Water balances of the northern catchments of Finland

Transcription

1 Northern Research Basins Water Balance (Proceedings of a workshop held at Victoria, Canada, March 2004). IAHS Publ. 290, Water balances of the northern catchments of Finland PERTTI SEUNA & JARMO LINJAMA Finnish Environment Institute, PO Box 140, FIN Helsinki, Finland pertti.seunafgjvmparisto.fi: iarmo.liniama@.vmparisto.fi Abstract This paper discusses the water balances of three basins in northern Finland, between latitudes 66 and 69 N. Drainage areas vary from 11.6 to 16.4 km 2. The Vâhâ-Askanjoki basin is completely covered by forest, the Iittovuoma basin is practically treeless, and the Laanioja basin is covered by forest to a major extent. Otherwise, conditions are quite similar in all three basins. Discharges are measured continuously, precipitation on a daily basis, and snow water equivalent (SWE) once or twice a month. Data series are years long. The average water balance components were P Conected = 640 mm, Q = 409 mm and ET = 231 mm. There was no trend in SWE in the north, but quite a strong decreasing trend in southern Finland (62 N). A slight increasing trend in the évapotranspiration of the two forested basins was seen. The estimation of true areal precipitation is the most critical parameter in water balance calculations. Keywords évapotranspiration; Finland; northern basins; precipitation; runoff; snow water equivalent; water balance INTRODUCTION Monitoring and research on small basins in Finland date back to the 1930s, when the first network was established. In the late 1950s the network was completely rebuilt. Measuring weirs were constructed and field surveys conducted on almost 40 basins. In the late 1980s and early 1990s the number of the basins amounted to about 90, due to comprehensive project studies aiming at the impacts of forest treatments, such as tree cutting and forestry drainage. At present, the national network consists of 45 catchments. The results of these catchments have been widely reported (see Mustonen, 1965a,b; Seuna, 1977, 1980, 1982, 1983a,b, 1989, 2003). This report discusses the water balance of three northern basins, namely Vâhâ-Askanjoki, Iittovuoma, and Laanioja. CATCHMENTS, METHODS AND MEASUREMENTS All three basins are located north of the Arctic Circle. The drainage areas are of the same order, and also climatological conditions, soil types and, to some extent, relief are quite similar. The biggest differences are in forest cover: the Vâhâ-Askanjoki basin is completely covered by forest, the Laanioja basin is partly forested, and the Iittovuoma basin is practically treeless. The characteristics of the basins appear in Table 1.

2 112 Pertti Seuna & Jarmo Linjama Table 1 Basin characteristics of the Vahâ-Askanjoki, Iittovuoma and Laanioja catchments. Characteristic Vahâ-Askanjoki Iittovuoma Laanioja Location 66 33'N, 27 40'E Drainage area 16.4 (km 2 ) Permafrost None extent Soils Coarse and fine sand moraines description (83%); Peat soils (17%) Vegetation Forest on firm land (83%); description mainly coniferous (91%) Climate Long cold winters, P mem : 600 mm year"', almost 40% as snow, T mem : 0 C Topography Variable relief with hills and low mountains ( m a.s.l.) Period of record (continues) Other Cont. Q, daily P, SWE, soi 1 frost TM, 'E 11.6 Q: discharge; P: precipitation; SWE: snow water equivalent. Sporadic permafrost may occur in the basin Stony moraines, rock exposures Almost treeless fields, except for dwarf birch Long cold winters, /'mean: 500 mm year" 1, 7mean: -1 C Low mountains ( m a.s.l.) (continues) Cont. Q, daily P, SWE 68 24TN,27 24'E 13.6 None Coarse moraines Dwarf birch, coniferous forest, partly treeless Long cold winters, Pmean; 500 mm year" 1, Tmean: -0.5 C Low mountains (220^*80 m a.s.l), treeless fjeld areas (continues) Cont. Q, daily P, SWE, soil frost Discharges are measured continuously using measuring weirs and water stage recorders. Rainfall is recorded on a daily basis, and snow water equivalent (SWE) once or twice a month, using snow courses and manual weighing. During the last three decades, different types of raingauges have been used. Until 1981 a Wild raingauge (500 cm 2 ) with a Nipher windshield was used. In , the gauge and the wind shield were changed to a Tretyakov gauge (200 cm 2 ), and in 1992 the collector was changed to a 200 cm 2 Holmberg & Heiskanen type, a modification of the Tretyakov gauge. In this study, the method of correction as elaborated by the Finnish Meteorological Institute (Solantie & Junila, 1995) was used with slight modifications, according to personal communications with Kaukoranta and Sankola. The wind correction is based on the exposures of the gauges compared with measurements from a completely sheltered gauge and a WMO pit gauge. The evaporation correction is based on the dimensions and structure of the precipitation gauge. For the raingauges used in this study, the total correction factors for the annual rainfall were estimated to vary from 1.17 to Two to four correction coefficients had to be used for each catchment due to the general change in gauge type in the 1980s, and, in some cases, due to a location change for an individual gauge. The water balances were calculated for hydrological years, starting with 1 November, using the corrected precipitation and measured discharges. RESULTS Water balances In Fig. 1(a), (b) and (c) the annual corrected precipitation, measured runoff and calculated (precipitation - runoff) évapotranspiration are given. The linear trends have

