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12 Streams in Semiarid Regions as Sensitive Indicators of Global Climate Change CLIFFORD N. DAHM and MANUEL C. MOLLES, JR. Revelle and Suess (1957) pointed out that human enterprise throughout our planet could yield "far-reaching insight into the processes determining weather and climate." Today, scientists worldwide continue to search in- tensively for this insight into global weather and climate as the rate of emission of radiatively significant trace gases continues to increase. Much has been learned about the processes that determine weather and climate, but substantial uncertainty remains as predictions are made relative to future global warming and climate change. Stream ecosystems will inevitably respond to climate change both ther- mally and hydrologically. Temperature changes over the next century have been predicted to be comparable to those that have occurred since the last glacial maximum 18,000 years ago (Schneider, 1989). Continental heart- lands, which include most of the arid and semiarid areas of the world, are predicted to be one of the regions where temperature increases will be greatest (Schlesinger and Mitchell, 1987; Hansen et ai., 1988; Schneider, 1989). Hydrologic changes in stream ecosystems as a consequence of global climate change are much more difficult to predict but are likely to be even more important to the biota than thermal changes (Karl and Riebsame, 1989; Poff, Chapter 5, this volume). Changes in the amount, the timing, and/or the form of precipitation (rain versus snow) can all have major effects on stream ecosystems. Coherent regional predictions of the effects of a doubling of atmospheric carbon dioxide on regional precipitation are still lacking. In fact, the present generation of global climate models (GCMs) cannot accurately simulate present day patterns of regional pre- cipitation in many areas of the world (Neilsen et ai., 1989). A more accu- rate representation of world oceanic circulation patterns and cloud forma- tion will be necessary before GCMs can provide dependable insight into how a warming planet will redistribute the predicted 7-11% increase in planetary evaporation from a warmer earth (Schlesinger and Mitchell, 1987). Understanding how precipitation patterns will change on a warmer earth is fundamental to assess the response of freshwater ecosystems. 250 P. Firth et al. (eds.), Global Climate Change and Freshwater Ecosystems © Springer-Verlag New York Inc. 1992

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12 Streams in Semiarid Regions as Sensitive Indicators of Global Climate Change

CLIFFORD N. DAHM and MANUEL C. MOLLES, JR.

Revelle and Suess (1957) pointed out that human enterprise throughout our planet could yield "far-reaching insight into the processes determining weather and climate." Today, scientists worldwide continue to search in­tensively for this insight into global weather and climate as the rate of emission of radiatively significant trace gases continues to increase. Much has been learned about the processes that determine weather and climate, but substantial uncertainty remains as predictions are made relative to future global warming and climate change.

Stream ecosystems will inevitably respond to climate change both ther­mally and hydrologically. Temperature changes over the next century have been predicted to be comparable to those that have occurred since the last glacial maximum 18,000 years ago (Schneider, 1989). Continental heart­lands, which include most of the arid and semiarid areas of the world, are predicted to be one of the regions where temperature increases will be greatest (Schlesinger and Mitchell, 1987; Hansen et ai., 1988; Schneider, 1989).

Hydrologic changes in stream ecosystems as a consequence of global climate change are much more difficult to predict but are likely to be even more important to the biota than thermal changes (Karl and Riebsame, 1989; Poff, Chapter 5, this volume). Changes in the amount, the timing, and/or the form of precipitation (rain versus snow) can all have major effects on stream ecosystems. Coherent regional predictions of the effects of a doubling of atmospheric carbon dioxide on regional precipitation are still lacking. In fact, the present generation of global climate models (GCMs) cannot accurately simulate present day patterns of regional pre­cipitation in many areas of the world (Neilsen et ai., 1989). A more accu­rate representation of world oceanic circulation patterns and cloud forma­tion will be necessary before GCMs can provide dependable insight into how a warming planet will redistribute the predicted 7-11% increase in planetary evaporation from a warmer earth (Schlesinger and Mitchell, 1987). Understanding how precipitation patterns will change on a warmer earth is fundamental to assess the response of freshwater ecosystems.

