Hydrological Sciences-Journal-des Sciences Hydrologiques, 46f 1 ) February 2001 27

Hydrological response to climate change in the Black Hills of South Dakota, USA

T. A. FONTAINE South Dakota School of Mines and Technology, 501 East St Joseph Street, Rapid City, South Dakota 57701-3995, USA c-maiI: Ubnunn@la/..sdsnn.edu

J. F. KLASSEN Duke Engineering and Services, 215 Sliuinan Boulevard, Suite 172, Naperville, Illinois 60563, USA

T. S. CRUICKSHANK Climate Chance Research Center, University of New Hampshire, Durham, New Hampshire 03824, USA

R. H. HOTCHKISS Washington State University, PO Box 642910, Pullman, Washington 99164-2910, USA

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 km2 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 km2) 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 2001

28 T. A. Fontaine et al.

Mots clefs conséquences des changements climatiques; analyse des scénarios climatiques; modification des débits; modélisation mathématique; hydrologie forestière; processus hydrologiques; Dakota du Sud. Etats Unis d'Amérique

INTRODUCTION

Significant impacts on natural resources and local economies could result from potential climate change. Jones (1999) contrasted the impacts of potential climate change with other factors that could determine the sustainability of water resources, such as popula­tion growth, urbanization, poor management of water resources, and maintaining water quality. Gleick (1999) summarized the current scientific perspectives in terms of what is known and what is still being explored. Generally accepted issues include the likelihood of changes in global mean atmospheric carbon dioxide (CO,), temperature and precipita­tion, and subsequent changes in hydrological conditions (e.g. streamflow, soil moisture, and groundwater levels). Fairly consistent estimates have been proposed for the direction and magnitude of global mean changes. Less understood are possible changes in regional and seasonal patterns of temperature and precipitation, and hence hydrological conditions. Implications for water resources, water quality, ecosystems, agriculture, and a variety of economic, social and legal issues have been identified, but uniformly accepted conclusions or expectations are not yet available. Improvements are required for general circulation models (GCMs), downscaling techniques, detailed analyses of local -scale hydrological processes, and evaluating the implications for water resources.

Anticipating these consequences is particularly important for regions with limited water supplies, such as the Great Plains of the north central USA for example. In a study of climate change effects on six rivers in the east, north central, and northwest regions of the USA, Lettenmaier et al. (1999) found that the Missouri River would experience the greatest reduction in annual streamflow. Estimated reductions of 6% to 34% would occur and would significantly impact objectives for hydropower, water supply, and navi­gation in the Missouri River. Ojima et al. (1999) described a variety of potentially negative impacts on the natural resources, agriculture, and ecology of the Great Plains region related to climate induced changes in hydrological processes. Improved under­standing of how climate change could influence the hydrological cycle is identified as a major research priority.

Hydrological simulation models are often used together with climate scenarios generated from GCMs to evaluate the impacts of potential climate change on water resources. Confidence in the results can vary greatly, and is related to the methods used for the climate scenario development and the hydrological modelling. For example, several different approaches have been used to develop climate change scenarios. An early method used output from GCMs that involved large grid cells with few details of land-atmospheric interactions at local scales. The analysis by Nash & Gleick (1991) of four large sub-basins of the Colorado River illustrated this regional approach.

Downscaling GCM output provides a more suitable approach for smaller scale hydrological analyses. The output from GCMs is used as input to regional climate models (RCMs) with much smaller grid cells and more detailed parameterization of local meteorological and hydrological processes. Bârdossy & Van Mierlo (2000) illustrate a downscaling method using stochastic models for precipitation and temperature in a large river basin m Germany. A variety of other downscaling methods are reviewed by Giorgi & Meams (1991). A common feature of these analyses involves

Hydrological response to climate change 29

GCM simulations with doubled CO, concentrations until equilibrium conditions occur. This approach has been widely used to illustrate possible impacts of potential climate change on agricultural systems and water resources. Examples include Giorgi et al. (1998) and Rosenberg et al. (1999).

