types of ruptures are apparent, one locally in 1891, due to artificial developments at the outlet of Lake Geneva, the second one at the end of the 1970’s, with the occurrence of wet decades throughout the basin, following a period (1940-1975) of lull. A new cycle rich in strong floods has occurred in recent years, similar to the late 21st period, but no effect of global change having been detected yet. |
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Sauquet et Haond 2003 in Bravard 2006 - P | |
Swiss Alps: In the Swiss Alps, in 2003, rivers fed by glaciers experienced higher annual average flow than normal and accumulation basins downstream of frozen sites filled rapidly. This can be explained by the acceleration in the melting of glaciers during the extreme hot weather period. The line at which the ice melt dominates rose indeed to altitudes never before recorded (over 3500m). In glacier free zones, river flow was, inversely, lover than normal. Lakes were influenced in the same was as rivers by the metrological conditions but their size had a modifying effect. Low capacity lakes whose tributaries were not fed by glaciers were the most affected. |
ProClim 2005 - R | ||
Switzerland: For the rivers located North of the Alps, with unbuilt catchments area, the peak flow increased in winter. |
Birsan & al. 2005 in Frei & Widmer 2007 - E | ||
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Southern Germany: The long-term behaviour of the highest runoffs can be characterised as follows: • When examining the annual series from 70 to 150 years duration, the majority of annual highest runoff levels do not show significant changes. • When examining the last approx. 30 years the highest runoffs show increasing trends at many gauges. • The frequency of winter floods has increased since the 70s with the exception of Southern Bavaria. • The monthly flood runoffs in the winter half year since the 70s are higher than in the time before the 70s. In summary, it can be established that only in the last 30 to 40 years do the examined runoff time series demonstrate regional increases in flood runoffs. |
Investigations of long series of hydrometeorological
and hydrological measurements available were systematically carried
out for Baden-Württemberg and Bavaria on the basis of a large data
set within the context of KLIWA. The long-term behaviour of the flood
runoffs, the mean runoffs, the regional and heavy precipitation, the
air temperature, the evaporation and the snow cover period were analysed
for time periods in the 20th century. The investigation of the long-term behaviour [of flood runoffs] included determination of any linear trends present in the time series of the annual and monthly highest runoffs. The annual and monthly highest runoff values at 107 gauges, which have long observation series since at least 1931, formed the basis for the trend investigations. Furthermore 51 gauges with shorter time series, i.e. with observation start after 1932, were included in the analysis. |
Hennegriff & al 2006 - A | |
Ecrins massif (Torrent du Glacier Noir): Comparison of 16-day monitoring periods in July 2003 and July 2004 showed that mean daily AT measured 1 km from the snout of the Glacier Noir was 1.2 °C higher in 2003, mean Q of the Torrent du Glacier Noir was 2.3 times greater and the SSL (suspended sediment loads) was between 3.1 and 4.1 times greater in July 2003, than for the same period in the 2004 ablation season. There is an increase in SSC during the 2004 observation period which is less apparent in 2003, most likely because higher ATs and consequently higher Q earlier in the 2003 melt season had removed available sediment before the study took place in July. It invites parallels to be drawn between the results from this study and likely impacts of higher alpine summer temperatures as a result of global warming. Such changes in SSLs have important implications for sediment transfer from mountain to piedmont zones, for stream ecology, for downstream channel stability and flood risk and for the hydro-electric industry. |
In this study streamflow (Q) and suspended sediment concentrations (SSC) in a proglacial stream in the Ecrins National Park were monitored in the summers of 2003 and 2004. SSC and Q were continuously monitored for 16-day periods during July 2003 and July 2004. SSC was monitored by automated pump sampling during diurnal events in each season and supplemented by a 10 min turbidity record. Q was monitored at a range of flows and a rating curve used to convert a 10 min water level record into Q. Air temperature (AT) was also logged at 10 min intervals throughout the study. |
Stott & Mount 2006 - A | |
World: On a world wide scale, the majority of maximum annual flow readings (70%) do not represent any significant statistical trend. The trend of the other readings is divided almost equally between an increase and a decrease. There is not therefore, at the present time, any convincing proof of any kind of long term increase in the force of river floods. |
Study of trends in long time series of annual maximum river flows at 195 gauging stations worldwide. |
Svenson & al. 2006 - P | |