250 P. Firth et al. (eds.), Global Climate Change and Freshwater Ecosystems© Springer-Verlag New York Inc. 1992

12. Streams as Sensitive Indicators of Global Climate Change 251

What types of stream ecosystems will be most susceptible to changes in average temperature and patterns in precipitation? We suggest that streams in arid or semiarid regions will be highly responsive to a changing climate. This is because streamflow amplifies variations in catchment pre­cipitation most strongly in semiarid regions and because many semiarid regions worldwide are directly affected by a major oceanic circulation pat­tern known as the EI Nino-Southern Oscillation (ENSO) phenomenon. Support for the amplification effect and the importance of the ENSO phe­nomenon will be provided in this chapter by examining streams and rivers in the southwestern United States.

Streams as Amplifiers of Climatic Change

Stream ecosystems in general are excellent candidates for research on glob­al climate change because small changes in average precipitation across a region will produce large changes in streamflow. This amplification effect occurs in all catchments, but it becomes considerably more pronounced in arid and semiarid environments. With arid lands making up 12% of the land surface on earth and semiarid lands making up 21 % of the land mass, the responses of these ecosystems to possible climate change are especially important to plant and animal communities dependent on the limited sources of fresh water in these regions. Approximately one-fifth of the world's human population lives in these arid and semiarid environments where small changes in precipitation can have major repercussions on the hydrology and ecology of streams and rivers.

One method for analyzing the hydrological sensitivity of lotic ecosystems to changing patterns of precipitation is by modeling discharge patterns in different catchments relative to mean long-term precipitation and flow pat­terns. Nemec (1986) provided an example of this approach using data from the Peace River at Vernon, Texas and the Leaf River near Collins, Missis­sippi (Figure 12.1). Changes in annual runoff can be predicted for either decreases or increases in annual precipitation. In addition, changes in vari­ables such as evapotranspiration can be entered into the model to further define how changing characteristics of a basin or the riparian zone might affect discharge.

The Leaf River in Mississippi drains a catchment with humid climate conditions. Mean precipitation is 1314 mm and the mean runoff is 409 mm. The drainage area is 1949 km2 at the gauging station near Collins, Missis­sippi. Annual runoff from the basin represents about 31% of annual pre­cipitation. A change in precipitation for this basin produces a nearly linear increase or decrease in discharge. For example, a 10% increase in pre­cipitation yields a 25% increase in runoff while a 10% decrease yields a 25% decrease in runoff (Figure 12.1).

The Peace River at Vernon, Texas drains a catchment with semiarid

252 Clifford N. Dahm and Manuel C. MoUes, Jr.

3 200

* A = Arid Climate • = Humid Climate /

6 /

...J t :§ / z / z « 200 / 100 Z / « w , ~ / LL LL 0 Z ::J a: 100 0 W CJ)

~ ii: " LL " 0 " Z _L ::J a: 0 -100

-25 -20 -10 0 10 20 25

PERCENT CHANGE IN PRECIPITATION

FIGURE 12.1. The ratio in percent of annual runoff to base annual runoff for changes in precipitation ranging from -25% to +25% of the annual mean for a stream in an arid climate compared to a stream in a humid climate. The changes assume no change in evapotranspiration. These data are modified from Nemec (1986).

climatic conditions. Mean precipitation is 540 mm and mean runoff is 11 mm. The drainage area is 9024 km2 at the gauging station. Annual runoff from the basin represents about 2% of annual precipitation. The response of this river system to changes in precipitation is nonlinear with large changes in runoff resulting from relatively small changes in annual pre­cipitation. For example , a 10% increase in precipitation produces a 70% increase in runoff while a 10% decrease results in a 50% decrease in river flow .

Hydrologic models of this type have been used to forecast changes in riverine discharge from global warming (e.g., Revelle and Waggoner, 1989). Most scenarios consider the coupled response of lotic ecosystems to warmer and dryer conditions. As an example , the Rio Grande in New Mexico and Texas has been predicted to have a 75.7% annual decrease in discharge under the conditions of global warming (2°C temperature in­crease) and an overall decrease of 10% in basin precipitation (Revelle and Waggoner, 1989). The characteristic semiarid climate of the region makes the Rio Grande one of the most sensitive rivers to climate change within the United States.

12. Streams as Sensitive Indicators of Global Climate Change 253

TESUQUE WATERSHEDS

TO P¢] ~ _~ SANTA FE .... .._---:::_-::=:=' . .,.,--. ;,--' ~

LlTILE TESUauE CREEK

FIGURE 12.2. Map showing the locations of the study watersheds in the Tesuque basin of the Sangre de Cristo Mountains near Santa Fe, New Mexico.

