A STUDY ON EVAPORATION IN IOANNINA, NW GREECE

Transcription

1 Proceedings of the 9 th International Conference on Environmental Science and Technology Rhodes island, Greece, 1 3 September 25 A STUDY ON EVAPORATION IN IOANNINA, NW GREECE A. V. KONTOGIANNI and A. BARTZOKAS Laboratory of Meteorology, Department of Physics, University of Ioannina, Greece abartzok@cc.uoi.gr EXTENDED ABSTRACT In this work, evaporation in Ioannina, NW Greece is studied for the 16-year period The data basis comes from the readings of the Wild evaporigraph charts of the meteorological station of Ioannina University. The total number of data consists of 7,8 bi-hourly values. At first, the main characteristics of evaporation in the study area are presented on annual, monthly and daily basis. The intra-annual variation of evaporation resembles to a simple sinusoidal curve with one maximum (July, 153mm) and one minimum (December, 16mm). The maximum inter-monthly evaporation change is found from August to September (-56mm) and the minimum from December to January (+6mm). The diurnal variation is also simple with a maximum at 14:-16: hour (.15mm in winter and.9mm in summer) and a minimum at 4:-6: hour (.3mm in winter and.5mm in summer). The study of frequency distribution reveals that, during winter, daily evaporation, in 9% of the cases, is less than 1.8mm. The maximum frequency appears for daily values less than.2mm, while the maximum values recorded are around 5mm. In summer, the curve is closer to the Gaussian distribution with maximum frequency of daily evaporation at around 4-6mm and maximum values above 1mm. A long-term study (test Mann-Kendal) revealed that there is a statistically significant (.5 level) negative trend during winter and autumn. For spring, summer and the year as a whole, trends were not found statistically significant. Examination of the number of days with evaporation above or below specific thresholds showed that in winter and autumn, the number of days with low evaporation increases while that with high evaporation decreases, in agreement with the general winter trend. The lower winter evaporation values during the recent years are related to the increase of precipitation in Ioannina and NW Greece after 199 due to the increase of cyclonic circulation in Central Mediterranean. Finally, the synoptic patterns over Europe and the Mediterranean, associated with cases of extreme evaporation values in NW Greece, are investigated. Factor Analysis was applied on the pressure space-series of the days of the upper decile for winter and lower decile for summer. The approximately 145 days (pressure patterns) of each category were grouped objectively to 4 factors in winter and 5 factors in summer, explaining around 85% of the total variance. In winter, it appears that the wind plays the major role since in the first 3 factors the main feature of the pressure pattern in the area of NW Greece is a dry and katabatic easterly or north-easterly flow. In summer, in the 4 stronger factors, a cyclonic flow appears over the Ionian sea. Key words: evaporation, Wild evaporigraph, extreme events, NW Greece Β-443

2 1. INTRODUCTION In meteorology, evaporation is called the change of liquid water or ice to water vapour. In certain usages the term signifies only the liquid to vapour phase change, as distinct from sublimation, which signifies the solid to vapour change. The rate of evaporation is controlled by the water and energy (mainly solar radiation) supplies and by the ability of the air to take up more water. The interpretation of direct measurements of evaporation presents certain problems. It is difficult to reconcile the results obtained from the different forms of evaporimeter or to relate them precisely to evaporation, which occurs from a free natural water surface. Evaporimeters are located either in meteorological enclosures as e.g. the common evaporation tank or inside meteorological screens, as e.g. the Piche evaporimeter, the Wild evaporigraph etc. Indirect measurements often use the relationship, evaporation equals rainfall minus run-off measures. Indirect assessments of evaporation can also be made using a theoretical formula based on incoming and outgoing radiation, wind and humidity conditions. For Greece, although there are various studies on evapotranspiration or soil evaporation [1], [2], evaporation from free water surface has not been studied extensively [3], [4], [5]. Especially for NW Greece, the existing studies are very few [6], [7]. In the present work, a survey of the evaporation regime in Ioannina, NW Greece will be presented based on Wild evaporigraph measurements. 2. DATA AND METHODS The data basis consists of 7,8 bi-hourly evaporation values, taken from the readings of the Wild evaporigraph (evaporigrams) of the meteorological station of Ioannina University for the 16-year period The instrument consists of an inclination balance carrying a metal bowl with an evaporating surface of 25cm 2 area and a recording device coupled with the balance, which records evaporation in mm of water level. The regime of evaporation in Ioannina is studied on daily, 5-day, monthly, seasonal and annual basis. In particular, the diurnal and intra-annual variations are studied by using the method of Harmonic Analysis while the long-term (inter-annual) variability is studied by using the Mann-Kendal test of randomness against trend. Also, the number of days with evaporation above or below specific thresholds is estimated for each year and the longterm changes are investigated [8], [9]. Furthermore, winter days with high evaporation (upper decile) and summer days with low evaporation (lower decile) are selected and studied. For these extreme evaporation days (approximately 145 days in each category), the pressure patterns over Europe (from 1W to 4E and from 3N to 6N - NCEP- NCAR data) are constructed and then, using Factor Analysis, the charts are grouped objectively [1]. Thus, the main pressure patterns associated with extreme evaporation values in NW Greece are revealed. 3. RESULTS AND DISCUSSION 3.1 The diurnal variation of evaporation The diurnal variation of evaporation resembles to a simple sinusoidal curve with a maximum at 14:-16: hour and a minimum at 4:-6: hour (local standard time). Β-444