3 Water balances of the northern catchments of Finland 113 mm mm O r O C D O J C M L O C O T - r f h - O C O C D C D C N l o t o c o t o h - r - r - c o o o o o c n c n O T O T o ^ _. _ T T T T T r T C M v T T _ T _ T _ T 1000 mm C D C O O C M r f t D C O O C N - r f C O C O O C N t CO CO CO CO CO O CT> OÏ Oî CD O O o o rj> en en oi ai o > o > 0 > o > o > o o T - T - T - T - T - T - T - T - f - T - T - T - C S C M h - o > T - M i o r - o ) T - f ) i o h - o ) T - o > o ) O T O > o > o ) O ï 0 > o î o > c n c r ) O T - T - T - ^ T - T - T - T - T - T - T - T - C M Fig. 1 Water balance components for (a) Vaha-Askanjoki basin, (b) Iittovuoma basin, and (c) Laanioja basin.

4 114 Pertti Seuna & Jarmo Linjama Table 2 The annual values of corrected precipitation, runoff and calculated (ET = R - P) évapotranspiration for the hydrological years (1 November-31 October). Hydr. Vahâ-Askanjoki: Iittovuoma: Laanioja: year P(mm) R (mm) E (mm) P(mm) R (mm) E (mm) P(mm) R (mm) E (mm) mean

5 Water balances of the northern catchments offinland 115 been calculated in these figures. Changes in the water storages are not taken into account, as these data are not available The annual and mean values of corrected precipitation, runoff, and calculated évapotranspiration are shown in Table 2. The runoff coefficient in the Vâhâ-Askanjoki basin varies from 0.36 to 0.87 (mean 0.65), in the Iittovuoma basin from 0.42 to 0.88 (mean 0.59), and in the Laanioja basin from 0.54 to 0.83 (mean 0.66). There was no clear correlation between the coefficient and corrected annual precipitation in any of the catchments. Trends The water balance series are too short to reach strong conclusions, except, perhaps for the Hovi snow series. However, it seems that precipitation shows a slight rising trend in all three basins from 1976 to A comparison of 5-year means at the beginning and end of the series shows that the increase could be as high as mm in all the basins. The longer period (from 1959) in the Vâhâ-Askanjoki basin shows some variability, but only 30 mm of growth. As to the évapotranspiration, the outcome is not unambiguous, but a slight rising trend could be detected in the Laanioja basin and in the second half of the Vâhâ-Askanjoki basin. Statistically, the trends are not significant. In order to look into the potential climate change effects, the longest snow observation series in Finland was used, namely the Hovi snow course (Fig. 2(a)). This 4-km snow course is located at the town of Vihti (62 N) in southern Finland and the record covers the period from 1940 to At Hovi the average of the annual maximum water equivalent of snow has amounted to about 100 mm, varying between 39 and 175 mm for The trend is not statistically significant, but indicates some 40 mm decrease in about 60 years; that is, from 120 mm to 80 mm. Linear extrapolation would mean snow-free winters from 2120 onwards or even earlier in southern Finland! Since the mid-1960s the decline has been quite strong. The decrease in snow cover in southern and western parts of Finland is very much in line with the climate change scenarios presented for these areas. In the Hovi series, one could identify an approximately 15-year cycle in snowy winters, but it is, of course, possible that it is a pure coincidence. It may be of interest to mention that observations on the ice break-up in the River Tornionjoki in northern Finland have been collected since According to those data, the date of ice break-up has become earlier by about two weeks (Kajander, 1995). The Hovi snow series can be compared with the Vâhâ-Askanjoki snow series, which covers the period from 1961 to 2003 (Fig. 2(b)). No clear trend in the Vâhâ- Askanjoki snow series can be detected. The date of the maximum snow water equivalent usually occurs around late March to early April. Maximum runoff Maximum daily runoffs were studied separately for spring and summer (see Figs 3 and 4). The spring maximum runoff occurred around late April to early June, depending on the snowmelt progress. The period for the summer maximum runoff began after the end of snowmelt and lasted to the end of October. The average values