An important assumption of the model type presented by Revelle and Waggoner (1989) is that there will be no change in basin evapotranspira­tion accompanying atmospheric enrichment of CO2 • Idso and Brazel (1984) have suggested that a direct antitranspirant response fromatmo­spheric CO2 enrichment, due to partial stomatal closure of many plants under conditions of increased CO2 , will strongly counteract the effects of global warming. Wigley and Jones (1985) have also pointed out that direct effects of higher CO2 levels on vegetation might lead to decreased rates of evapotranspiration and increase surface and subsurface runoff to streams worldwide. Predictions of major decreases in runoff for streams experienc­ing modest warming and lowered precipitation must be tempered with the realization that direct vegetation effects might be working in the opposite direction.

In any case, smaller basins within semiarid regions are quite sensitive to minor changes in precipitation. A long-term study of a series of small catchments within the Tesuque Watershed study area near Santa Fe, New Mexico provides hydrologic data to determine discharge response to changes in precipitation (Figure 12.2). The discharge from the catchments

254 Clifford N. Dahm and Manuel C. Molles, Jr.

is analyzed using the technique described by Nemec (1986) for the Leaf and Peace rivers. The watersheds vary in elevation with increasing mean annual precipitation occurring as elevation increases. All the catchments are in the Sangre de Cristo Mountains of New Mexico and are underlain by crystalline Precambrian granite and gneiss (Gosz, 1978).

An increasingly nonlinear response in annual flow occurs within the basins as mean precipitation decreases (Figure 12.3). Watershed W15 is the highest elevation basin (3231-3734 m) and the catchment is dominated by spruce-fir (Picea engelmannii-Abies lasiocarpa) forest. Mean precipita­tion during the period of record (1972-1980) was 859 mm. The best fit of the change in annual flow to a change in annual precipitation was a linear model with a slope of 1.59 and an R2 value of 0.77. Discharge increased or decreased approximately 1.6% for each 1 % change in precipitation, while total precipitation varied by a factor of 2 X during the period of measure­ment. This high elevation watershed displayed a flow response characteris­tic of a mesic catchment.

Watershed AWl is an intermediate elevation basin (2950-3525 m) dominated by aspen (Populus tremuloides) forest. Mean precipitation in this basin during the same period of record was 705 mm. The best fit of flow change in this basin to variable precipitation was also a linear model (Figure 12.3). The slope of the line was 2.41 with an R2 of 0.66. Annual discharge increased or decreased about 2.4% for each 1 % change in annual precipitation. The amount of change in flow to a change in mean annual precipitation was greater than the higher elevation watershed (WI5) and the response was more variable although a linear response was still appar­ent.

Watershed W5 is a lower elevation site (2804-3444 m) and the catch­ment is in a mixed conifer forest. This is the driest site with mean precipita­tion of 578 mm. A change in precipitation in this catchment produces a nonlinear response in stream flow (Figure 12.3). Relatively small increases in precipitation yield large increases in total annual flow. Decreases in pre­cipitation produce a rather variable flow response with the driest year only about 25% lower in discharge over the year.

The relationships of precipitation and runoff across these basins conform quite well to the results obtained by Nemec (1986). The annual change in flow shows a linear response to changes in precipitation in the wetter sites (W15 and AWl) and a nonlinear response to precipitation variations at the driest site. The degree of amplification increases as the sites become drier.

The model presented by Nemec (1986) and the empirical data from the Tesuque Watersheds both suggest a leveling off in the percent change in runoff as a large decrease in annual precipitation occurs. In other words, when substantial drying occurs in a semiarid basin, an equally dramatic decrease in annual runoff may not occur. This counterintuitive response can be better understood when the character of semiarid fluvial systems are considered. A period of sustained drought in small semiarid basins com-

12_ Streams as Sensitive Indicators of Global Climate Change 255

MEAN PRECIP = 859 mm

200 y = - 2.8909 + 1.5907x R 2 = 0.770

150 W15

100

50

0

-50

-100 - 4 0 - 2 0 o 20 40

MEAN PRECIP = 705 mm

200 y = - 0.19789 + 2.4052x R 2 = 0.664

150 AWl

~ 3 100 0 ...J

50 u. ~ w 0 (!) z « -50 J: 0

-100 - 4 0 - 20 o 20 40

MEAN PRECIP = 578 mm

200 y = -17.650 + 3.4948x + 0.097x 2 R 2 = 0.815

150 W5

100

50

0

-50

-100 - 4 0 -20 o 20 40

CHANGE IN PRECIPITATION (%)

FIGURE 12.3. Change in annual flow in percent versus the change in precipitation over the long-term mean for three small basins in the Tesuque watersheds. The best fit regression equation for each set of data is shown.