France: At the French scale, no coherence in the spatial distribution of the detected anomalies, or generalised evolution of discharges or floods, have been found. No coherent and generalised climatic trend has been detected in the evolution of the hydrometric extremes pattern. Evolutions have been detected for only 5 of the 15 hydro-climatic regions: weaker low water levels in the Alps and the nival wave seems to be more premature in the Northern Alps; increase of the nival module of the glacial stations and earlier occurrence of the maximum basic discharge; Stronger low water in Pyrenées and Basque country. Pluvial floods also seem to decrease in Pyrenées; increase of annual maxima in the Northeast region; Stronger floods in oceanic region (Northern France), apparently related to the filling of the fluvial water tables. In the Northern Alps, changes concern snow cover floods and low waters. The melting wave occurs earlier and the melting peak decreases. Low waters are less severe, with a significant increase of the annual daily minima and a significant decrease of the volume deficit. The duration of low waters also seems to decrease, although not significantly. According to the climatic evolution maps of the IMFREX project, the observed warming is significant for a large number of stations and for several parameters (notably the number of days warmer than 25°C or the number of frosty days). On the other hand, the precipitation evolution is much less marked and is significant only for rainfall amounts on several days. The observed climatological evolutions are insufficient to explain the extreme hydrometric evolutions, notably because of the strong non-linearity of the rain-temperature-discharge relation. |
Set of 192 long series of daily discharges, only with series considered as not influenced, covering at least 40 years of observation. Around thirty samples were extracted for every station from descriptive variables of high-waters, low-waters and water pattern. Every sample was subjected to a stationnarity test chosen according to the series autocorrelation, the expected type of distribution and the sample length. A series of 15 hydro-climatic regions was established (Renard, 2006) by crossing a classification based on floods and low waters seasonality with a pluviometric zoning of Météo France. The regional coherence was studied by using regional variable, from the regional version of the Mann and Kendall test (Douglas et al., 2002; Yue and Wang, 2002), or with the regional test of deviation proposed by Renard. |
Lang & Renard 2007 - P | |
| Switzerland: Precipitations show a slight increasing trend (about 113 mm over hundred years) for 303 precipitation record series covering the 1901-2005 period. However, this trend is not significant because of the large variability of this parameter. Given that the evaporation trend is almost identical to the precipitation one (increase of 99 mm per hundred years ), discharge remained almost constant over the last hundred years. Since the 1960s, temperature of all Swiss large rivers shows a clear increasing trend. This evolution is more marked for the Plateau streams than for the Alpine ones (Jakob et al., 2007). The increase in water temperature occurs at the same time as the average atmospheric temperature increase. |
Bibliographic review | North & al. 2007 - R: OFEV | |
Swiss Alps (upper Aare and upper Rhône basins): Annual total runoff from the (near-) ice-free basins of Allenbach and Grande Eau generally reflected the temporal variation of precipitation, rising to maxima in the late 1970searly 1980s and late 1960s respectively. P 1110 at Sion was reasonably correlated with Q 112 of both Grande Eau and Allenbach. Year-to-year fluctuations of runoff in the two basins were broadly parallel until the 1990s. Mean decadal flow in the Allenbach and Grande Eau declined by 11% and 16% respectively from 197786 to 19972006. Runoff in the Massa, which drains the most highly icecovered of the glacierized basins (~66% glacierized), mimicked the cyclical pattern of variation in T 59 at Sion. The correlation between T 59 and Q 112, whilst generally strong, was greater the shorter the time period considered. High discharge in 1928 resulted from a warm spring with precipitation slightly above average. Rising temperatures, together with increasing precipitation, led to runoff generally increasing over subsequent years, with a maximum in 1947. Declining summer air temperatures next reduced runoff to a minimum in 1978. During the second warming cycle (1980s-2006), temperatures exceeded those experienced in the 1940s. However, mean discharge in the warmer 1990s2000s failed to exceed that of the 1940s50s. The Rhône drains the second most highly glacierized basin of the study. Correlation of T 59 at Sion with runoff in the Rhône was not as strong as for flow in the Massa, whereas correlation of P 1110 with runoff was more positive for the Rhône, for respective periods. Runoff in the two rivers was strongly correlated. Discharge in the Rhône also increased from the cooler 1960s70s to the warmer 1980s2000s but by only 12%, whereas quinquennial average annual runoff in the Massa increased by almost 50% between 197478 and 200105, accompanying a rise in mean T 59 from 16.1°C to 18.1°C. In fact, runoff at Gletsch increased from 197786 to 198796, before decreasing in 19972006. The pattern of temporal variation of discharge in the Lonza (~38% glacierized) was to an extent intermediate between that in the Allenbach (0%) and the Rhône (~54%). Runoff in the Lonza increased between 197786 and 198796, before decreasing in 19972006. |
The study basins are located in the upper Aare and upper Rhône basins in Switzerland. Percentage glacierization of basin area was taken from the contemporary annual Hydrologisches Jahrbuch der Schweiz (e.g. BAFU, 2006). The Allenbach basin is ice-free. The Grande Eau basin is almost ice-free. Total glacier-covered areas of about 32 and 22 km2 in the Lonza and Rhône basins declined by about 10% and 7.5% respectively between 1977 and 2002. Percentage glacierization of the Massa basin (recalculated to take into account catchment area change) reflects a loss of total icecovered area of about 8 km2 (6%) from 136.6 km2 between 1934 and 2002, of which about three-quarters had disappeared by 1957. The Rhône was gauged in a natural cross-section at Gletsch for 1894-1903 and 1920-1928 periods, before use of a flume structure from 1956. The Lonza is gauged from 1956. Following dam construction, from 1965 the Massa was gauged at Blatten-bei-Naters, upstream of the former station at Massaboden, reducing basin area by 3.47%. Runoff is unlikely to have been proportionally reduced. Total annual runoff was taken as the total for the calendar year (Q 112). Mean MaySeptember air temperature (T 59) was used to indicate heat-energy availability for melting. Total annual precipitation between November in one year and October in the next (P 1110) reflects build-up of winter snowpack and summer rainfall which affect total annual discharge between January and December (Q 112) in the second year. |
