By trapping sediment in reservoirs, dams interrupt the continuity of sediment transport through rivers, resulting in loss of reservoir storage and reduced usable life, and depriving downstream reaches of sediments essential for channel form and aquatic habitats. With the acceleration of new dam construction globally, these impacts are increasingly widespread. There are proven techniques to pass sediment through or around reservoirs, to preserve reservoir capacity and to minimize downstream impacts, but they are not applied in many situations where they would be effective. This paper summarizes collective experience from five continents in managing reservoir sediments and mitigating downstream sediment starvation. Where geometry is favorable it is often possible to bypass sediment around the reservoir, which avoids reservoir sedimentation and supplies sediment to downstream reaches with rates and timing similar to pre-dam conditions. Sluicing (or drawdown routing) permits sediment to be transported through the reservoir rapidly to avoid sedimentation during high flows; it requires relatively large capacity outlets. Drawdown flushing involves scouring and re-suspending sediment deposited in the reservoir and transporting it downstream through low-level gates in the dam; it works best in narrow reservoirs with steep longitudinal gradients and with flow velocities maintained above the threshold to transport sediment. Turbidity currents can often be vented through the dam, with the advantage that the reservoir need not be drawn down to pass sediment. In planning dams, we recommend that these sediment management approaches be utilized where possible to sustain reservoir capacity and minimize environmental impacts of dams.

No abstract

No abstract

Water from the Missouri River Basin is used for multiple purposes. The climatic change of doubling the atmospheric carbon dioxide may produce dramatic water yield changes across the basin. Estimated changes in basin water yield from doubled CO2 climate were simulated using a Regional Climate Model (RegCM) and a physically based rainfall‐runoff model. RegCM output from a five‐year, equilibrium climate simulation at twice present CO2 levels was compared to a similar present‐day climate run to extract monthly changes in meteorologic variables needed by the hydrologic model. These changes, simulated on a 50‐km grid, were matched at a commensurate scale to the 310 subbasin in the rainfall‐runoff model climate change impact analysis. The Soil and Water Assessment Tool (SWAT) rainfall‐runoff model was used in this study. The climate changes were applied to the 1965 to 1989 historic period. Overall water yield at the mouth of the Basin decreased by 10 to 20 percent during spring and summer months, but increased during fall and winter. Yields generally decreased in the southern portions of the basin but increased in the northern reaches. Northern subbasin yields increased up to 80 percent: equivalent to 1.3 cm of runoff on an annual basis.

The potential impacts of climate change on water yield are examined in the Upper Wind River Basin. This is a high‐elevation, mountain basin with a snowfall/snowmelt dominated stream‐flow hydrograph. A variety of physiographic conditions are represented in the rangeland, coniferous forests, and high‐elevation alpine regions. The Soil Water Assessment Tool (SWAT) is used to model the baseline input time series data and climate change scenarios. Five hydroclimatic variables (temperature, precipitation, CO2, radiation, and humidity) are examined using sensitivity tests of individual and coupled variables with a constant change and coupled variables with a monthly change. Results indicate that the most influential variable on annual water yield is precipitation; and, the most influential variable on the timing of streamflow is temperature. Carbon dioxide, radiation, and humidity each noticeably impact water yield, but less significantly. The coupled variable analyses represent a more realistic climate change regime and reflect the combined response of the basin to each variable; for example, increased temperature offsets the effects of increased precipitation and magnifies the effects of decreased precipitation. This paper shows that the hydrologic response to climate change depends largely on the hydroclimatic variables examined and that each variable has a unique effect (e.g., magnitude, timing) on water yield.

Water yield responses to two climate change scenarios of different spatial scales were compared for the Missouri River Basin. A coarse‐resolution climate change scenario was created from runs of the Commonwealth Scientific and Industrial Research Organization General Circulation Model (CSIRO GCM). The high‐resolution climate change scenario was developed using runs of the Regional Climate Model RegCM, for which the GCM provided the initial and lateral boundary conditions. Water yield responses to the high‐ and low‐resolution climate change scenarios were investigated using the Soil and Water Assessment Tool (SWAT). Basin‐wide water yield increased for both GCM and RegCM scenarios but with an overall greater increase for the RegCM scenario. Significant differences in water yields were found between the GCM and RegCM climate scenarios.

No abstract

The hydrological response due to potential CO,~forced climate change in the Black Hills of South Dakota was investigated using modelling techniques that include variations to atmospheric CO,, temperature, and precipitation. The Soil and Water Assessment Tool (SWAT) was used to model the 427 km 2 Spring Creek basin hydrology and simulate the impact of potential climate change. As expected, modelling results of precipitation and temperature change demonstrated that increased temperature caused a decrease in water yield while increased precipitation caused an increase in water yield. Increased CO, and precipitation caused the largest increase in yield. Modelling results of increased atmospheric CO, indicate that average annual water yield increased by W7c. This increase is attributed to a suppression of transpiration processes due to increased levels of atmospheric CO,. Simulation results demonstrate that increased concentrations of atmospheric CO, act to dampen water yield loss due to the effects of increased temperature or decreased precipitation alone.Key words climate change impacts; climate scenario analysis; yield changes; mathematical modelling; forest hydrology; hydrological processes; South Dakota, USA Réponse hydrologique au changement climatique dans les Collines Noires du Dakota du Sud Résumé La réponse hydrologique aux modifications potentielles du forçage climatique du au CO, dans les Collines Noires du Dakota du Sud a été étudiée grâce aux techniques de modélisation incluant des variations du CO, atmosphérique, de la température, et des précipitations. Un outil d'évaluation du sol et de l'eau (SWAT) a été utilisé pour modéliser l'hydrologie du bassin de Spring Creek (427 km 2 ) et aussi pour simuler l'impact d'une éventuelle modification du climat. La modélisation des effets d'une modification des précipitations et de la température montre que l'augmentation de la température provoque une diminution de la production d'eau alors que l'augmentation des précipitations provoque une augmentation la production d'eau. L'augmentation conjointe du CO, et des précipitations provoquent la plus importante augmentation de la production. La modélisation de l'augmentation du CO, atmosphérique montre que la production moyenne annuelle d'eau augmente alors de 16%. Cette augmentation est attribuée à la diminution de la transpiration qui est attribuable à l'augmentation du niveau de CO, atmosphérique. Les résultats des simulations démontrent que l'augmentation de la concentration du CO, atmosphérique amortissent les pertes de production de l'eau dues à l'augmentation de la température ou à la seule diminution des précipitations. Open for discussion until I August 200128 T. A. Fontaine et al.