The effects of climate change on discharges and snow cover in Finland

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1 Hydrological Sciences Journal ISSN: (Print) (Online) Journal homepage: The effects of climate change on discharges and snow cover in Finland BERTEL VEHVILÄINEN & JARI LOHVANSUU To cite this article: BERTEL VEHVILÄINEN & JARI LOHVANSUU (1991) The effects of climate change on discharges and snow cover in Finland, Hydrological Sciences Journal, 36:2, 19121, DOI: 1.18/ To link to this article: Published online: 29 Dec 29. Submit your article to this journal Article views: 218 View related articles Citing articles: 22 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 31 December 217, At: 23:53

2 Hydrological Sciences Journal des Sciences Hydrologiques, 36,2, 4/1991 The effects of climate change on discharges and snow cover in Finland* BERTEL VEHVILÂINEN & JAM LOHVANSUU National Board of Waters and the Environment, Water Research Institute, Hydrological Office, PL 436, 11 Helsinki, Finland Downloaded by [ ] at 23:53 31 December 217 Abstract The effects of climate change obtained through model simulations on runoff and snow cover in Finland are presented in this study. The results are based on hydrological model simulations using the information from one climatic general circulation model (GISS) scenario with doubled atmospheric carbon dioxide as input. According to the GISS model scenario, the temperature increase in Finland will be 26 C, the precipitation increase 13 mm per month and the evaporation increase 53 mm per month. The effects of these climatic changes on runoff and snow cover have been evaluated in twelve different watersheds by hydrological watershed models. According to the results the mean discharge (MQ) increases by 25%, the mean minimum discharges (MNQ) increase considerably in winter due to a two to three months shorter snow cover period and the mean maximum discharges (MHQ) decrease due to diminished maximum snow water equivalents. In southern Finland persistent winter snow cover will vanish. Les effets du changement de climat sur les débits et la couverture de neige en Finlande Résumé Les effets du changement de climat sur l'écoulement et la couverture de neige en Finlande, simulés au moyen d'un modèle, sont présentés dans cette étude. Les résultats sont obtenus à partir des simulations fournies par un modèle hydrologique où on utilise le scénario de modèle climatique de GISS avec une concentration d'anhydride carbonique double de la concentration existante comme entrée. D'après le scénario du modèle GISS l'augmentation de température en Finlande sera de 2 à 6 C, l'augmentation de précipitation sera de 1 à 3 mm par mois et l'augmentation d'évaporation sera de 5 à 3 mm par mois. Les effets de ces changements climatiques sur l'écoulement et la coverture de neige sont ensuite évalués dans douze bassins versants différents au moyen des modèles hydrologiques de bassins versants. Selon les résultats le débit moyen (MQ) augmentera de 2 à 5%, le débit minimum moyen (MNQ) * Paper presented in the Hydrology Programme of the EGS Assembly, Copenhagen, Denmark, 2327 April 199. Open for discussion until 1 October