256 Clifford N. Dahm and Manuel C. MoUes, Jr.

monly converts formerly perennial streams and rivers to intermittent sys­tems. Long periods of little or no precipitation can eliminate surface flow within the basins, but flow associated with storms may actually be accentu­ated. Long periods with little or no antecedent moisture may accentuate the fluvial response of the basins in semiarid environments through a varie­ty of hypothesized mechanisms. These include:

1. A decrease in overall vegetation biomass, which acts to increase over­land flow and reduce percolation of rainfall. Groundwater recharge is thereby diminished and rapid, often erosive, surface flow occurs.

2. A hydrophobic layer develops on the soil surface during extended periods with little or no precipitation. This layer also decreases ground­water recharge and routes more of the precipitation into the stream channel.

3. Daytime surface soil temperatures in desiccated soils can reach substan­tially higher temperatures during summer heating than moist soils. This temperature differential may set up more active convective thunder­storm systems, which produce more intense occasional rainfall events that yield large short-term runoff from the catchments.

These hypothetical mechanisms, either individually or in concert, pro­vide an explanation for why decreasing precipitation may not necessarily yield a major decrease in total runoff within semiarid ecosystems. A major change, however, from perennial to intermittent stream systems and to a more flashy hydrology would accompany a major period of drying within such semiarid catchments.

El Nino-Southern Oscillation and Semiarid Aquatic Ecosystems

A second attribute of many streams and rivers in semiarid environments worldwide is their linkage to a globally important atmospheric/oceanic circulation pattern known as the EI Nino-Southern Oscillation (ENSO) phenomenon (Ropelewski and Halpert 1987, Nicholls 1988). The ENSO phenomenon is associated with variations in sea surface temperature and barometric pressure across the tropical and subtropical Pacific Ocean (Ras­mussen, 1985; Enfield, 1989). The name "EI Nino" derives from the com­mon appearance of warmer sea surface temperatures off the coast of South America around the Christmas season. The term "Southern Oscillation" refers to fluctuations in barometric pressure across the Pacific Ocean. Dur­ing an EI Nino episode, the eastern tropical Pacific has elevated sea surface temperatures and reduced mean monthly barometric pressure while the western tropical Pacific has decreased sea surface temperatures and in­creased mean monthly barometric pressure. Under the opposite condi­tions, now dubbed La Nina, the eastern tropical Pacific has lower than

12. Streams as Sensitive Indicators of Global Climate Change 257

average sea surface temperatures and increased mean monthly barometric pressure while surface water temperatures are warmer and barometric pressure lower in the western tropical Pacific (Rasmussen and Wallace, 1983; Enfield, 1989).

Variations in precipitation attributable to ENSO phenomena have been documented for semiarid environments as diverse as the Sahel countries of Africa, most of Australia, northern Mexico and the southwestern United States, and much of western South America (e.g., Conrad, 1941; Ropelewski and Halpert, 1987; Nicholls, 1988). For example, EI Nino epi­sodes are characterized by excessive precipitation in many of the arid and semiarid regions in the mid-latitudes of western North and South America while much of Australia suffers through drought. Precipitation distribu­tions are reversed during the La Nina side of the ENSO phenomenon, and Australia receives enhanced inputs of precipitation while the mid-latitudes of western North and South America experience drought. On a worldwide basis, ENSO-affected locations have annual rainfall variability that is typi­cally one-third to one-half higher than sites outside the influence of ENSO phenomenon (Nicholls, 1988). Years of plenty and years of drought occur commonly in ENSO-affected regions.