Collins 2008 - A | |
German Alps: Caspary (2004) underlines that the discharge regimes of the rivers Donau, Enz, Kocher and Alp in South West Germany show statistical instationarities in their time series because of the relative accumulation of extreme events since the 1990s. E.g. a discharge event with a reoccurrence interval of 100 years in the reference period 1932-1976 of the river Enz at the gauge of Pforzheim equals a discharge event with a reoccurrence interval of 30 years in the reference period 1932-2002. |
Bibliographic review | Staffler & al. 2008 - A | |
Alps: The water amount in alpine watersheds is determined by numerous factors: the climatic variability at a decadal scale (NAO, AO ); the temperature increase at a centennial scale; the extreme events; the water use for diverse socioeconomic activities. Changes in the one of these factors, notably induced by a warmer climate, will modify the water quantity and quality, as well as the seasonal character of discharges, which is currently observed in the alpine range. |
Beniston 2007 - C | ||
French Alps: In the Northern Alps, the detected changes concern nival floods and low waters. The melting wave occurs earlier and the melting peak decreases. Low waters are less severe, with a significant increase of the annual daily minima and a significant decrease of the volume deficit. The duration of low waters also seems to decrease, although not significantly. In the Southern Alps, the severity of low water events also decreases; on the other hand the nival melting wave does not show coherent changes for the five tested stations. Three glacial regime stations show a decrease of the basic discharge during the melting peak, and a rather clear increase of the nival season total module. This last result is coherent with the glacier retreat in the French Alps (Vincent, 2002). Additional tests from the MORDOR simulation model allowed bringing into light a link between the temperature increase and a seasonality modification of stream flows related to melting. |
A statistical analysis of 200 long hydrometric series has been carried out within the framework of a PNRH national project and the B. Renard's doctoral thesis (2006). 13 hydrometric stations were studied for the French Alps sector. |
Lang 2007 - C1 | |
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Modeling |
Europe: A major question is the extent to which future flooding may be exacerbated by climate change. Arnell (1999) has modeled potential changes in hydrological regimes for mainland Europe, using four different GCM-based climate scenarios. While differences exist between the four scenarios, each indicates a general reduction in annual runoff in Europe south of around 50°N and an increase polewards of that. The decreases could be as great as 50% and the increases up to 25%. |
Bibliographic review of studies using GCM. |
Arnell 1999 in Goudie 2006 - A |
Europe: Calculations on a European scale indicate that under most climate change scenarios, northern Europe would see an increase in annual average stream flow, but southern Europe would experience a reduction in stream flow. In much of mid-latitude Europe, annual runoff would decrease or increase by about 10% by the 2050s, but the change resulting from climate may be smaller than “natural” multidecadal variation in runoff. |
IPCC 2001 - R | ||
Rhine
River: Climate-change simulations project a warming in the Rhine River basin of 1.0-2.4°C over present values by the middle of the century (IPCC 2001). Hydrological simulations suggest that this warming will shift the Rhine river basin from a combined rainfall and snowmelt regime to a more rainfall-dominated regime, resulting in an increase in winter discharge, a decrease in summer discharge, increases in the frequency and height of peak flows, and a longer and more frequent periods of low flow during the summer. |
Middelkoop 2001 in Barnett & al. 2005 - A | ||
| World / extratropical basins larger than 200,000
km²: Under idealized CO2 quadrupling, relative change in annual mean discharge range from -12% to +76%, with a median value of +30%. Relative changes in the 100-yr monthly maximum discharge generally are smaller and less variable, with a median of +15%. Danube basin (station: Orsova, Romania), after quadrupling of atmospheric CO2 : - Relative change in annual mean discharge (dq) : -3%. - Relative change in 100-yr annual maximum monthly discharge (dQ) : +16%. - Annual probability of 100-yr flood (P, defined with respect to the control experiment) : 4.6% |
Milly & al. 2002 - A | ||
French