3 Bertel Vehvilàinen & Jari Lohvansuu 11 INTRODUCTION augmentera considérablement en hiver par suite d'une période de couverture de neige plus courte de deux à trois mois, et le débit maximum moyen (MHQ) diminuera à cause d'une diminution d'équivalent en eau de la couverture de neige. Dans le sud de Finlande le couverture de neige ne persistera plus tout l'hiver. Downloaded by [ ] at 23:53 31 December 217 The basis of this study is the results from a report prepared for the European Workshop on International Bioclimatic and Land Use Changes compiled by Koster & Lundberg (1987). The changes in the climatic variables in the Fennoscandian area presented in that report are based on the simulation results from a climatic model developed by Hansen et al. (1984) at the Goddard Institute for Space Studies (GISS). With the GISS model the effects of doubling the atmospheric C 2 concentration on temperature, precipitation and evaporation have been evaluated. Those changes have been input values to hydrological watershed models. The reason why the GISS model was chosen are mainly those presented by Bach (1987). Bach chose the GISS model as the best choice of general circulation model (GCM) in his scenario analysis. The criteria for the choice were: the GCM should be based on a realistic geography and topography; it should have a high spatial resolution and adequate temporal resolution; it should incorporate a coupled model of the atmosphereocean circulation and simulate realistically the patterns of the observed climate. However, sensitivity analysis by Hansen et al. (1984) showed that higher spatial resolution and improvements in modelling various physical processes are still needed in the GISS model. The hydrological model used to evaluate the effects of climate change on runoff and snow is the Finnish version of the HBV model (Bergstrôm, 1976), which is widely used in Fennoscandian countries for the forecasting of discharges (over 5 applications) and for the simulation of snow in real time (Vehvilàinen, 1989). The study areas are presented in Fig. 1. The Sâkylân Pyhâjârvi and Loimijoki basins in southern Finland and the Lapuanjoki, Àhtâvânjoki, Perhonjoki and Siikajoki basins on the western coast of Finland are flat with an altitude range of 5 m; about 5% is forested area, 23% fields and the rest is open or thinly forested marshland. The three northern basins (Ylâ Kemijoki, Ounasjoki and Ivalonjoki) have a larger altitude range (about 2 m) and more forested area with open marshland (together over 9%). The percentages of lakes are less than 5% in all the study basins. The three basins in central and eastern Finland (Iisalmen reitti, Koitajoki and Hyrynsalmen reitti) have larger elevation ranges (12 m) and larger lake percentages (81%) than the other basins. They also consist mainly of forested areas. THE CHANGE OF CLIMATIC VARIABLES The results from the GISS model concerning changes of temperature,

4 Downloaded by [ ] at 23:53 31 December 217 Ill Effects of climate change on discharges and snow cover in Finland Fig. 1 The research areas: 1 = Loimijoki; 2 = Sàkylân Pyhâjârvi; 3 = Lapuanjoki; 4 = Àhtàvânjoki; 5 = Perhonjoki; 7 = Siikajoki; 8 = lisalmen reitti; 18 = Koitajoki; 19 = Hytynsalmen reitti; 11 = YlàKemijoki; 12 = Ounasjoki; and 14 = Ivalonjoki.

5 Bertel Vehvilâinen & Jari Lohvansuu 111 precipitation and evaporation due to doubled atmospheric carbon dioxide were used as input values for hydrological watershed models. The GISS model gives changes of temperature, precipitation and evaporation for different periods of the year. These changes were added to the daily values of each climatic variable in the simulation period of 51 years length in the 197s and 198s The watershed models were then simulated over this period with both the original and the changed data. Comparisons were then made between the hydrological model outputs (discharges, snow water equivalents) of those two different simulations. The changes of climatic variables and their mean values are presented in Tables (16). The temperature increase of 4 C for global warming calculated by the Downloaded by [ ] at 23:53 31 December 217 Table 1 The increase of daily mean temperature ( XJ) due to doubling carbon dioxide concentration in Finland Southern Finland Northern Finland Winter Spring Table 2 Mean temperature (XJ) in different basins Loimijoki Pyhâjârvi Lapuanjoki Ahtàvànjoki Perhonjoki Iisalmen reitti Koitajoki Hyrynsalmen reitti Siikajoki YlâKemijoki Ounasjoki Ivalonjoki Summer Autumn Period Winter Spring Summer Autumn XIIII IIIV VIVIII IXXI Table 3 The increase of monthly precipitation (mm) due to doubling carbon dioxide concentration in Finland Winter Spring Summer Autumn Southern Finland Northern Finland

6 113 Effects of climate change on discharges and snow cover in Finland Table 4 Monthly precipitation (mm) in different basins Period Winter Spring Summer Autumn xnn inv vivm ixxr Loimijoki Pyhàjàrvi Lapuanjoki Ahtàvànjoki Perhonjoki Iisalmen reitti Koitajoki Hytynsatmen reitti Siikajoki YlâKemijoki Ounasjoki Ivalonjoki Downloaded by [ ] at 23:53 31 December 217 Table 6 basins Loimijoki Pyhàjàrvi Lapuanjoki Ahtàvànjoki Perhonjoki Iisalmen reitti Koitajoki Hyrynsalmen reitti Siikajoki YlâKemijoki Ounasjoki Ivalonjoki Table 5 The increase of monthly evaporation (mm) due to doubling carbon dioxide concentration in Finland Southern Finland Northern Finland Winter 3 1 Spring Summer Autumn Cumulative class A evaporation (mm) over three months in different Period Winter Spring Summer Autumn XIIII IIIV VlVin IXXI GISS model (Hansen et al., 1984) falls in the upper part of the temperature range given in some international reports as the effect of C 2 increase: C from a report by the International Meteorological Institute in Stockholm or C from the UNEP/COMCMCSU International Conference in Villach, 1985, according to Heino (1987). The results for changes in precipitation differ considerably in different