3 The mean seasonal curves are presented in Figure 1a. They exhibit maximum bi-hourly values of.15mm in winter and.9mm in summer and minimum values of.3mm in winter and.5mm in summer. Fourier Analysis shows that the first harmonic term presents its maximum approximately at 15: explaining 77% of the total variance in winter and 84% in summer. This difference must be due to the very small and almost constant evaporation values during winter nights. The results of the analysis for the first harmonic are presented in Table 1. In Figure 1b the mean bi-hourly evaporation changes during a day are presented for each season. It is seen that the maximum evaporation increase appears from 11: to 13: [(1-12)-(12-14)] (.5mm in winter and.29mm in summer) and the maximum decrease from 17: to 19: (-.5mm in winter and -.27mm in summer). 1,,4 EVAPORATION,9,8,7,6,5,4,3,2,1 WINTER SPRING SUMMER AUTUMN EVAPORATION CHANGE,3,2,1, -,1 -,2 (2-4)-(-2) (4-6)-(2-4) (6-8)-(4-6) (8-1)-(6-8) (1-12)-(8-1) (12-14)-(1-12) (14-16)-(12-14) (16-18)-(14-16) (18-2)-(16-18) (2-22)-(18-2) (22-)-(2-22) (-2)-(22-), ,3 TIME DIFFRENCE Figure 1: (a) The diurnal variation of evaporation in Ioannina, for each season (1988-3), (b) Mean bi-hourly evaporation changes during a day. Table 1. The first harmonic term of the diurnal variation of evaporation in Ioannina Mean value Amplitude Time of maximum Variance explained (%) Winter :38' 77.3 Spring :9' 81.9 Summer :1' 84.6 Autumn :9' The intra-annual variation of evaporation The intra-annual variation of evaporation also resembles to a simple sinusoidal curve with one maximum (July, 153mm) and one minimum (December, 16mm) (Figure 2a). High EVAPORATION JAN FEB MAR APR MAY JUN JUL AUG SEP OKT NOV DEC EVAPORATION CHANGE JAN-FEB FEB-MAR MAR-APR APR-MAY MAY-JUN JUN-JUL JUL-AUG AUG-SEP SEP-OKT OKT-NOV NOV-DEC DEC-JAN TIME DIFFERENCE Figure 2: (a) The intra-annual variation of evaporation in Ioannina ( ), (b) Mean inter-monthly evaporation changes during a year. Β-445

4 inter-monthly evaporation increase appears from May to June (35mm) and from June to July (33mm), while the maximum decrease appears from August to September (-6mm) (Figure 2b). The intra-annual variation is analysed by using Fourier analysis twice. First, on the 12 monthly evaporation totals and then on the 73 5-day interval totals (Figure not shown). As was expected, the first harmonic term explains most of the variance (Table 2), and it is stronger in the latter case. Table 2. The first harmonic term of the intra-annual variation of evaporation in Ioannina Mean value Amplitude Time of maximum Variance explained (%) 12 monthly values July day values July Frequency distribution The study of frequency distribution reveals that, during winter, daily evaporation, in 9% of the cases, is less than 1.8mm. The maximum frequency appears for daily values less than.2mm, while the maximum values recorded are around 5mm (Figure 3a). In summer, the curve is closer to the Gaussian distribution with maximum frequency of daily evaporation at around 4-6mm and maximum values above 1mm (Figure 3b). Spring resembles more to summer while autumn resembles more to winter (figures not shown) FREQUENCY FREQUENCY ,2,6 1 1,4 1,8 2,2 2,6 3 3,4 3,8 4,2 4,6 5 EVAPORATION,5 1,5 2,5 3,5 4,5 5,5 6,5 7,5 8,5 9,5 1,5 11,5 12,5 EVAPORATION Figure 3: Frequency distribution of daily evaporation values in Ioannina ( ) (a) in winter and (b) in summer (.2 means -.2mm,.5 means -.5mm). 3.4 Extreme daily evaporation events Examination of the number of days with evaporation above or below specific thresholds showed that during the study period, in winter, the number of days with low evaporation increases while that with high evaporation decreases (Figures 4a, b). These findings are in agreement with the increase of precipitation in Ioannina and NW Greece after 199 due to the increase of cyclonic circulation in Central Mediterranean. The results were further enhanced by the Mann-Kendal test, which was applied on the daily winter evaporation values and revealed that there is a statistically significant (.5 level) negative trend. A statistically significant negative trend was also found for autumn. In summer, the two diagrams of high and low thresholds appear supplementary. For example, in 1995 and 22, which are summers with low temperature and high precipitation, there appear maxima in Figure 5a (low evaporation thresholds), while in 1998, a high temperature and low precipitation summer, there appears a maximum, in Figure 5b (high evaporation thresholds). Β-446