Alps: The ratio between future and actual mean annual discharges presents a strong northward gradient, identical to the precipitation anomaly. In the North, the discharge increase lies between 10 and 50%. In the central part of the basin and over the Alps, the ratio is close to 1. In the south, the stream flow diminishes by 20 to 40%. In the northern part, the discharges are stronger in winter (because the liquid precipitation increases) and spring. They reach values comparable to actual ones at the end of spring and during summer and are moderately lower during autumn. In the southern part, the low altitude rivers are influenced by a general reduction of the discharge during the whole year, and especially during autumn and spring. Precipitation, evaporation and runoff increase (respectively of 13, 10 and 16%), but this average evolution is much contrasted between north and south, and depending from seasons. Runoff increases in the north (+50 to +150 kg m-2 per year) and decreases in the south (-50 to -150 kg m-2 per year). Evaporation anomaly is more homogeneous and less strong (-50 to +50 kg m-2 per year). The higher increase in evapotranspiration occurs in the Alpine catchments, where it reaches 20% (reduction of snow cover duration due to higher air temperature and reduction of snow fall ranging between -18 and -36%). The snow pack is the most sensitive hydrological component to the atmospheric warming. The ratio of solid precipitation over total precipitation decreases significantly over the Rhone basin (-21%), particularly for low and medium altitude watersheds, and the duration of the snow cover reduces. The snow pack is strongly reduced in the southern part of the Alps, the maximum snow water equivalent (SWE) and snow cover duration diminish by 50%. In the northern Alps, the SWE reduction is weaker (-20%). The snow pack anomalies strongly depend on the elevation and are significant below 1000 m. The mean snow depth diminishes by 25 cm under 2000 m (-31%) but only by 16 cm at 3000 m (-11%). The contrast is particularly strong between 1500 and 2400 m. The reduction of the snow cover lifetime is 50 days (-26%) under 2000 m versus 40 days (-3%) at 3000 m. For the Durance at La Clapière (2150 m), the snow pack appears a month later and disappears a month earlier. The SWE reaches a maximum value reduced by 30% and begins to decrease in March instead of April. The winter discharges are not modified, but the melt flash floods differ completely, occurring a month earlier and their amplitude diminishes by 30%. The summer discharges are lower (stronger evaporation). The same evolution affects the Buech river (in the southern Alps) and the Isere, but for the last, the amplitude of flash floods is unchanged (increase of spring rainfall). |
The Rhone basin has been modeled at a resolution of 8 m using CIM (CROCUS-ISBA-MODCOU), a coupled system including a Soil-Vegetation-Atmosphere transfer scheme (SVAT) and a macroscale hydrological model. This tool has been validated from 1981 to 1994 by comparing the daily river flows simulated by the models with measurements from 145 gauging stations (daily discharge and alpine snow depth observations). Comparisons have shown that the mean annual discharge is reproduced with an error lower than 5% and the efficiency is higher than 0.8 for the largest subcatchments. The climatic results are obtained from ARPEGE (period 2054-2064) with a 2*CO2 scenario. |
Etchevers & al. 2002 - A | |
| Isère: The Isère river catchments area would experience a decrease in maximum snow accumulation of between 30 and 50% with a quasi-complete disappearance during the summer season. The winter flow would increase with the precipitation increase and the melting peak shift of a month. The spring flows magnitude would be reduced in most cases. At the year scale, the scenarios are showing a slight increase or decrease. Durance: The impacts of climate change on snow cover in the Haute Durance river catchments are similar, with a lesser reduction due to the lower altitude. The annual flow would decrease in a faster way than in the North (from 0 to 20%). The flows would increase significantly in autumn and winter and would decrease during summer. The spring flows would occur a month earlier, their magnitude should remain constant or would decrease (from 10 to 25%), depending on the scenarios. In both cases, snow cover will tend to decrease and the nival characteristic of the river catchments tend to soften. |
This method uses the SAFRAN tools for the meteorological parameter analysis, the hydrological model MODCOU integrating the surface scheme ISBA and a simplified snow module inspired by CROCUS. These tools were validated on the Rhone river watershed, before being used in a climatic scenario (GICC-Rhône project). Six different 2*CO2 scenarios were used to estimate the impact of climate change on nival hydrology of certain watershed with nival regime. Four models were used among which two supplied scenarios with low (LR) and high (HR) resolution. |
Etchevers & Martin 2002 - P | |
Europe: A major question is the extent to which future flooding may be exacerbated by climate change. The proposed decrease in annual runoff in southern Europe is confirmed by Menzell and Burger's (2002) work in Germany, for they also suggest that peak flows will be very substantially reduced. A model by Eckhardt and Ulbrich (2003) for the Rhenish Massif in Germany found that in summer mean monthly groundwater recharge and stream flow may be reduced by up to 50%. |
Bibliographic review of studies using GCM... | Menzell & Ulbrich 2003 in Goudie 2006 - A | |
Rhine: Models suggest that Rhine's discharge will become markedly more seasonal by the end of the century, with mean discharge decreases of about 30% in summer [Shabalova et al., 2003]. The decrease in the summer discharge is related mainly to a predicted decrease in precipitation combined with increases in evapotranspiration. Increases in winter discharge will be caused by a combination of increased precipitation, reduced snow storage and increased early melt. Glacier melting in the Alps also contributes to the flow of the Rhine. Once these glaciers begin to disappear, this contribution will diminish sharply, and will eventually cease. |
Bibliographic review of studies using GCM...