A recent refinement to this approach is to replace the equilibrium 2xCO, condition with a transient scenario that allows CO, concentration to increase at rates near 1% per year (typically doubling CO, in about 70 years). Additional details are available in Doherty & Mearns (1999) and Wood et al. (1997), for example. The GCM simulations are typically coupled with land, atmosphere and biological models to provide feedback during the transient conditions. For example, Liu & Avissar (1999) illustrated the importance of land-atmosphere process interaction for developing climate change scenarios with the use of a GCM (CCM2) coupled with the Biosphere-Atmosphere Transfer Scheme (BATS, a soil-vegetation-atmosphere process model). Bounoua et al.

(1999) used a coupled atmosphere-biosphere model (SiB2) to describe interactions between climate, CO, and vegetation.

Significant improvement of GCMs is still required before reliable assessments can be completed however, as illustrated by Wolock & McCabe (1999) with a comparison of climate change effects predicted by two major transient GCM scenarios on major river basins in the United States. All three methods described here: GCM only, downscaled GCM, and transient GCM, have been used recently in different climate change assessments. It is important to interpret the results in light of the limitations associated with the development of the climate scenario.

The second component for evaluating climate change impacts on water resources normally involves simulating the effect of changed meteorological conditions on the drainage basin hydrology system using one of several types of hydrological models. For example, a regression model was developed and applied by Duell (1994) on three rivers in the Sierra Nevada Mountains. Simple water balance models have been developed to investigate climate change and the Sacramento River (Gleick, 1987), the major river basins in the USA (Wolock & McCabe, 1999), and 33 of the major river basins in the world (Miller & Russell, 1992).

Regression and simple water balance methods provide useful results; however, the most common approach involves conceptual and physically-based hydrological runoff models that simulate each of the important hydrological processes involved in the rainfall-runoff system. Runoff models are often applied as part of a sensitivity analysis where baseline conditions of climate and streamflow are established, and then used to compare the effect on streamflow of changes in C02, precipitation, temperature, and other climate variables. These studies provide examples of the direction and magnitude of expected change in streamflow for specific drainage basins, as well as providing insight into which climate and hydrological variables are most significant in predicting streamflow changes. For example, sensitivity analyses using runoff models have been applied to evaluate climate change impacts to the Colorado River (Nash & Gleick, 1991), the Broye River in Switzerland (Bultot et al., 1994), to 28 drainage basins in Australia (Chiew et al., 1995), and the Cottonwood River in Minnesota (Hanratty & Stefan, 1998).

More rigorous analyses have been used that attempt to couple the hydrological model with some of the atmospheric processes. As climate change begins to alter the land-surface condition and processes (e.g. vegetative cover, soil moisture, energy and water fluxes, and momentum), the changed land processes influence the atmospheric

30 T. A. Fontaine et al.

system, and hence the climate scenario. An overview of some of these issues is dis­cussed by Rodriguez-Iturbe (2000). Wood et al. (1992) reviewed issues related to the development of biosphere-atmosphere models for coupling with GCMs, and illustrated the use of a variable infiltration capacity (VIC) model for parameterizing land-surface hydrology in GCM simulations. Leung et al. (1996) described how detailed physical parameterization in a coupled atmospheric-hydrological model could ultimately provide explicit feedback between land-surface processes and atmospheric processes as climate change proceeds. Climate change impacts on a Rocky Mountain drainage basin were evaluated by Baron et al. (2000) using a sensitivity analysis with hydrological and eco­logical simulation by the Regional Hydro-Ecological Simulation System (RHESSys) model.

Using a variety of these approaches for climate scenarios and hydrological model­ling, several studies in the Missouri River basin have evaluated the impact of climate change on the resources of the north central USA. Lins & Stakhiv (1998) reported that streamflow in the Missouri River would probably be reduced by future climate conditions, but that the uncertainty in the climate and hydrological analyses must be reduced before these observations will have significant impacts on river management. Strzepek et al. (1999) applied climate change forecasts using hydrological, agricultural and water planning models to evaluate the future availability of water in the US cornbelt region, which includes the Missouri River basin. Their long-term results indicated that increasing demand for water and changes in water availability will require improved water management and planning.