6 116 Pertti Seuna & Jarmo Linjama (a) o fo to <j) c N m c o T - T t r ^ o c n t D c n c N (b) (Û to h - N - h - C O C O C O O l C n C D O T O Fig. 2 The annual maximum snow water equivalent (a) at Hovi, southern Finland (62 N) and (b) at Vahâ-Askanjoki, northern Finland (66 N). of the spring maximum specific discharge for Vahâ-Askanjoki, Iittovuoma and Laanioja were 140.6, 141.0, and s" 1 km" 2, respectively, and those of the summer maxima were 71.0, 45.9, and s" 1 km" 2, respectively. None of these are very high. It may be noted that the highest daily runoff ever in the Finnish small catchments has been recorded during snowmelt and amounts to s" 1 km" 2. The highest instantaneous runoff has been recorded during a thunderstorm in early September and amounts to s" 1 km" 2. Both values were recorded in Hovi, southern Finland. For comparison, the formula for the Probable Maximum Flood (PMF) proposed by Chinese researchers at the WMO-CHy meeting held in Abuja, November 2000, may be noted. According to this simple formula, PMF = 1830x/ 7 316, where PMF is discharge in m 3 s" 1 and F is the drainage area in km 2. The formula leads to very high values. Values are about times higher than ever recorded in Finland in small catchments, and the formula is possibly not so universal as proposed. Obviously, it is not suitable for small catchments.

7 Water balances of the northern catchments offinland Fig. 3 Spring maximum runoff at Vâhâ-Askanjoki, Iittovuoma and Laanioja. l/s/km' Vâhâ-Askanjoki - Iittovuoma -Laanioja Fig. 4 Summer maximum runoff at Vâhâ-Askanjoki, Iittovuoma and Laanioja. DISCUSSION The calculated values of the water balance components vary relatively strongly from year to year, and this also concerns évapotranspiration (ET). The lowest évapotranspiration values (calculated as a difference between precipitation and runoff) for all the three basins are below 100 mm year" 1, and the highest annual values exceed 400 mm, with the average of about 230 mm year" 1 (see Fig. 1 and Table 2). From the records it can be concluded that water availability plays an important role in the amount of évapotranspiration. In the north there is a potential for substantial amounts of solar energy available in summer. This is confirmed by the calculations of potential évapotranspiration (Seuna, 1977; Finnish Meteorological Institute, 2003, personal communication). The Penman-Monteith values for Ivalo (Laanioja region) have shown

