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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.

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.

A simulation analysis of contaminated sediment transport involves model selection, data collection, model calibration and verification, and evaluation of uncertainty in the results. Sensitivity analyses provide information to address these issues at several stages of the investigation. A sensitivity analysis of simulated contaminated sediment transport is used to identify the most sensitive output variables and the parameters most responsible for the output variable sensitivity. The output variables included are streamflow and the flux of sediment and Cs137. The sensitivities of these variables are measured at the field and intermediate scales, for flood and normal flow conditions, using the HSPF computer model. A sensitivity index was used to summarize and compare the results of a large number of output variables and parameters. An extensive database was developed to calibrate the model and conduct the sensitivity analysis on a 6.2 mi2 catchment in eastern Tennessee. The fluxes of sediment and Cs137 were more sensitive than streamflow to changes in parameters for both flood and normal flow conditions. The relative significance of specific parameters on output variable sensitivity varied according to the type of flow condition and the location in the catchment. An implications section illustrates how sensitivity analysis results can help with model selection, planning data collection, calibration, and uncertainty analysis.

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The Great Plains of the United States, drained primanly by the Missouri River, are very sensitive to shifts in climate. The six main stem dams on the Missouri River control more than one‐half of the nearly 1.5 million square kilometer basin and can store three times the annual inflow from upstream. The dams are operated by the U.S. Army Corps of Engineers using a Master Manual that describes system priorities and benefits. The complex operational rules were incorporated into the Soil and Water Assessment Tool computer model (SWAT). SWAT is a distributed parameter rainfall‐runoff model capable of simulating the transpiration suppression effects of CO2 enrichment. The new reservoir algorithms were calibrated using a 25‐year long historic record of basin climate and discharge records. Results demonstrate that it is possible to incorporate the operation of a highly regulated river system into a complex rainfall‐runoff model. The algorithms were then tested using extreme climate scenarios indicative of a prolonged drought, a short drought, and a ten percent increase in basin‐wide precipitation. It is apparent that the rules for operating the reservoirs will likely require modification if, for example, upper‐basin precipitation were to increase only ten percent under changed climate conditions.

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