Several case studies of climate change impacts on water resources in the Missouri River region have applied the Soil Water Assessment Tool, or SWAT (Arnold et al., 1998), to the Ogallala Aquifer (Rosenberg et al, 1999), the Wind River (Stonefelt et al, 2000), the main stem reservoirs (Hotchkiss et al, 2000), and snowmelt in alpine areas (Cruickshank, 1999). An outcome of several of these studies has been the need for more research into the hydrological response of forested, hilly terrain to climate change, and how well SWAT performs in this type of application. This landform is common along the eastern slopes of the mountain ranges that provide much of the headwater regions of the Missouri River basin, as well as the Black Hills region of South Dakota. A significant portion of the annual discharge of the Missouri River originates in these areas, and therefore the impact of potential climate change on this hydrological system is important. To provide insight into this potential impact, a case study was conducted on a medium-sized drainage basin typical of the Black Hills. The primary objective of the analysis was to evaluate the impact on water yield in a forested drainage basin with steep sloping terrain from typical climate change conditions for the Missouri River region. The investigation was also used to develop additional insight into how the SWAT model performed for assessments of climate change impacts on water resources, in order to help interpret the results of the other studies in this region.

STUDY AREA

Spring Creek was chosen for this project because it is representative of the climate, geology, vegetation, and land use of major streams in the Black Hills of South Dakota. The drainage basin, located in the eastern Black Hills, has an area of 427 km". Elevation ranges between 1311 and 2195 m. The climate is characterized by warm

Hydrological response to climate change 31

summers and dry cold winters. Hill City, South Dakota, located near the centre of the drainage basin, has an average annual precipitation of 50.0 cm and an average annual temperature of 6.6°C. Precipitation is heaviest in late spring and early summer.

The drainage basin has four major soil types described in the Pennington Country Soil Survey: the soils are all classified as hydrological Group B and C soils that have moderately fine to moderately coarse texture and slow to moderate infiltration rates when saturated. The Black Hills Forest is primarily composed of Ponderosa Pine with smaller, scattered hardwood forest and grassland areas. The high density of conifer forest has a significant influence on the drainage basin hydrology. The large surface area of conifer canopies significantly reduces available surface and soil moisture due to transpiration and to evaporation of intercepted precipitation.

HYDROLOGICAL MODEL

The model selected for the Spring Creek analysis had to be the same as the model used for the larger, Missouri River basin analysis (Fontaine et ai, 1999). Therefore, the model had to work well on large basins without the detailed data required for a standard calibration and validation effort. An interface for large GIS data sets, and efficient computations for long-term, continuous simulation at daily time steps were required. The model had to include algorithms for the critical hydrological processes involved with the Missouri River and Spring Creek, as well as a mechanism for simulating the impact of changed CO,. A well documented model that had been tested and validated was also desired. The Soil Water Assessment Tool (SWAT) (Arnold el ai, 1998) met all of these criteria.

The current version of SWAT is related to several other models, including the Field Scale Model for Chemicals, Runoff, and Erosion from Agricultural Management Systems (CREAMS) (Knisel, 1980), Erosion Productivity Impact Calculator (EPIC) (Williams et al., 1984) and Simulator for Water Resources in Rural Basins (SWRRB) (Arnold et al, 1990; Arnold et al., 1993). The SWAT model is a comprehensive, continuous river system scale hydrological model that simulates interactions of major hydrological components on a daily time step. Simulation algorithms especially useful for this analysis included biomass production, plant growth, évapotranspiration (ET), and the capability to deal with transpiration suppression effects due to atmospheric CO, enrichment (Arnold et al, 1995; Stockle et al., 1992). The SWAT model has been extensively documented and illustrated in other articles. A detailed description of SWAT, including extensive equations, a flow chart, and a discussion of model limita­tions, is given by Arnold et al. (1998). Arnold & Allen (1996) provide a good descrip­tion of the model along with applications to three small drainage basins in Illinois. Equations, data requirements, application guidelines, and component descriptions are also available from several user's manuals, such as Arnold et al. (1995).

