The impact of climate change on evaporation from the water surface in Ukraine

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The impact of climate change on evaporation from the water surface in Ukraine

Viktor I. Vyshnevskyi National Aviation University

Abstract

Based on the monitoring data, the features of long-term changes of evaporation from the water surface are determined. Data from relatively small evaporators and evaporation basins located in different regions of Ukraine were processed. It was found that during the first part of the observation period, which began in the 1950s, evaporation had the tendency to decrease, while in the second part it increased significantly. To determine the factors of these changes, the existing calculated dependences were analyzed. In most of them, evaporation is determined by three arguments: the partial pressure of saturated water vapour, which corresponds to the water temperature, the actual water vapour pressure, and wind speed. It was determined that the main factor of the modern increase in evaporation is the increase of water temperature, which is accompanied by a significant increase in the partial pressure of saturated water vapour.

In particular, the mean water temperature in the Dnipro Reservoirs in May- September during 1977-2020 increased at an average rate of 0.65-0.70 °C per decade, and the air temperature at 0.75 °C per decade. It is important that the relationship between water temperature and the partial pressure of saturated water vapour, which corresponds to it, is nonlinear. Wind speed does not significantly affect evaporation. In addition, in recent decades there has been a tendency to its decrease. An empirical dependence of evaporation on air temperature is proposed. Its nonlinear form indicates a significant increase in evaporation due to the temperature increase. Currently (1991-2020), evaporation from the water surface near Kyiv during the ice-free period is approximately 650 mm, in the south of Ukraine it reaches 1000 mm. The increase in evaporation results in additional water losses and a reduction in available water resources.

Keywords: evaporation, water surface, water and air temperature, water vapour pressure, wind

Анотація

В.І. Вишневський, Національний авіаційний університет

Спираючись на дані моніторингу, встановлено закономірності багаторічних змін випаровування з водної поверхні. Опрацьовано дані з порівняно невеликих випаровувачів і випаровувальних басейнів, розташованих у різних регіонах України. З'ясовано, що протягом першої частини періоду спостережень, розпочатих у 1950-х роках, випаровування зменшувалося, натомість у другій частині - помітно зростає. Для встановлення чинників цих змін проаналізовано наявні розрахункові залежності. У більшості з них випаровування визначають три аргументи: парціальний тиск насиченої водяної пари, що відповідає температурі води, фактичний тиск водяної пари, а також швидкість вітру.

Встановлено, що основним чинником сучасного збільшення випаровування є підвищення температури води, яке супроводжується значним зростанням парціального тиску насиченої водяної пари. Зокрема середня за травень-вересень температура води у дніпровських водосховищах протягом 1977-2020 рр. зростала із середньою швидкістю 0,65-0,70 °С за десятиліття, а температура повітря 0,75 °С за десятиліття. Важливо, що залежність між температурою води і парціальним тиском насиченої водяної пари, що їй відповідає, нелінійна. Швидкість вітру на випаровування істотно не впливає. До того ж в останні десятиліття існує тенденція її зменшення. Запропоновано емпіричну залежність випаровування від температури повітря. Її нелінійний вигляд свідчить про значне зростання випаровування внаслідок підвищення температури. Нині (1991-2020) випаровування з водної поверхні біля Києва у безльодоставний період приблизно становить 650 мм, на півдні України - сягає 1000 мм. Наслідком зростання випаровування є додаткові втрати води і зменшення наявних водних ресурсів.

Ключові слова: випаровування, водна поверхня, температура води і повітря, тиск водяної пари, вітер

evaporation water surfa temperature pressure

Introduction

Evaporation from the water surface is an important factor that often determines the very existence of water bodies. It is no coincidence that this issue has long been the focus of many scientists. In many studies (Abdul, 2012, Jensen, 2010, Guidelines, 1969, Kaganer and Diukel, 1980, Kohut et al, 2014, Vuglinskyi and Abdul, 2016) the fact was determined that evaporation depends on the difference between the saturated water vapour pressure corresponding to the water temperature, the actual water vapour pressure and wind speed. However, the empirical dependences obtained by different researchers have differences. There are studies (Postnikov, 2014) in which it is proposed to calculate evaporation by simplified dependences, in particular using the data of air temperature. In addition, there are studies (Golubev et al, 2001, Shereshevskyi and Synitska, 2000), devoted to the long-term changes in evaporation.

Climate change, first of all the increase in air temperature, determines the growing urgency of this issue, because the amount of water resources and the ecological condition of water bodies significantly depend on evaporation. Therefore, the main purpose of this study is to clarify the long-term changes in evaporation from the water surface in Ukraine, as well as to find dependencies that can be used in practice.

