-10 -5 0 5 10 15 20 25
11.1 2.3 21.4 10.6 30.7 18.9 7.11 27.12 15.2
Temperature [°C]
Kitkajoki
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3.2 Diurnal variation of water temperature
During the summer season surface water temperature has usually a diurnal variation and a vertical stratification, and the difference between morning and afternoon temperatures can be notable. Based on data used in this study, diurnal variation of water temperature was strongest during mid-summer months (July and August) in 2014 when there were most intense air temperature daily cycle. The minima of surface water temperature occurred usually night time or before sunrise and maxima during late afternoon or early evening. Based on the pressure transducer water temperature data, apparent diurnal water temperature variation is present in the sensor depth. In Kyyvesi, temperature variation cycle in July in 1.7 meters depth (Figure 6) was very similar to nearby surface water (Haukivesi 20 cm depth) station diurnal variation (Figure 7).
During summer 2014 only a couple of official surface water temperature devices logged temperatures for an hour interval. This allows to study diurnal variation of water temperature near the surface. As an example surface water temperature variation in July in lake Haukivesi observation site is presented in Figure 7. In early July weather was rather cool and there was no clear diurnal variation present. Later weather type changed to more typical summer weather with clear diurnal cycle. This was also seen from the water temperature data. In late July typical diurnal variation of surface water temperature was 1-3 Celcius degrees. The biggest change in surface water was cooling of water by 6 Celcius degrees in a day.
Figure 6. Diurnal variation of water temperature at 1.7 meters depth in July-August 2014 in Kyyvesi. Purple signs show minima of the day, green triangles maxima of the day. Blue squares indicate morning values of official water temperature site.
16 18 20 22 24 26 28
Temperature [°C]
20.7.-3.8.2014
Kyyvesi
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Figure 7. Hourly temperature information from one of the official surface water temperature sites (Haukivesi, near Kyyvesi) in July 2014.
Surface waters start usually to cool in early August (Korhonen 2002). During summer 2014 surface water maxima was recorded in most of the country in late July (23rd – 27th). A small diurnal variation persisted until end of the September. After that effect of solar radiation did not warm surface waters anymore during the day time.
Official daily surface water temperatures have been measured manually during the 20th century. Observers are guided to perform measurements in the morning at 8 AM, but there is no guarantee that all measurements during the decades are really done in the morning according the procedures. If measurements have been done sometimes during the late afternoon, there might be even 2-4 degree bias for higher than real values during the mid-summer time. Of course in the case of cooling period bias can be towards lower values. This might affect trend analysis of maximum values. Nowadays, since almost all sites have automatic measurements this kind of uncertainty is not present.
4. CONCLUSIONS
Study showed that pressure sensors which are situated directly in a lake or a river within 2 meter depth from the surface can provide usable information on surface water temperature in that specific area. Bias between 20 cm depth’s official morning values and the values of pressure sensor’s depth varied more in the early summer than during the late summer as was expected due to depth of thermocline. Usually difference was less than 2 degrees. In the late summer difference between 20 cm depth and pressure sensor’s depth (1-2 m) was even smaller since both measurements were done in the same warm mixed surface layer. Even though pressure sensors are measuring deeper than just surface water they provide extra information, which can be used as input data for hydrological models and numerical weather prediction models, evaporation and heat budget calculations. These sensors also provide additional information for ecology research, fishing and recreational users. For this kind of use of data the accuracy of measurements is feasible. However, it is always advisable to first
14
1.-31.7.2014 one hour measuring interval
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evaluate data from each station with the nearby official surface water temperature stations depending on purposes of data use. Nevertheless, these pressure transducer data should not be used for long-term climate change studies since the depth and installing site of the sensor is not prearranged for water temperature measurements purposes and this might affect the results.
There is a significant diurnal fluctuation of water temperature in the surface water layer during warmest summer months when the air temperature is having clear diurnal variation and solar radiation is high. Actually, official surface water temperature values measured at 8 am are during warmest summer months (July and August) near the timing of lowest values of the day and temperatures in the afternoon can be even 2-4 degrees higher. This fact should be taken into account while using the official surface water temperatures for different purposes.
5. REFERENCES
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Elo, A. & Koistinen, A., 2002 Evaluating temperature of lake surface and lake evaporation in Mäntyharju watershed area. XXII Nordic Hydrological conference, Röros. NHP Report no. 47. Nordic Hydrological Programme. 417-426.
Elomaa, E. 1977 Pääjärvi Presentative Basin Finland: Heat Balance of a Lake. Fennia 149.
Järvinen, J. & Huttula, T. 1982 Estimation of Lake Evaporation by Using Different Aerodynamical Equations. Geophysica 19(1): 87-99.
Kheyrollah Pour, H., Duguay, C., Solberg, S. & Rudjord, Ø. 2014 Impact of satellite-based lake surface observations on the initial state of HIRLAM. Part I: evaluation of remotely-sensed lake surface water temperature observations. Tellus A2014, 66, 21534.
Korhonen, J. 2002 Water temperature conditions of lakes and rivers in Finland in the 20th century. Finnish Environment 566. (In Finnish, summary in English). Finnish Environment Institute, Helsinki. 116 p.
Rontu, L., Eerola, K., Kourzaneva, E. & Vehviläinen, B. 2012 Data assimilation and parametrizations of lakes in HIRLAM. Tellus A 2012, 64, 17611.
Seppälä, O. 2015 Järvien ja jokien veden lämpötilan vaihtelut - paineanturihavaintojen käytettävyystarkastelu. = Water temperature variations in lakes and rivers - usability study of pressure transducer temperature data. Bachelor’s Thesis, Aalto University. 36 p. (In Finnish only)
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