The SWAT model has been used for large-scale impact assessment research such as the Hydrologie Unit Modelling of United States (HUMUS) (Srinivasan et al., 1993), which modelled the regional hydrological response of the entire USA using the State Soil Geographic Data Base (STATSGO) (SCS, 1992) sub-basins which average 2000 to 5000 km2. The SWAT model was part of the basis for the Soil and Water Integrated Model (SWIM) developed at the Potsdam Institute for Climate Impact Research and used to simulate climate change and environmental impacts in Germany (Krysanova et

32 T. A. Fontaine et ai

al., 1998; Krysanova et al., 1999). Further, SWAT has been applied to evaluate the impact of potential climate change on streamflow and water quality in Minnesota (Hanratty & Stefan, 1998).

Because of the numerous articles describing SWAT and the variety of recent applications, including climate change assessments, only a brief summary of the model is given here. The relevant components include hydrology, weather, soil, crop (vegetation) growth, and land management. The hydrological component is based on a water balance equation where the change in soil water content is defined by:

SW=(R-Q-ET-P-QR)

where R is precipitation, P is percolation, Q is runoff, QR is return flow, and ET is évapotranspiration.

Predictions of surface runoff from daily rainfall and snowmelt are based on a procedure similar to the CREAMS runoff model (Knisel, 1980). The runoff volume is estimated using a modified Soil Conservation Service (SCS) curve number method (SCS, 1972), which incorporates soil, land-use and management information. The curve number is adjusted at each time step based on the amount of soil water present.

Percolation of water is regulated by saturated conductivity and the ratio of soil water to field capacity in each soil layer. Lateral subsurface flow is determined simul­taneously with percolation using a nonlinear function of lateral flow travel time. Groundwater accumulates in shallow aquifers at the bottom of the root zone and contributes to recharge of the stream based on specific yield and a recession constant used to lag flows from aquifer to stream.

Plant and soil evaporation are computed separately. Plant transpiration is a linear function of potential évapotranspiration (ET) and leaf area index, limited by soil water content. The Penman-Monteith method is used to determine potential ET. Potential soil evaporation is estimated as a function of potential ET and leaf area index, with actual soil water evaporation estimated using exponential functions of soil depth and water content.

MODEL CALIBRATION

An original design objective for SWAT was to provide a model for continuous simula­tion on large ungauged basins without requiring large calibration efforts and data sets (Arnold & Allen, 1996). A series of calibration and validation studies involving a wide variety of drainage basins and hydrological conditions have demonstrated that SWAT and the GIS interface can set parameter values automatically based on easily available information such as soil, land-use, weather, and topography (Arnold et al., 1999; Srinivasan et al., 1998; Manguerra & Engel, 1998). These studies illustrate that good results are possible using automatic calibration of SWAT, and that if additional accuracy is required, all that is usually needed is detailed calibration of one or two parameters.

Following the approach of these examples, SWAT was set up and calibrated for Spring Creek by defining 13 sub-basins (Fig. 1) using a GIS programme based on hydrological processes and topography in the basin. Daily precipitation and maximum and minimum daily temperature data were obtained from the National Weather Service station at Hill City. Other time series input data were generated from the SWAT

Hydrological response to climate change

A USGS streamgauge I I Watersheds A / Streams

* TV

10 20 Kilometers

Fig. 1 Spring Creek drainage basin sub-division.

weather database using weather statistics from the Rapid City station, except for dewpoint and windspeed, which were from Hot Springs. Information was collected for surface and subsurface soil conditions, topography, land-use and management, and other parameters for each sub-basin. Soil parameters were obtained from the SWAT Soils-5 database (Arnold et al, 1995), from STATSGO (SCS, 1992), and from the Pennington County Soil Survey (SCS, 1990). The Soils-5 database includes soil bulk density, water capacity, saturated conductivity, and other physical properties. The SWAT crop database contains crop-specific parameters for 37 different crops, inclu­ding forest and grassland. Plant growth is simulated by considering a harvest index, temperatures for plant growth, leaf area index, and root depth. In general, warmer weather increases plant growth and cooler weather decreases plant growth.