Materials and methods of research

The main source of data for this study was the materials of the hydrometeorological service observations, started in the 1950s. Evaporation from the water surface was studied according to the devices installed on the network: evaporation basins and evaporators DGI-3000. The evaporation basin has a depth of 2.0 m and a water surface area of 20 m². The evaporator is much smaller - its depth is approximately 70 cm, a water surface area - 3000 cm².

Until recently, the relevant observations have been carried out at a fairly large number of meteorological stations. The complexity of the operation with the evaporation basin has led to the fact that a significant part of them have broken down and today (on 01.01.2021) the relevant observations in Ukraine remain only at two meteorological stations: in Svitlovodsk and Bolgrad. In addition to these meteorological stations, in the study data were used from some other meteorological stations where observations have since stopped: Pechenihy, Novodnistrovsk, Zaporizhzhia, Nova Kakhovka and Klepinine. The geography of observations with the use of evaporators is much wider, nowadays their total number is 11. In addition to Svitlovodsk, observations with their use are carried out at meteorological stations Svitiaz, Sarny, Brody, Rava-Ruska, Velykyi Bereznyi, Pokoshychi, Kaniv, Pomichna, Nova Kakhovka and Vilkove. It is important, that at these meteorological stations, in addition to evaporation, air and water temperature, actual vapour pressure, wind speed and precipitation are measured. In our study we used mean monthly data.

The data on water temperature in the Dnipro Reservoirs at hydrological stations in Kyiv and Nova Kakhovka were used in the study as well. The observations at these stations have a long duration and they are representative for the whole Dnipro Cascade. In addition, observations of meteorological parameters at Kyiv meteorological station were used, which made it possible to obtain the relationship between air and water temperature. The observation points, the data of which were used in the study, are shown in Fig. 1.

Fig. 1. Location of observation points for meteorological elements and water temperature: 1 - Pokoshychi, 2 - Kyiv, 3 - Pechenihy, 4 - Svitlovodsk, 5 - Novodnistrovsk, 6 - Zaporizhzhia, 7 - Nova Kakhovka, 8 - Bolgrad and 9 - Klepinine

The available data were processed using a statistical method, in particular regression analysis.

Results and discussion

Observations on the hydrometeorological network indicate that during the period from the end of the XIX century the mean annual temperature in Ukraine increased by about 3.0 °C, which is more than the global level (Fig. 2).

The warmest year for the whole observation period was 2020, when the mean annual temperature in Kyiv reached 10.9 °C, in Odessa - 13.0 °C. This increase in air temperature affected its norm, which is usually determined over a 30-year period. Compared with 1961-1990, in 1991-2020 it increased everywhere. In particular, in Kyiv it was 7.7 and 9.0 °C, in Odesa - 10.2 and 11.3 °C.

The increase in air temperature has affected many natural processes, including evaporation from the water surface. However, these changes can only be determined since the 1950s, when the relevant observations began.

Fig. 2. The long-term changes in mean annual air temperature: 1 - in Kyiv, 2 - in Odesa

These changes have been studied according to evaporators, as most meteorological stations have no other information. Considerable attention is paid to the mean monthly data at the meteorological stations Pokoshychi and Nova Kakhovka, which are located in the north (Chernihiv region) and south (Kherson region) of the country. In both cases, data for the five warmest months from May to September were processed. The choice of this period is due to the fact that observations are carried out only in the ice-free period. At the Pokoshychi meteorological station, located in the north, observations often do not cover the whole of April or October. In any case, the selected period corresponds for most of the total evaporation.

According to available data, the total evaporation initially decreased, but from the late 1980s, and mainly from the 1990s, began to increase. This is especially noticeable according to observations at the meteorological station Nova Kakhovka (Fig. 3).

Note that these data correspond, at least in the majority of cases, but only for part of the ice-free period. In recent decades, this has lengthened. This means that the total evaporation is slightly higher. At the same time, it is known that the data of the evaporator DGI-3000, compared to the evaporation basin, are slightly inflated (Abdul, 2012). Therefore, the data shown in Fig. 3 are quite representative for the long-term changes of evaporation.

Fig. 3. Long-term changes of evaporation from water surface in May-September, observed with the use of evaporators at the meteorological stations Pokoshychi (1) and Nova Kakhovka (2)

The largest evaporation was recorded in 2020, which was caused by the abnormally warm spring and autumn. In that year, the total evaporation according to the evaporation basin in Svitlovodsk was 841 mm. In the same year, the total evaporation according to the evaporator at the meteorological station Pokoshychi reached 628mm, Nova Kakhovka - 1282 mm.