5 <,9 >3, <,8 >3,2 <,7 <,6 <,5 <,4 <,3 <,2 <,1 >3,4 >3,6 >3,8 >4, >4,2 >4,4 >4, >4, Figure 4: (a) Number of days with evaporation below specific thresholds in winter, (b) number of days with evaporation above specific thresholds in winter. <1,6 >7 <1,4 >7, <1,2 <1 <,9 <,8 <,7 <,6 <,5 23 <, >8 >8,5 >9 >9,5 >1 >1,5 >11 >11,5 > Figure 5: As in Figure 4 but for summer. 3.5 Pressure patterns associated with extreme evaporation events In order to reveal pressure patterns related to extreme evaporation events in NW Greece, Factor Analysis was applied on the pressure space-series of the days of the upper decile for winter (high evaporation) and lower decile for summer (low evaporation). The approximately 145 days (pressure patterns) of each category were grouped objectively to 4 factors in winter and 5 factors in summer, explaining around 85% of the total variance. In winter, it appears that the wind plays the major role since in the first 3 factors the main feature of the pressure pattern in the area of NW Greece is a dry and katabatic easterly or north-easterly flow (Figure 6a). The relationship between evaporation and wind speed was further examined by correlating the daily evaporation values with the daily wind speed values taken from the anemometer of the classical evaporation pan. It was found that the correlation is high enough, r=.7. Factor 4, exhibits an extended anticyclonic circulation over the whole Mediterranean, associated with sunshine (Figure 6b). In summer, in the 4 strongest factors, a cyclonic flow appears over the Ionian Sea, either due to an extension of the SW Asia thermal low or due to central European depressions or due to Mediterranean cut-off lows (Figures 6c, d). Β-447

6 (a) (b) (c) (d) Figure 6: Pressure patterns (standardized factor scores) for extreme evaporation days in Ioannina. (a) winter-factor 1, (b) winter-factor 4, (c) summer-factor 1, (d) summer-factor CONCLUSIONS This work, deals with the meteorological phenomenon of evaporation in NW Greece, for the 16-year period It is shown that evaporation presents simple sinusoidal diurnal and intra-annual variation. The first harmonic terms, derived from Fourier Analysis, explains approximately 8% of the total variance. Frequency distributions in winter and in summer appear different. In winter, there is a peak in low evaporation values while in summer the curve resembles to the Gaussian distribution. The number of days with high evaporation in winter decreases in recent years while that of low evaporation increases. Finally, it was found that the synoptic situations associated with days of high evaporation, in winter, are characterized by strong easterly katabatic winds while days with low evaporation, in summer, are characterized by cyclonic conditions in the Ionian Sea. REFERENCES 1. Dalezios N.R., Loukas A. and Bamzelis D. (22) Spatial variability of reference evapotranspiration in Greece, Physics and Chemistry of the Earth, 27 (23-24), Kosmas C., Marathianou M., Gerontidis S., Detsis V., Tsara M. and Poesen J. (21) Parameters affecting water vapor adsorption by soil under semi-arid climatic conditions Agricultural Water Management, 48 (1), Livadas G.C. and Machairas P.C. (1972) Evaporation in Thessaloniki-Greece, Meteorologika, 18, Machairas P.C. (1973) Evaporation and weather types, Meteorologika, 3, Metaxas D.A. and Repapis C.C. (1977) Evaporation in the Mediterranean Rivista di Meteorologia Aeronautica, XXXVII, Romero J.R., Kagalou I., Imberger J., Hela D., Kotti M., Bartzokas A., Albanis T., Evmirides N., Karkabounas S., Pappigannis J., Bithava A. (22) Seasonal water quality of shallow and eutrophic Lake Pamvotis, Greece: implications for restoration, Hydrobiologia, 474, Μαλδογιάννης Θ. (1971) Το Κλίµα των Ιωαννίνων, ιδακτορική ιατριβή, Παν/µιο Ιωαννίνων. 8. Domonkos P. (1998) Statistical Characteristics of Extreme Temperature Anomaly Groups I Hungary, Theoretical and Applied Climatology, 59, Domonkos P. (21) Temporal accumulations of extreme daily mean temperature anomalies, Theoretical and Applied Climatology, 68, Bartzokas A. and Metaxas, D.A. (1996) Nοrthern hemisphere gross circulation types. Climatic change and temperature distribution, Meteorologische Zeitschrift, 5 (3), Β-448