The model was calibrated with nine years (1987-1995) of measured streamflow data from the US Geological Survey (USGS) gauge near Rockerville, and daily precipitation and temperature data. This period was selected because it represents the range of natural variability between wet and dry years, and the relatively modest changes used for the climate change scenario are well within the conditions that occurred during these nine years. Initial conditions, such as existing soil moisture, groundwater storage, and snow accumulation were calculated by SWAT based on the first year data set. Runoff generated from each sub-basin is routed through the channel system to the outlet of the drainage basin at the USGS gauge.

The parameter set initially selected by the model resulted in undersimulated average annual yield. Model performance was increased using additional calibration based on consultation with the model developer, additional data acquired by field trips, and a sensitivity analysis. The improved simulation primarily resulted from decreasing hillslope lengths, increasing curve numbers, and adjusting hillslope steepness. Parameter values remained within reasonable ranges given in the users manual (Klassen, 1997). Model parameters were varied until the best fit was achieved on an annual, and then monthly, basis. The final calibrated model simulates yield fairly well, reproducing the pattern of high and low flows without significant bias (Fig. 2). Results

34 T. A. Fontaine et al.

(a)

60

-o 5 0

•2 „ 4 0 > F v- § 3 0 -(D fc ra 20

5 10

0 ^ ,

Spring Creek Basin Monthly Water Yield

Measured v. Simulated Final Calibration

<̂ ^_ù__>^.—JL;^™-*^—~^.-.,Jr-

Measu ed

Simulated

^ ^ : . V ^ T ^ L I

Month

(b)

n a

<u fc

120 r

100 j

80 }

60 |

Spring Creek Basin Annual Water Yield

Measured v. Simulated Final Calibration

JK /,«

— • — Measured

• • * • Simulated

40 i-

20 t

0 -i—

Year Fig. 2 Calibration results: (a) monthly and (b) annual.

of a linear regression analysis on the final calibrated model output and observed streamflow indicates a strong correlation for annual water yield (/?" = 0.94) and a fair con-elation for monthly water yield (R2 = 0.62). Although a single precipitation gauge was used, it is located near the centre of the drainage basin, and the calibration of the model did not appear to be hindered by the lack of additional gauges.

CLIMATE CHANGE SCENARIO

Since the focus of this work involves the hydrological and water resource impacts of potential climate change, and not the details of how and why climate could change, general climate change scenarios were adopted from other investigators to use as input to SWAT. The Spring Creek climate change scenarios were required to be consistent with general conditions for the entire Missouri River basin because of the objectives of the larger project. Therefore, this impact assessment was based on a series of simulations by Giorgi et al. (1994) illustrating the type of climate changes that could result in this region from doubled CO,. Changes in temperature and precipitation were estimated using the NCAR/Penn State mesoscale model (MM4) for a regional climate model (RCM), nested in and driven by the NCAR Community Climate Model (CCM). The temperature and precipitation output from the RCM was presented in two formats: as regional averages listed in tables, and as contour maps for the continental USA. These results indicate that, for the Black Hills region, a reasonable climate change scenario would involve average annual increases of 4.6°C in temperature and 24% in

Hydrological response to climate change 35

precipitation. The results of Giorgi et al. (1994) were not intended to be accurate predictions of future climate conditions; rather, they serve as one example of plausible changes in temperature and precipitation that can be used for the type of sensitivity analyses described in this paper,

The final values adopted for the Spring Creek case study were influenced by the need to use a climate change scenario that was consistent for the entire Missouri River basin; therefore, an increase in average annual temperature of 4.0°C and a ±10% change in precipitation were selected. Both an increase and decrease in precipitation were used because of the higher uncertainty associated with the magnitude and direc­tion of changes in precipitation compared to temperature. These values are consistent with more recent climate change scenarios for this region that have been based on more rigorous climate modelling, as described in Giorgi et al. (1998), Rosenberg et al. (1999) and Strzepek et al. (1999).

CLIMATE CHANGE SIMULATIONS

The impact on streamflow of changed CO,, temperature, and precipitation was esti­mated using a sensitivity analysis with the calibrated SWAT model. The baseline streamflow condition is represented by the final calibration simulation. For the first part of the analysis, separate model simulations were performed for +4°C, ±10% pre­cipitation, and doubled atmospheric CO, (Fig. 3). The second part of the analysis simulated the hydrological response for combinations of doubled atmospheric CO, and 4°C increased temperature, doubled atmospheric CO, and ±10% precipitation, and +4°C and +10% precipitation (Fig. 4). The average annual water yield and évapo­transpiration (ET) for each condition are compared with the baseline case in Table 1.