The identified changes in evaporation determine the relevance of the question of the factors that caused them. In this case, it is advisable to analyze the existing dependencies for its calculation. The Braslavsky-Vikulina formula (Abdul, 2012, Guidelines, 1969), which is also called the DGI formula, became widespread in the former USSR:

E = 0.14 (e0 - e)·(1 + 0.72·W200), (1)

where E - the daily evaporation layer (mm), e₀ - the saturated vapour pressure at water temperature (hPa), e - the actual vapour pressure (hPa), W₂₀₀ - wind speed at a height of 2 m (m/sec).

To calculate the monthly evaporation layer, its daily value is multiplied by the number of days in the month. Observational materials using evaporation basins indicate that the DGI formula generally provides reliable results. The correlation coefficient between the actual (Eact) and the calculated data (Ec) is greater than 0.9 (Fig. 4).

Fig. 4. Relationship between actual evaporation from the evaporation basin (Eact) and calculated evaporation (Ec) according to the DGI equation: a - meteorological station Svitlovodsk, b - meteorological station Nova Kakhovka

The rather close relationship between the calculated and actual data indicates that the DGI formula makes it possible to calculate the evaporation with due accuracy. It is no coincidence that this formula is used in the calculations of the water balance of the Dnipro Reservoirs.

The available observational data allow us not only to determine the reliability of the DGI formula, but also to make corrections in it. The relationship between actual and calculated data becomes closer in the case of small changes of empirical coefficients:

E = 0.16 (e₀ - e) (1 + 0.6 r₂oo). (2)

Although the obtained dependence has become closer, there is a serious problem of its use due to the lack of data necessary for the calculation. It will be recalled that regular wind speed monitoring at a height of 2 m is carried out in Ukraine only at some meteorological stations. Therefore, it is often necessary to use the wind speed at the height of the weather vane with a coefficient that is not constant.

Fig. 5. The relationship between the actual data of evaporation from the evaporation basin (Eact) and the calculated evaporation (Ec) according to the proposed dependence: a - meteorological station Svitlovodsk, b - meteorological station Nova Kakhovka

The existing problem of lack of data determines the relevance of finding dependencies that require less source data. It turned out that a fairly close relationship between evaporation from the water surface exists in the case of using difference between the partial pressure of saturated water vapour, which corresponds to the water temperature, and the actual water vapour pressure (Fig. 6).

Comparison of Fig. 6 with Fig. 4 and 5 indicates that the wind speed has almost no effect on evaporation from the water surface. This can be explained by the fact that the components of water vapour are more important than the wind speed. Indeed, the available data from observations at hydrological stations indicate that the water temperature rises (Vyshnevskyi, 2020a, 2020b), and the corresponding partial pressure of saturated water vapour increases accordingly. This is observed in all months of the year and, in particular, during May- September, when the water temperature is the highest. This increase is especially noticeable in the last three to four decades. An example is the data of observations on the Dnipro Reservoirs, namely in Kyiv and Nova Kakhovka (Fig. 7).

Fig. 6. Relationship between actual data of evaporation from the basin (Eact) and the difference between the partial pressure of saturated water vapour, which corresponds to the water temperature and the actual water vapour pressure (e₀ - e): a - meteorological station Svitlovodsk, b - meteorological station Nova Kakhovka

Fig. 7. Long-term changes in mean water temperature in May-September at hydrological stations Kyiv (1) and Nova Kakhovka (2)

During the period after the filling of the Kanivske Reservoir (1977), the change in the mean water temperature in May-September at the Kyiv station averaged 0.69 °C per decade, and during the same period for the Kakhovske Reservoir at the Nova Kakhovka station - 0.67 °C per decade. It can be assumed that for the entire Dnipro Cascade it is 0.65-0.70 °C per decade.

The increase in water temperature during 1977-2020 is at least 2.5 °C.

Importantly, the relationship between water temperature and the water vapour pressure corresponding to it is nonlinear. As a result, even a relatively small increase in water temperature is accompanied by a significant increase in the partial pressure of saturated water vapour, which corresponds to it. This indicates that the main factor in increasing evaporation from the water surface is the increase in water temperature.