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Shabalova & al 2003 in Goudie 2006 - A | |
Thur and upper Ticino river basins (Swiss Alps): Winter (DJF) runoff increased on average by 14% (Thur basin) and 31% (Ticino basin), whereas summer (JJA) runoff reduced by 16% (Thur basin) and 33% (Ticino basin). The annual mean runoff was reduced by about 6% (Thur basin) and 10% (Ticino basin) relatively to the control run. Note that, depending on the selected scenario, the projection results differed strongly; for the annual mean runoff, the changes were between -17 and +7% for the Thur basin and between -22 and +4% for the Ticino basin. In general, significant changes in runoff were projected in spring (early summer), mainly due to clearly changed snow conditions. As a consequence, the projected runoff peaks occurred earlier and substantially reduced relative to the peaks in the control runs. Note, furthermore, that changes in runoff were largest for areas where runoff is most strongly controlled by snow dynamics. When using the SDT_P15 climate scenarios, the uniform all season increase in P (+15%) almost compensates the potential T-induced decrease in ET/PET and SWC. The SDT_P15-driven projections showed positive runoff changes not only for winter but also for summer, independently of the choice of scenario. |
Possible future changes in the natural water budget relative to the 1981-2000 (Thur) and 1991-2000 (Ticino) baselines were investigated by driving the distributed catchment model WaSiM-ETH with a set of 23 regional climate scenarios for monthly mean temperature (T) and precipitation (P). The scenarios referred to 2081-2100 and were constructed by applying a statistical-downscaling technique to outputs from 7 global climate models. |
Jasper & al. 2004 - A | |
World: The projected changes in temperature strongly imply future changes of seasonal runoff patterns in snowmelt-dominated regions. In a warmer climate, snow will melt earlier in the year than it did before and in some places [North America] this has already happened. Taken together, these impacts mean less snow accumulation in the winter and an earlier peak runoff in the spring. On a global scale, the largest changes in the hydrological cycle due to warming are predicted for the snow-dominated basins of mid- to higher latitudes, because adding or removing snow cover fundamentally changes the snow pack's ability to act as a reservoir for water storage. Studies in various regions of the globe indicate that the stream-flow regime in snowmelt-dominated river basins is most sensitive to wintertime increases in temperature. |
Barnett & al. 2005 - A | ||
| Rhone: According to simulations of CO2 concentration doubling, the whole of the Rhone, Saône and l’Ardèche regions would encounter a decrease in average and low water levels, but high flow rate would increase. For the river Durance, with a nival regime, a decrease in the melting peak and a less thin winter are observed. However these conclusions vary greatly between the different scenarios. The flows are likely to decreases during the period May to November, but the evolution in winter alters between the various scenarios. The decrease in spring flow, resulting from the decrease in the nival components, is the strongest conclusion to be drawn from the study. As snow caps begin to melt earlier and snow fall reduces, the strong spring follows are generally reduced and they occur 1 month earlier. Winter flows increase significantly (higher rainfall) whereas summer flows reduce to 50% (drying out of the ground). These general tendencies are observed throughout the different scenarios to varying degrees. |
Evaluation of global climate change with a 2*CO2 scenario and 4 models of atmospheric circulation (low and high resolution). Hydrological modeling and determination of 6 scenarios combining observed climatic variables (data provided by 131 stations distributed in the watershed for the 1981-1997 period) with simulated anomalies for temperature and precipitation. Application of climate forcing scenarios to 5 different hydrological models (Modcou, ISBA-Modcou, Marthe, CEQUEAU, Ecomag) covering the French part of the Rhone river watershed. |
GICC Rhône 2005 - R | |
Europe: The answer of the European continental hydrological cycle is caracterised by an earlier nival melting peak and an intensification of the seasonal cycle of surface humidity, because of the intensification of the seasonal precipitation cycle. The evolution of the surface hydrological cycle under the influence of the climate change has also been analysed by Douville et al. (2002). They conclude that the earlier snow cover melting should lead to an earlier runoff spring peak in Europe. |
Within the framework of the international Coupled-Model Intercomparison Project (CMIP), Covey et al. analysed the results of 18 ocean-atmosphere coupled models simulating the climatic impact of a 1% per year increase of the atmospheric concentration of CO2. |
Planton & al 2005 - A | |
Rhone catchments (France):
A detailed study has been performed on the potential impacts of climate change on the runoff regimes of 11 small catchments having glacier surfaces comprised between 0 and 50%, at altitudes ranging between 1340 and 2940 m, under different hydrological regimes (Horton et al., 2005 ; Schaeffli, 2005). Predictions were developed for a scenario of +1°C (expected for 2020-2049) and two scenarios considering two increased green house gas emissions (period 2070-2099: + 2.4 to 2.8 °C and +3.0 to 3.6°C, with rates higher in summer than for the average). The conclusion are the following for the +1°C scenario: - A decrease of annual precipitations - An increase of winter precipitations, with the risk of higher flood peaks - A decrease of summer precipitations - A strong decrease of ice-covered areas, due to the strong increase of summer temperatures. The regimes will be mainly driven by snow-melt during the Late 21 st century. - A decrease in the amplitudes of discharge - A significant decrease of annual discharge (5-15% for the +1°C scenario) due to the reduction of precipitation, the increase of evapo-transpiration, the long term decrease of glacier surface and discharge. Horton et al. (2005) predicted “a significant decrease of the total annual discharge and a shift in the monthly maximum discharge to earlier periods of the year due to the temperature increase and the resulting impacts on the snow melt processes”. At lower altitudes, “the influence of precipitations is more pronounced and the variability of the predicted climate change impact is mainly due to the large range of predicted regional precipitation change”. |