1987 1988 1989 1990 1991 1992 1993 1994 1995

Fig. 3 Annual water yield for climate change scenarios (+4°C, ±10% precipitation, and doubled atmospheric CO,).

36 T. A. Fontaine et al.

125

1987 1988 1989 1990 1991 1992 1993 1994 199S

Fig. 4 Annual water yield for climate change scenarios (combinations of doubled atmospheric CO,, 4°C increased temperature, and ±10% precipitation.

Table 1 Impact of climate change conditions on water yield and ET.

Average annual values:

Baseline (calibrated) +4°C + \Q'/< precipitation -10% precipitation 2xCO. 2xCO, & +4"C 2xCO,&+10vi precipit ttion 2xCO, & -10% precipitation +4"C and +10% precipit Uion

Water yield:

(*)

-38.8 43.6

-34.3 16.3

-26.6 52.9

-23.4 -9.6

(mm)

31.2 19.1 44.8 20.5 36.3 22.9 47.7 23.9 28.2

ET: (7c)

2.6 7.8

-8.1 -1.2

1.8 4.9

-8.8 11.4

(mm)

484.4 497.2 522.0 445.2 478.7 493.2 508.2 441.6 539.6

Temperature

Increasing each measured temperature value by 4°C significantly decreases annual water yield. The maximum annual water yield decrease was 30.7 mm (33.4%) in 1995 while the average annual water yield decreased 12.1 mm (38.8%). Increased air temperature increases the vapour pressure deficit from leaf surface to the leaf surface boundary layer, increasing the evaporative demand and reducing basin water yield.

Precipitation

Increasing each measured precipitation value by 10%) significantly increases annual water yield. A maximum water yield increase of 32.8 mm (35.8%) occurred in 1995.

Hydrological response to climate change 37

The average annual increase was 13.6 mm (43.6%). A 10% decrease in precipitation significantly decreases average annual water yield. The maximum water yield decrease of 27.8 mm (30.3%) occurred in 1995, while the average annual water yield decrease was 10.7 mm (34.3%).

Atmospheric CO,

Doubling atmospheric CO, from 330 to 660 ppmv increased annual water yield for all of the years studied. A maximum water yield increase of 14.4 mm (24.4%) occurred in 1993, while the average increase in annual water yield was 5.1 mm (16.3%). The increased water yield is related to decreased leaf transpiration caused by partial closing of leaf stomata in response to elevated atmospheric CO, (Graham et al., 1990; Kimball et ai, 1993). Under extremely dry conditions, as in 1989 and 1990, forest response to increased atmospheric CO, remained relatively minor due to low water availability.

CO, and temperature

Doubled atmospheric CO, concentrations and increased temperature conditions produced a smaller water yield reduction compared to the simulations when only temperature was increased. The largest decrease in annual water yield was 21.3 mm (36.0%) in 1993. The average annual decrease for increased CO, and temperature was 8.3 mm (26.6%). When only temperature was increased, average annual water yield decreased 12.1 mm (38.8%).

C02 and precipitation

Doubled atmospheric CO, and 10% increased precipitation significantly increased basin water yield in most years. Maximum water yield increased 43.9 mm (74%) in 1993, while average annual water yield increased by 16.5 mm (52.9%). When the precipitation increase was not combined with a doubling of CO,, the average annual water yield increased only 13.6 mm (43.6%).

The combination of 10% decreased precipitation and doubled atmospheric CO, decreased water yield. The maximum annual water yield decrease of 20.5 mm (22.4%) occurred in 1995 while the 9 year average decrease was 7.3 mm (23.4%). The magnitude of decreased water yield caused by reduced precipitation is partially offset by reduced ET loss triggered by increased CO, and resultant partial closing of leaf stomata.