At the same time, the wind speed is decreasing throughout Ukraine. In some cases, it is due to construction in the areas adjacent to the meteorological stations and the increase in the height of the trees located nearby. However, this decrease is also observed where these factors do not exist. Decrease in wind speed has been reported in many studies in different regions (Deng et al, 2018, Guo et al, 2011, Lialko et al, 2019, Spinoni et al, 2015, Vyshnevskyi and Donich, 2021). It is important that decrease in wind speed is observed throughout the entire year (Fig. 8).

Evaporation from the water surface can also be determined from observations using the DGI-3000 evaporator, the number of which is much larger than the evaporation basins. However, the evaporation ratio for the two instruments for each of the seven meteorological stations is variable. For the selected period from May to September, it averages 1.11. Otherwise - evaporation from the evaporation basin is approximately 0.9 from the data of the evaporator DGI-3000.

Actually during the ice-free period, the relationship between the data for the two devices is not constant. In spring, the water in a relatively small evaporator heats up faster than in an evaporation basin, and in autumn it cools down just as quickly.

These data show that it is quite problematic to calculate evaporation - primarily due to the lack of necessary data, as well as due to their significant spatiotemporal variability. The only parameter which is quite stable in the territory is the air temperature. At the same time, the closeness of the dependence of evaporation on air temperature is worse than in the case of use several parameters (Fig. 9).

Fig. 8. Long-term changes of mean annual wind speed at meteorological stations Svitlovodsk (1) and Nova Kakhovka (2)

Fig. 9. Dependence between evaporation from the water surface according to the evaporation basin and air temperature at meteorological stations Svitlovodsk (a) and Nova Kakhovka (b)

Fig. 9 indicates that the relationship between air temperature (as well as water) and evaporation from the water surface is nonlinear. This once again shows that a relatively small increase in temperature is accompanied by a significant increase in evaporation - especially in the southern part of the country, where temperatures are the highest.

Analysis of Fig. 9 gives reason to believe that there is a possibility to calculate the evaporation on a single basis for the whole country. Such dependence exists, but its accuracy is worse than for individual meteorological stations (Fig. 10).

Fig. 10. Exponential dependence of daily evaporation layer on air temperature according to the evaporation basins at the meteorological stations Pechenihy, Svitlovodsk, Novodnistrovsk, Zaporizhzhia, Nova Kakhovka, Bolgrad and Klepinine

The obtained dependence can be used for calculations, because the correlation coefficient between the calculated and actual data is close to 0.9. However, its use at low temperatures causes overestimation of values, and at high - their underestimation. In this regard, an empirical curve is drawn, which better corresponds to the actual values (Fig. 11).

To simplify the calculations of evaporation from air temperature, the required data are given in Table 1.

Fig. 11. Empirical dependence of daily evaporation layer on air temperature according to the evaporation basins at the meteorological stations Pechenihy, Svitlovodsk, Novodnistrovsk, Zaporizhzhia, Nova Kakhovka, Bolgrad and Klepinine

Table 1. The relationship between air temperature and the daily layer of evaporation from the water surface in Ukraine

The use of the data in Table 1 allows us to calculate evaporation for most of the country. It is possible to use data from a certain year or mean value of several years. In particular, the necessary data for the calculation of evaporation in Kyiv can be found on the website of the Central Geophysical Observatory named after Borys Sreznevsky http://cgo-sreznevskyi.kyiv.ua/. According to the mean monthly values of air temperature in the last 30 years, the estimated evaporation in Kyiv during the months with a positive temperature is 650 mm.

This approach is close to that proposed in the study (Postnikov, 2014), in which the main argument is the sum of positive mean monthly temperatures divided by 12. According to the dependences given in this articler, the estimated evaporation in northern Ukraine (the Kyivske Reservoir) for the period 1991-2020 is about 800 mm, in the south (the Kakhovske Reservoir) - about 1090 mm. These values exceed the actual data.

Which is primarily caused by the increase in air and water temperature. At the same time, there has been a significant decrease in wind speed, and consequently the role of this factor.

There is a fairly close relationship between actual and calculated evaporation according to the DGI formula, but its use requires data which are not usually available. At the same time, it is possible to calculate evaporation only by air temperature. The nonlinearity of this dependence determines that the increase in temperature causes a significant increase in evaporation. According to the actual data of observations using evaporation basins, an empirical curve and a corresponding table were obtained, which allows us to carry out calculations of evaporation for most of Ukraine only on the basis of the mean monthly air temperature. In 1991-2020 the mean layer of evaporation from the water surface near Kyiv in the ice-free period was approximately 650 mm, in the south of Ukraine it reached 1000 mm.

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