Horton et al. 2005 ; Schaeffli 2005 in Bravard 2006 - P | ||
Rhone
catchments (France): - In the Doubs basin, the snow-rain regime shifts to rain regime with an increase of discharge in December and January, and a decrease in spring, without a significant change of the total yearly discharge. - In the Saône basin (Mâcon), the rain regime remains the same, but discharges decrease in summer. - In the Isère basin (Northern Alps), the maximum shifts from April to March, the winter maximum increases, and the summer minimum decreases by 50%. - In the Southern Alps, during contemporary dry years, the Durance basin experiences a “precocious and excessively rapid snow melt… resulting in an early peak and correspondingly very weak summertime flows”. The simulated change forecasts “an annual reduction of river discharge and of the soil moisture, decreasing by as much as 30% below the present values”. - However, if “the reduction of snowfall and earlier snow melting (increased air temperature) induced a decrease of the average snow depth by 50% and of the snow duration by more than one month”, snow pack at high altitude is less affected because even with the air warming, the average air temperature would remain below 0°C. - The Ardèche river basin experienced a “significant reduction in summer flow” and a strong reduction of the soil water content, … “reflecting the heavy reduction of precipitation in that area” The GICC-Rhone programme extended these conclusions drawn from sub-watersheds to the larger area of the French part of the Rhone basin (Leblois et al., 2005): - Average yearly discharge and low flows decrease (from May to November), but high discharges increase. Low flows may be reduced by 40-50% close to the outlet of the Rhone. - Spring flow related to snow-melt decreases since the warming of the climate reduces snow depth and the duration of snow cover, and snow melt occurs one month earlier. - The behaviour of rivers in the winter depends on the different scenarios, but generally the increase of winter rainfall induces an increase of winter discharges. |
The
coupled ISBA-MODCOU model was used in three sub-watersheds and on the entire
Rhone basin for a selected warm year, then tested for the prediction of
change (Noilhan et al., 2000; Etchevers et al., 2001; Etchevers & Martin, 2002; Leblois, 2002; Leblois & Grésillon, 2005)
|
Bravard 2006 - P | |
Swiss Alps: Runoff changes The simulated annual discharges show a significant decrease compared with the control period, for all catchments and for all climate experiments. The median decrease ranges between -26% (Drance de Bagnes) and -12% (Vorderrhein) under the B2 scenario, and between -30% (Drance de Bagnes) and -14% (Vorderrhein) under scenario A2. It should be noted that the simulated mean discharge decrease may be overestimated as a result of the highly simplified simulation of the glacier retreat. The considerable decrease of the mean annual discharges has several reasons, the most evident being the decrease in mean annual precipitation. Another significant part of the discharge reduction is due to the predicted increase of evapotranspiration. This increase is expected to be less important in summers than in winters: even if there is a pronounced temperature increase in summer, summer evapotranspiration will be limited due to reduced precipitation. The important variability of annual precipitation changes simulated by the different climate models and the significant variability of simulated changes in evapotranspiration explain the different magnitudes of mean annual runoff decrease. Note that if the maximum predicted discharge decrease is significantly higher for the A2 scenario than for the B2 scenario, then the median and minimum decreases obtained for both scenarios are quite similar. The variability highlighted is mainly due to inter-model differences. Hydrological regime changes All simulations under the future regional climate scenarios show the same significant changes of the hydrological regimes and the same trends of these changes. Summer discharge is found to reduce significantly, winter discharge increases, and the snowmelt-induced peak is shifted to earlier in the year and decreases in most cases. Owing to the substantial reduction of the glacierized areas, pure glacial discharge regimes tend to disappear. The changes are very similar for catchments having the same present regime, independently of their geographical situation. The changes predicted for the B2 scenario tend to be enhanced for A2. The median simulated shift of the maximum monthly or semi-monthly discharge is, for example, around half a month for B2 and an entire month for A2. The results are, however, highly variable for the different RCM experiments. For many catchments the main difference compared with the present regime types was the appearance of a small secondary peak in autumn that does not exist in present intra-alpine regimes, but only in south-alpine regimes. It should be noted that the shift of a glacier- or snowmelt-driven hydrological regime to a future regime more driven by snowmelt and/or rainfall will be associated with a modification of the year-to-year variability of the discharges. For high-elevation catchments (catchments with current proportions of glacier cover higher than 15%), the variation coefficient of annual discharges is, for example, predicted to increase by around 85% for scenario A2. The interannual variability of seasonal discharges also changes significantly due to an important modification of the temporary water storage properties of these mountainous catchments. For catchments with high elevations, the regime modifications are highly conditioned by the changes of the seasonal temperatures. The simulated maximum monthly discharge is, for example, highly correlated to the change in mean spring temperature. For catchments at lower altitudes, the influence of precipitation modification is more pronounced and the variability of the climate-change impacts is mainly due to the large range of predicted regional precipitation changes. |