The present generation of GCMs is not capable of providing accurate regional estimates of precipitation changes that might accompany a war­mer earth. In fact, GCMs are often unable to effectively simulate present­day regional precipitation patterns (Nielsen et aI., 1989). Two important limitations of current GCMs include problems with accurately representing cloud formation and the use of a static ocean. These limitations directly affect the ability of GCMs to predict regional precipitation patterns and future trends. The next generation of GCMs should begin to address some of the present limitations and hopefully develop better prognostic powers relative to the redistribution of precipitation on a warming earth. Recent GCM runs predict a 7-11 % increase in global precipitation with a doubling of atmospheric CO2 concentrations (Schlesinger and Mitchell, 1987) and the regional distribution of this additional moisture will be critical for assessing the impact of global warming on various stream and river eco­systems.

Reorganization of major weather patterns due to a shift in oceanic cir­culation patterns during the last glacial episode has been hypothesized to have had worldwide climatic effects (e.g., Broecker et aI., 1989; Gasse et aI., 1990). The effects of global warming from increasing atmospheric trace gases on ocean circulation remain unknown in global greenhouse scenar­ios. A change in the intensity, periodicity, or geographic extent of the ENSO phenomenon would have far-reaching climatic effects. Stream flow in many of the semiarid regions affected by ENSO activity would be a sensitive indicator responding to any changes in such a global ocean/ atmosphere circulation system.

Molles and Dahm (1990) have shown that spring discharge from two

258 Clifford N. Dahm and Manuel C. MoUes, Jf.

rivers in New Mexico in the southwestern United States is strongly in­fluenced by the El Nino and La Nina extremes of the ENSO phenomenon. Mean spring runoff was 6.0 to 7.4 x higher in EI Nino years compared to La Nina years. The amplification effect was also apparent as differences in precipitation were 2.1 to 2.8 x higher in EI Nino years compared to La Nina years. A good deal of the interannual variability in precipitation and discharge in ENSO-affected regions can be explained by considering the state of the ENSO phenomenon. Such analyses even extend to the scale of the Amazon River (Richey et aI., 1989).

The changing composition of the radiatively active trace gases in our atmosphere is changing the heat balance of the earth (e.g., Dickinson and Cicerone, 1986; Schneider, 1989). These changes are predicted to increase global temperatures and to alter atmospheric and oceanic circulation pat­terns. GCMs are showing reasonable agreement on the extent of global warming that might occur from doubling CO2 concentrations (e.g., Han­sen et aI., 1988; Schneider, 1989). Much greater uncertainty exists about how regional precipitation patterns will change (Schlesinger and Mitchell, 1987). Such changes, if they occur, will be fundamental to the structure and function of stream and river ecosystems. We suggest that stream and river ecosystems that drain semiarid catchments in ENSO-affected regions of the world are excellent candidates for long-term research on global climate change. The linkage of these lotic ecosystems to a well-characterized and thoroughly researched ocean/atmosphere circulation pattern, the ENSO phenomenon, will assist freshwater ecologists in assessing the regional re­sponse of streams and rivers to potential global climate changes. In addi­tion, this link provides a direct tie to the research efforts of climatologists, oceanographers, and atmospheric scientists working at global scales and aquatic ecologists working at basin scales.

Conclusions

Stream and river ecosystems play pivotal roles in the environmental health of the planet as we face the specter of global climate change. This is espe­cially true in semiarid regions of the world where relatively small amounts of warming coupled with a slight decline in annual precipitation would pro­duce substantial decreases in runoff. Streams and rivers in semiarid zones are therefore excellent candidates for research on global climate change because they integrate spatial and temporal variability in basin precipita­tion, their flow responses to a changing precipitation regime are strongly nonlinear, and they represent critical resources for the overall economic health of these regions. In addition, many streams and rivers in semiarid regions are influenced by the ENSO phenomenon. This globally important driver of weather explains much of the variability in annual precipitation patterns in many semiarid environments. There is also a growing under-

12. Streams as Sensitive Indicators of Global Climate Change 259

standing of the functioning of the ENSO phenomenon, which allows some predictability at mesoscale time frames of regional precipitation. As GCMs become better able to represent ocean-atmosphere interaction, regions of the earth influenced by global atmospheric circulation systems such as ENSO may be particularly responsive and predictable elements of the landscape in the face of a changing global climate.