7 Bertel Vehvilâinen & Jari Lohvansuu 114 Downloaded by [ ] at 23:53 31 December 217 climatic models, but the overall tendency is an increase of precipitation. The results for changes in evaporation differ considerably in different climatic models. The possible feedback effects caused by the response of vegetation (forest growth) to climate change are not taken into account either in the GISS model or in the hydrological HBV model. These feedback effects may have an effect especially on transpiration from vegetation. Warmer climate enhances forest growth and increases evaporation. But the temperature increase of O.TC or more per decade may cause adaptation problems to forests and decrease forest growth and transpiration. Thus the change of evaporation or évapotranspiration can be taken only as preliminary. The actual evaporation in the HBV model is simulated as a function of soil moisture and potential evaporation, which is taken approximately as 7% of the class A pan value. The increase of evaporation is introduced into the simulation as an increase in potential evaporation. Seasonal variability is taken into account to some extent by the GISS scenario as can be seen from Tables 1, 3 and 5. To the question of how extreme values of precipitation, temperature or evaporation change, the GISS model gives no quantitative information as judged from the information available. RESULTS The change of discharges The changes of temperature, precipitation and evaporation due to the doubling of carbon dioxide have been added to the observed input data in the simulation period. The hydrological watershed model has been run with the original data and with the changed data for the twelve watersheds. The comparison of the results for monthly mean discharges between the original climatic data and the changed data due to doubled carbon dioxide are presented in Figs 27 for three basins; one each in southern, central and northern Finland. According to those results, winter discharges increase considerably in the doubled C 2 situation in the whole of Finland. The spring high flows decrease considerably and in southern Finland the time of the maximum discharge moves from spring to autumn. The smallest changes are found in the summer in both southern and northern Finland. Maximum discharges decrease, especially in northern Finland, due to the smaller snow storage in the spring. In southern Finland maximum discharges diminish less or not at all and their time changes from the spring to the autumn. The values of the mean maximum discharge (MHQ) and mean minimum discharge (MNQ) are presented in Tables 8 and 9. MHQ decreases considerably in northern and eastern Finland and moderately or not at all in southern Finland as presented in Table 8. MNQ, as seen in Table 9, increases considerably in northern and eastern Finland and moderately or not at all in southern Finland. The changes in mean discharges (MQ) over the whole simulation period

8 115 Effects of climate change on discharges and snow cover in Finland Downloaded by [ ] at 23:53 31 December 217 Fig. 2 Loimijoki, period : simulated monthly mean discharges with original meteorological data (1 x C 2 ) and changed meteorological data in the doubled carbon dioxide situation (2 x C 2 ). SNOW WATER EQUIVALENT Fig. 3 Loimijoki, period : simulated monthly mean areal snow water equivalent with original meteorological data (1 x CO 2) and changed meteorological data in the doubled carbon dioxide situation (2 x C 2 ).

9 Bertel Vehvilâinen & Jari Lohvansuu Inflow m3/s 15^ 1. 2XC2 Downloaded by [ ] at 23:53 31 December I II,1XC2 1 III IV V VI VII VIII IX X 1 XI ] 1 1 XII Fig. 4 Koitajoki, period : simulated monthly mean inflow (Koitere lake) with original meteorological data (1 x CO^ and changed meteorological data in the doubled carbon dioxide situation (2 x CO 2). 15. Snow water equivalent XC i2xc2 III IV VI VII VIII IX XI XII Fig. 5 Koitajoki, period : simulated monthly mean areal snow water equivalent with original meteorological data (1 x C 2 ) and changed meteorological data in the doubled carbon dioxide situation (2 x CO 2).