8 118 Pertti Seuna & Jarmo Linjama about 450 mm of potential évapotranspiration in This is twice as much as the calculated actual ET. The variation in ET values in Fig. 1 is partly due to the fact that the changes in the water storages (soil water content and groundwater storage) are not taken into account, as these data are not available. Another reason is that the precipitation correction did not take into account the fact that, when most ofthe annual precipitation falls as snow, the annual correction should be higher than in the years when the portion of snow has been small. This means that the individual annual values given may be too high or too low, depending on the storage conditions. In the averages of several years these under- or overestimations are eliminated. In the treeless Iittovuoma basin, the drift of snow from or to the basin caused by strong wind can cause small under- or overestimations in precipitation. The biggest uncertainties for the water balance calculations originate from the precipitation values. The corrections, especially due to wind, are high up to 30% as estimated here on the annual basis (Finnish Meteorological Institute, 2003, personal communication). This under-catch of gauges occurs even with the presence of windshields, and it can be higher in winter. In the Vahâ-Askanjoki basin, the water equivalent of snow from the snow course was compared with uncorrected precipitation. The comparison was made during the winter periods, with no occurrences of melting before the maximum accumulation of snow. By this comparison the correction factor for raingauges was 1.49 for the Wild and 1.34 for the Tretyakov, on the average. The factor may be even higher, because snow retained in frees has not been taken into account in the snow course measurements. Until now somewhat lower correction factors have been recommended for northern Finland. In Finland, the precipitation correction studies have been carried out in the southern part of the country, but due to the different structure of snow in northern and southern Finland, the exposure effect is very likely to differ in the north and the south. This effect is very difficult to take into account precisely. Another fact significant for the accuracy of the precipitation values is the rather scarce network of raingauges in Lapland, where the population is sparse. Therefore precipitation values of fairly long distances have to be used. In the short term, it is an important source of error, but less important when averaging several years. The balance calculations strongly underline the importance of precipitation measurements combined with sufficient snow cover measurements in order to achieve reliable input values for water balance and other hydrological studies in northern conditions. CONCLUSIONS Based on the study, following conclusions can be made: 1. The average water balance components for the three basins were as shown in Table There might be a slight rising trend in the precipitation values of all the three basins, but it was not determined to be significant. 3. Despite the storage differences between individual years, a slight increasing trend seems probable in the évapotranspiration of the two forested basins, Vahâ- Askanjoki and Laanioja. In the unforested field basin, Iittovuoma, the runoff showed a slight rising trend.

9 Water balances of the northern catchments of Finland 119 Table 3 Average water balance components for the basins studied. Basin -^corrected R ET Runoff coefficient (mm) (mm) (mm) Vâhâ-Askanjoki ( ) Iittovuoma ( ) Laanioja ( ) 4. There was no trend in the maxima of the northern snow water equivalents, but quite a strong decreasing trend in the long-term series from south Finland. 5. The estimation of the true areal precipitation is the most critical parameter in water balance calculations. REFERENCES Kajander, J. (1995) Cryophenological records from Tornio. Mimeograph series of the National Board of Waters and the Environment, 552. National Board of Waters and Environment, Helsinki, Finland. Mustonen, S. E. (1965a) Hydrologie investigations by the Board of Agriculture during the years 1957 to Soil Hydrotechnical Investigations 11. Engineering Department of the Board of Agriculture, Finland. Mustonen, S. E. (1965b) Effects of météorologie and basin characteristics on runoff. Soil and Hydrotechnical Investigations 12. Engineering Department of the Board of Agriculture, Finland. Mustonen, S. E. (1965c) Effect of météorologie terrain factors on water equivalent of snow cover and on frost depth. Ada Forestalia Fennica 79. Seuna, P. (1977) On the hydrometeorological factors affecting irrigation. Publications of the Water Research Institute 24. National Board of Waters, Helsinki, Finland. Seuna, P. (1980) Long-term influence of forestry drainage on the hydrology of an open bog in Finland. In: The Influence of Man on the Hydrological Regime with Special Reference to Representative and Experimental Basins (Proc. Helsinki Symp, June 1980), IAHS Publ IAHS Press, Wallingford, UK. Seuna, P. ( 1982) Frequency analysis of runoff of small basins. Publications of the Water Research Institute 48. Frequency National Board of Waters, Helsinki, Finland. Seuna, P. (1983a) Small basins a tool in scientific and operational hydrology. Publications of the Water Research Institute 51. National Board of Waters, Helsinki, Finland. Seuna P. (1983b) Influence of physiographic factors on maximum runoff. Infiltration and its dependence on some physiographic factors. Publications of the Water Research Institute 50. National Board of Waters, Helsinki, Finland. Seuna, P. (1989) Effects of clear-cutting forestry drainage on runoff in the Nurmes-study. Publications of the Academy of Finland 4. National Board of Waters, Helsinki, Finland. Seuna, P. (2003) Hydrological trends after agricultural sub-drainage at the Hovi experimental catchment, southern Finland. Poster in IUGG XXI11 General Assembly, Sapporo. Solantie, R. & Junila, P. (1995) Precipitation correction with comparative measurements of Tretyakov and Wild raingauges (in Finnish). and