The 11 catchments analysed (Drance de Bagnes, Saaser Vispa, Lonza, Rhône at Gletsch, Weisse Lütschine, Minster, Tamina, Vorderrhein, Dischmabach, Rosegbach and Verzasca) were selected to represent the seven different hydro-climatic zones of the Swiss Alps and the different mountainous hydrological regimes occurring in these zones. The simulation of catchment behaviour for the different time periods was carried out using a hydrological model, for the precipitation-runoff transformation, and a land-cover evolution model. The only land-cover change that is explicitly accounted for is the modification of the glacier-covered surface. The glacio-hydrological model needs three input time-series, namely daily precipitation, daily potential evapotranspiration PET and mean daily temperature. For all catchments, the precipitation and temperature data are obtained from nearby meteorological stations. The hydrological discharge simulation is carried out at a daily time-step through a conceptual reservoir-based model called GSM-SOCONT. The model has seven parameters to calibrate (four for catchments without a glacier): two degree-day factors for the ice and snowmelt computation, four time constants of the reservoirs, and the maximum storage capacity of the slow reservoir. Calibration and validation were undertaken for two consecutive 10-year periods (1971-1980 and 1981-1990 respectively, for the majority of the catchments). The calibrated model performs well for all catchments for both the calibration and the validation period. Each RCM experiment consists of a simulation for the period 1961-1990 (control run) and a simulation for the period 2070-2099 (future run). For each RCM experiment, the boundary conditions were obtained from one of the three AOGCMs used in PRUDENCE. The AOGCM experiments were completed for the two SRES emission scenarios A2 and B2. In this study, nine RCMs are included. Outputs from 19 RCM experiments are available in total: 12 for the A2 scenario and seven for B2. |
Horton & al. 2006 - A | |
France: The hydrological models show that the observed discharge evolution (increase of annual maxima and intensification of floods) seems coherent with the precipitation behavior for the Northeast region, the rain-discharge transformation being apparently not responsible for this change. For the Alps, a link between the temperature increase and the seasonality modification of stream flows related to melting was highlighted. |
Comparison of the observed evolution of discharges with that obtained from hydrological model: for the continental region, the pluvial floods evolution was obtained from the GR4J model (Perrin, 2000) and for the Alps, the nival floods evolution was obtained from the MORDOR model (Paquet, 2004). |
Lang & Renard 2007 - P | |
Alps: Remarkably increases in runoff and discharge volumes were computed for the Lavanttal region (Austria) when considering possible effects of climate changes (Regional Office of Carinthia, Department of Water Economy 2008). An indirect effect of the increase of mean temperature is the rising altitude level for the limit between rainfall and snowfall. In areas of the Northern Alps below 1500 m a.s.l., an increase of flood peaks is expected in winter due to higher soil water contents, the rising of the rainfall/snowfall limit level and due to an increased liquid precipitation (KOHS, 2007). Autonomous Province of Bolzano South Tyrol (Italian Alps): A high spatial variability of the sensitivity of the torrent catchments to specific impacts of climatic changes has been highlighted: - The runoff of nearly all torrent catchments is expected to increase in summer; - The runoff in winter is expected to increase only in torrent catchments having a high percentage of their total surface area below 2000 m; - The bed load transport in summer is expected to increase in high mountain areas and is expected to decrease in catchments at submontane levels; - The bed load transport in winter increases in a few mountain torrent catchments and does not change in the most catchments; - In some catchments eroding younger deposits (weathered material), a decrease in extreme events is highlighted; - In some catchments eroding older deposits an increase in extreme events is pointed out; - The frequency of small scale debris flow and sediment transport processes is expected to increase in most of the torrent catchments. |
Bibliographic review for Alps. Basic assumptions for potential future climate conditions: The daily mean temperatures in summer and winter are increasing; The mean sum of precipitation in summer is decreasing or remains constant; The mean sum of precipitation in winter is increasing; The intensity and frequency of short extreme rainfall events in summer and autumn is increasing. It was assumed that the following factors are varying spatially and are relevant for the sensitivity of the torrent catchments to climate changes: Percentage of areas located between 1000 and 2000 m a.s.l. A threshold value of 50% of these areas respective to the total catchment area was chosen. Characteristics of bed load source areas: If the percentage of areas with older deposits to the total bed load source area exceeds 30% of the total catchment area, the torrent catchments were classified as torrents eroding older deposits. Available bed load source areas: The sensitivity of torrent activity against climate changes increases with a higher proportion of bed load source areas respective to the total catchment area. Permafrost degradation: Catchments were classified as sensitive, if more than 30% of the total catchment area is subjected to permafrost degradation. Areas with elevated surface runoff: The sensitivity of torrential rivers and rivers to climate changes increases with a higher proportion of areas with reduced water storage capacities respective to the total catchments area. |
Staffler & al. 2008 - A | |
|
Hypothesis |