10 117 Effects of climate change on discharges and snow cover in Finland 1 m3/s DISCHARGE 8 _ 7 _ 6 5 _ Downloaded by [ ] at 23:53 31 December 217 4_ XC2 1XC2 1 I 1 II ' III IV V VI VII 1 _ 'VIII IX. X 1 XI ] 1 l xii! Fig. 6 YlàKemijoki, period : simulated monthly mean discharges with original meteorological data (1 x C 2 ) and changed meteorological data in the doubled carbon dioxide situation (2 x C 2 ). SNOW WATER EQUIVALENT 6 r VII VIII Fig. 7 YlâKemijoki, period : simulated monthly mean areal snow water equivalent with original meteorological data (1 x CO 2) and changed meteorological data in the doubled carbon dioxide situation (2 x CO 2).

11 Bertel Vehvilâinen & Jari Lohvansuu 118 are presented in Table 7. MQ increases by 25% (mean 4%) in all basins. This is due to the increased precipitation in the doubled carbon dioxide scenario. The increase of evaporation counterbalances the increase of precipitation only in the summer (Tables 3 and 5). Table 7 The observed and simulated mean discharges (MQ) over the whole simulation period with original (1 x C 2 ) and changed (2 x CO ^ climatic data; the increase is calculated between simulated values Downloaded by [ ] at 23:53 31 December 217 Loimijoki Pyhàjàrvi (1) Lapuanjoki Lappajarvi (1,4) Perhonjoki Iisalmen reitti (1) Koitajoki (1) Lylykoski (2) Mohko (2) Hyrynsalmen reitti Siikajoki Uljua (1,3) YlàKemijoki (1) Ounasjoki Ivalonjoki Period Observed 1 x CO, 2 x CO. Change in MQ l z MQ MQ MQ? 7 3 7? 7 nr s* m J s m J s % (1) (1) inflow; (2) subbasin of Koitajoki; (3) subbasin of Siikajoki; (4) subbasin ofàhtâvânjoki. Table 8 The observed and simulated mean maximum discharges (MHQ) over the whole simulation period with original (1 x CO 2) and changed (2 x CO 2) climatic data; the increase is calculated between simulated values Period Observed 1 x CO 2 x C Change in MHQ MHQ MHQ MHQ l m 3 s' 1 m 3 s' 1 m 3 s' 1 % Loimijoki Pyhàjàrvi (1) Lapuanjoki Lappajarvi (1,4) Perhonjoki Iisalmen reitti (1) Koitajoki (1) Lylykoski (2) Mohko (2) Hyrynsalmen reitti Siikajoki Uljua (1,3) YlàKemijoki (1) Ounasjoki Ivalonjoki (1) (1) inflow; (2) subbasin of Koitajoki; (3) subbasin of Siikajoki; (4) subbasin ofàhtâvânjoki.

12 119 Effects of climate change on discharges and snow cover in Finland Table 9 The observed and simulated mean minimum discharges (MNQ) over the whole simulation period with original (1 x CO 2) and changed (2 x C 2 ) climatic data; the increase is calculated between simulated values Period Observed MNQ 3 1 m s 1 x CO MNQ l 3 1 m s 2 x CO, / MNQ 3 1 m s Change in MNQ % Downloaded by [ ] at 23:53 31 December 217 Loimijoki Pyhàjàrvi (1) Lapuanjoki Lappajârvi (1,4) Perhonjoki lisalmen reitti (1) Koitajoki (1) Lylykoski (2) Mohko (2) Hyrynsalmen reitti Siikajoki Uljua (1,3) YlàKemijoki (1) Ounasjoki Ivalonjoki (1) * * (1) inflow; (2) subbasin of Koitajoki; (3) subbasin of Siikajoki; (4) subbasin ofàht'àv'ânjoki; * negative due to evaporation from lake. Table 1 Maximum monthly snow water equivalent in observed period (1 x CO 2) and in 2 x CO 2 scenario Loimijoki Sàkylàn Pyhàjàrvi Lapuanjoki Àhtàvànjoki Perhonjoki lisalmen reitti Koitajoki Hyrynsalmen reitti Siikajoki Uljua* YlàKemijoki Ounasjoki Ivalonjoki * subbasin of Siikajoki. Period 1 x CO (mm) l x C 2 (mm) Change % The change of snow cover The increase in temperatures by 56 C in the winter greatly shortens the time of snow cover and the snow accumulation period. Also the maximum amount of snow during these short winters is diminished by 8% in southern and central Finland and by 5% in northern Finland (Table 1). The monthly