Acknowledgments. We thank Stuart Fisher and Penny Firth for inviting us to participate in the symposium, Troubled Waters of the Greenhouse Earth: Global Climate Change, Water Resources, and Freshwater Ecosys­tems, at the NABS meeting in Blacksburg, Virginia and encouraging us to write this paper. We appreciate the help of Tad Crocker and Jim Gosz in preparing the manuscript. An anonymous reviewer helped significantly in revising this manuscript by providing a thorough and constructive review. This study was partially supported by research Grants BSR-8616438 and BSR-9020561 from the National Science Foundation. This is publication number 13 of the Sevilleta National Wildlife Refuge Long-Term Ecological Research project.

References

Broecker WS, Kennett JP, Flower BP, Teller IT, Trumbore S, Bonani G, Wolfli W (1989) Routing of meltwater from the Laurentide ice sheet during the Younger Dryas cold episode. Nature (London) 341:318-32l.

Conrad V (1941) The variability of precipitation. Monthly Weather Rev 69:5-1l. Dickinson RE, Cicerone RJ (1986) Future global warming from atmospheric trace

gases. Nature (London) 319:109-115. Enfield DB (1989) EI Nino, past and present. Rev Geophys 27:159-187. Gasse F, Tehet R, Durand A, Gibert E, Fontes J-C (1990) The arid-humid transi­

tion in the Sahara and the Sahel during the last deglaciation. Nature (London) 346:141-146.

Gosz JR (1978) Terrestrial contribution of nitrogen to stream water from forests along an elevational gradient in New Mexico. Water Res 12:725-734.

Hansen J, Fung I, Lacis A, Rind D, Lebedeff S, Ruedy R, Russell G (1988) Global climate changes as forecast by Goddard Institute for Space Studies three­dimensional model. J Geophys Res 93:9341-9364.

Idso IB, Brazel AJ (1984) Rising atmospheric carbon dioxide concentrations may increase streamflow. Nature (London) 312:51-53.

Karl TR, Riebsame WE (1989) The impact of decadal fluctuations in mean pre­cipitation and temperature on runoff: A sensitivity study over the United States. Clim Change 15:423-447.

Molles MC, Jr, Dahm CN (1990) A perspective on EI Nino and La Nina: Global implications for stream ecology. J North Am Benthol Soc 9:68-76.

Neilsen RP, King GA, Develice RL, Lenihan J, Marks D, Dolph J, Campbell B, Glick G (1989) Sensitivity of ecological landscapes and regions to global climate change. U.S. Environmental Protection Agency Research Laboratory Report, Global Climate Change Research Team, Corvallis, Oregon.

260 Clifford N. Dahm and Manuel C. Molles, Jr.

Nemec J (1986) Hydrological Forecasting. D Reidel, Boston. Nicholls N (1988) EI Nino-Southern Oscillation and rainfall variability. J Clim

1:418-421. Poff NL (1991) Regional hydrologic response to climate change: An ecological

perspective. In Penelope Firth and Stuart G. Fisher, eds., Global Climate Change and Freshwater Ecosystems, pp. 88-115. Springer-Verlag, New York.

Rasmussen EM (1985) EI Nino and variations in climate. Am Sci 73:168-177. Rasmussen EM, Wallace JM (1983) Meteorological aspects of the EI Nino/

Southern Oscillation. Science 222:1195-1202. Revelle R, Suess H. (1957) Carbon dioxide exchange between the atmosphere and

ocean, and the question of an increase in atmospheric CO2 during the past decade. Tellus 9:18-27.

Revelle RR, Waggoner PE (1989) Effects of climatic change on water supplies in the western United States. In DE Abrahamson, ed, The Challenge of Global Warming, pp 151-160. Island Press, Washington DC.

Richey JE, Norbre C, Deser C (1989) Amazon River discharge and climate variability: 1903 to 1985. Science 246:101-103.

Ropelewski CF, Halpert MS (1987) Global and regional scale precipitation patterns associated with the EI Nino/Southern Oscillation. Monthly Weather Rev 115:1606-1626.

Schlesinger ME, Mitchell JFB (1987) Climate model simulations of the equilibrium climatic response to increased carbon dioxide. Rev Geophys 25:760-798.

Schneider SH (1989) The greenhouse effect: Science and policy. Science 243:771-780.

Wigley TML, Jones PD (1985) Influences of precipitation changes and direct CO2

effects on streamflow. Nature (London) 314:149-152.