Swiss Alps and pre-Alps: The effects of global warming on the hydrological balance of the catchment area of a torrent may be summerized as fallow: • with precipitation tending to fall increasingly as rain rather than snow, increased discharge during the winter half of the year at altitudes between 1000 and 1500 m are likely. Thus pre-Alpine regions in particular are affected; • significant effects on the water cycle are observed in spring (March to April) owing to the reduced volume of melt water; • summer and autumn are still critical times, characterized by storms and concentrated heavy rainfall. This storm activity may be expected to bring an increase in peak flow rates, transportation of solids and mudflow events; • evapotranspiration will increase greatly (up to 30%) with higher air temperatures. A lack of snow in spring will also encourage premature evapotranspiration. |
Bader & Kunz 2000f - R: PNR31 | |
Europe: Glacier retreat has implications for downstream river flows. In rivers fed by glaciers, summer flows are supported by glacier melt (with the glacier contribution depending on the size of the glacier relative to basin area, as well as the rate of annual melt). If the glacier is in equilibrium, the amount of precipitation stored in winter is matched by melt during summer. However, as the glacier melts as a result of global warming, flows would be expected to increase during summer—as water is released from long-term storage—which may compensate for a reduction in precipitation. As the glacier gets smaller and the volume of melt reduces, summer flows will no longer be supported and will decline to below present levels. The duration of the period of increased flows will depend on glacier size and the rate at which the glacier melts; the smaller the glacier, the shorter lived the increase in flows and the sooner the onset of the reduction in summer flows. |
IPCC 2001 - R | ||
World (cold regions): Following a global analysis of future hydrological trends, Nijsssen et al. (2001) suggested that the largest changes in the hydrological cycle are predicted for the snow-dominated basins of mid to higher latitudes, and in particular marked changes are likely in the amplitude and phase of the annual water cycle (Arora and Boer, 2001). |
Bibliographic review of studies using GCM. | Nijsssen & al 2001 ; Arora & Boer 2001 in Goudie 2006 - A | |
Alps: With the warming of climate, “minimal and maximal discharges will be observed more frequently than in present times during other periods of the year than it is presently expected”. In others words, prediction will be more difficult and the authors recommend the adoption of a probabilistic approach (Krasovskaia et al., 2002). However specialists consider that discharge regimes have not changed enough to justify any change in the policy of dam management (D. Duband, oral comm.). |
Krasovskaia et al., 2002 in Bravard 2006 - P | ||
World:
The consequences for river runoff (of precipitation changes) are likely to affect not only the watersheds within the mountains themselves, but also in the lowland regions that are heavily dependent on these mountain resources. For every °C increase in temperature, the snowline will rise by about 150 m. Shifts in snow-pack duration and amount as a consequence of sustained changes in climate will be crucial factors in water availability for hydrological basins. |
Beniston 2003 - R | ||
Europe:
Even as summers become drier, the incidence of severe precipitation [on summertime flooding] could increase. |
Christensen & Christensen 2003 - A | ||
Rhine River : Climate-change simulations project a warming in the Rhine River basin of 1.0-2.4°C over present values by the middle of the century (IPCC, 2001). Hydrological simulations suggest that this warming will shift the Rhine river basin from a combined rainfall and snowmelt regime to a more rainfall-dominated regime, resulting in an increase in winter discharge, a decrease in summer discharge, increases in the frequency and height of peak flows, and a longer and more frequent periods of low flow during the summer. |
Barnett & al. 2005 - A | ||
Alps:
The warm winter spells may have impacts, especially if the temperatures exceed the freezing point, inducing a melting of snow enhanced early runoff into river basins that originates in the mountains, avalanche risk for the exposed slopes. |
Beniston 2005a - A | ||
World:
For rivers fed largely by ice melt, reduction in glacier volumes will have a particularly strong impact on dry-season river flows, and on the provision of downstream water. |
Kääb & al. 2005 - A | ||
Alps:
The specific annual discharge of mountain rivers is higher than the specific discharge of extended watersheds including lowland areas. This results from higher precipitations, low evaporation rates, and by conditions favouring runoff. “The hydrological regime is strongly influenced by water accumulation in the form of snow and ice and the corresponding melting processes resulting in a pronounced annual cycle of the discharge. A modification of the prevalent climate and especially of the temperature can therefore considerably affect the hydrological regime and induce important impacts on the water management” (Horton et al., 2005). The recent increase in temperatures has probably already had consequences on river regimes. Swiss
Alps: Climate warming will increase the average discharge of rivers flowing from glaciers at first during the period of retreat, but then will decrease summer discharge, as rivers will progressively lose their glacial-type hydrological regime. |
Horton
2005; Beniston et al. 2003 in Bravard 2006 - P
|
||
Swiss
Alps: Paradoxically, the impacts associated with future extreme rainfall may be reduced because of the buffering effects of snowfall on rapid runoff. This is because future springs and falls will be colder than today’s summers with lower freezing levels. |
Stoffel & Beniston 2006 - A | ||
Alps: A temperature increase might have strong consequences on the water supply in alpine watersheds at a large-scale. The snow cover decrease, the generalised glacier retreat and the summer precipitation decrease can limit the buffer effect in terms of hydrological balance and thus the water availability of these European watersheds. Extreme rainfalls could increase in a warmer climate, but with a seasonal shift, so the snow would have a buffer effect and modulate the future flood risk. |
Beniston 2007 - C |
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Legend of bibliographical
references:
- * : study taken into account in WP7.
- A : Article (Peer reviewed publication)
- C : Comment
- E : Scientific study (unpublished)
- P : Proceedings
- R : Report
- Re : Experience Feedback
- T : Thesis
- W : Website