13 Bertel Vehvilâinen & Jari Lohvansuu 12 means of snow water equivalent simulated with the original and the changed data in three watersheds are presented in Figs 3, 5 and 7. In southern Finland, persistent snow cover vanishes and only occasionally are there periods with continuous snow cover lasting for one month in February or March. The same is true also for the Lapuanjoki basin on the western coast of Finland. In the northern river basins, the snowcover time will diminish by one or two months from the beginning and at the end of the winter. The acute part of the winter period with no melt exists only in February. The mean maximum snow storage diminishes to 15% of its previous value in southern Finland, to 2% in central Finland and to 5% in northern Finland. DISCUSSION Downloaded by [ ] at 23:53 31 December 217 The most prominent effect of the climate change caused by a carbon dioxide increase is the nearly total vanishing of the acute part of the winter in southern Finland and the strong shortening of winters and the snow accumulation period in central and northern Finland. This causes a strong increase in discharges over the whole winter period and especially at the beginning and end of the winter. The increase of precipitation strengthens further the effect of temperature in increasing the runoff during the winter. The increase of evaporation counterbalances the effect of the precipitation increase during the summer and the change in discharge is smallest in the summer. The results of this study were obtained by using the output of a climatic model (GISS) as an input for a hydrological model (HBV). Thus the results rely heavily on the accuracy and relevance of those models. One must be aware of this when using the above results. Still, there is no basic reason why the overall effects of climate change should not be like those presented. The climate model GISS gives a larger temperature increase than many other climate models, but all climate models give a definite increase in temperature as one effect of a carbon dioxide increase in the atmosphere. The present climate models do not give information about possible changes of probabilities of daily maximum precipitation or the change of variation of daily temperatures, which may have considerable effects on maximum discharges and flooding. Thus the decrease of maximum discharge (HQ) values were obtained supposing that the maximum precipitation increases in the same proportion as the mean precipitation in different seasons and that the variation of daily temperature is not changed in the doubled carbon dioxide simulation. REFERENCES Bach, W. (1987) Scenario analysis. In: European Workshop on Interrelated Bioclimatic and Land Use Changes, Vol. A: Climate Scenarios, Noordwijkerhout, The Netherlands, 1721 October, 1987, 11. Bergstrôm, S. (1976) Development and application of a conceptual runoff model for Scandinavian catchments. SMHI Rapporter Hydrologi oeh Oceanografi Nr RHO 7. Swedish Meteorological and Hydrological Institute, Norrkôping, Sweden. Hansen, J., Lacis, A., Rind, D., Russell, G., Stone, P., Fund, I., Ruedy, J. & Lerner, J. (1984) Climate sensitivity: analysis of feedback mechanisms. In: Climate Processes and Climate

14 Downloaded by [ ] at 23:53 31 December Effects of climate change on discharges and snow cover in Finland Sensitivity (ed. J. Hansen & T. Takahashi). Maurice Ewing Series, 5, AGU, Washington, DC, USA, Heino, R. (1987) Climatic scenarios. In: European Workshop on Interrelated Bioclimatic and Land Use Changes, Vol. D, Impact analysis of climatic change in the Fennoscandian part of the boreal and subarctic zone (ed. E. A. Koster & H. Lundberg), Noordwijkerhout, The Netherlands, 1721 October, 1987, 234. Koster, E. A. & Lundberg, H. (1987) Impact analysis of climate change in the Fennoscandian part of the boreal and subarctic zone. European Workshop on Interrelated Bioclimatic and Land Use Changes, Vol. D, Noordwijkerhout, The Netherlands, 1721 October Vehvilainen, B. (1989) Operational snow accumulation and snowmelt modelling. In: New Directions for Surface Water Modelling ed. M. L. Kawas (Proc. Baltimore Symp. May 1989). IAHS Publ. no. 181, Received 11 May 199; accepted 13 September 199