Temperature and precipitation

Increasing temperature (+4°C) and precipitation (+10%) produced a varied response in yield. The maximum water yield decrease of 14.1 mm (23.9%) occurred in 1993. On average, annual water yield decreased by 3.0 mm (9.6%); however, during 1987, 1988, 1989 and 1994 water yield increased slightly. The variation in water yield response from year to year is likely due to varying patterns of precipitation and temperature that are different each year. However, basin response (particularly forest response) to the condition of a 4°C increase in temperature and a 10% increase in precipitation may be a function of factors other than ET. Net radiation effects, timing of precipitation during the

38 T. A. Fontaine et al.

year, and changes in the forest canopy may all play significant roles in controlling average annual water yields (Graham et al, 1990).

IMPLICATIONS

These results indicate that significant changes in annual yield could occur under the potential climate change scenario adopted. A maximum reduction of 38% and a maxi­mum increase of 52.9% represent the most extreme conditions. Even the condition with the smallest impact (9.6%) represents a significant change in average annual streamflow. Spring Creek represents a typical hydrological and meteorological system in the Black Hills, where potential ET exceeds annual precipitation. Relatively mild temperatures with adequate rainfall usually occur in spring and early summer. Dry and often hot weather lasts through summer and into autumn. Several wet years can be followed by several dry years. The uneven seasonal and year-to-year distribution of precipitation causes runoff to be uncertain. Excess water yield causes flooding and agricultural losses from high soil moisture conditions. Droughts cause crop damage, reduce the reliability of surface and groundwater supplies, and increase forest fire risk. It is important to know how annual streamflow patterns could change in the Black Hills, so that negative effects on water resources and the economy can be anticipated and mitigated.

These results also provide an example of the potential change in streamflow that could occur in other parts of the western USA. The forested hilly terrain and the hydro-logical response of Spring Creek are similar to many foothills areas along the mountain ranges that form the western boundary of the Missouri River basin. A significant portion of the annual discharge in the Missouri River originates in these mountain and foothills areas. Therefore, if the climate change scenario adopted for this study were to occur in the future, significant changes in the Missouri River system could occur.

This evaluation has been based on an example of potential climate change, used with a hydrological modelling sensitivity analysis. The results are not intended to represent an accurate prediction of climatic and hydrological conditions that will occur in the future. The analysis simply illustrates the relative direction and magnitude of streamflow changes in a drainage basin similar to Spring Creek, related to possible changes in CO,, temperature, and precipitation. Additional analyses are needed that extend this work with refined climate change models, hydrological models that include more direct coupling between the land surface and the atmosphere, and evaluation of seasonal patterns in addition to average annual changes.

CONCLUSIONS

Changes in streamflow related to potential climate change were illustrated using a sensi­tivity analysis with the SWAT hydrological model in a medium sized, forested drainage basin in the Black Hills. Significant changes occurred in average annual streamflow for Spring Creek. Decreases of 39% and increases of 53% indicate that the most extreme conditions would have important impacts on streamflow. Of the individual climate variables changed, streamflow was most sensitive to the 10% increase and decrease in precipitation and the 4°C increase in temperature, and was less sensitive to doubled CO,. Of the combined conditions, doubled CO, and 10% increased precipitation had the greatest impact on streamflow, doubled C02 and 4°C increased temperature had an intermediate

Hydrological response to climate change 39

impact, and 4°C increased temperature and 10% increased precipitation had the least impact. Similar results could be expected in many other areas of the Black Hills and the foothills regions along the western boundary of the Missouri River basin, where hydro-logical and meteorological conditions are similar to Spring Creek. These results are not intended to be accurate predictions of future climate or hydrological conditions. They are an example of the direction and relative magnitude of changes in streamflow that could be associated with various conditions of the potential climate scenario adopted.

Acknowledgements The majority of this research (80%) was funded by the US Department of Energy's National Institute for Global Environmental Change (NIGEC) through the NIGEC Great Plains Regional Center at the University of Nebraska-Lincoln (DOE Cooperative Agreement No. DE-FC03-90ER61010). The South Dakota School of Mines and Technology provided additional funding. Jeff Arnold, the author of the Soil and Water Assessment Tool, provided valuable software support. Scott Kenner gave important insight into the hydrology of Spring Creek.

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Received 30 March 1999; accepted 18 September 2000