WATER RESOURCES OF THE LOWER DANUBE RIVER
AND THEIR USE WITHIN THE TERRITORY OF UKRAINE

V.I. Vyshnevskyi, S.A. Shevchuk, T.V. Matiash

Institute of Water Problems and Land Reclamation, Kyiv, Ukraine

Corresponding author: V.I. Vyshnevskyi, Institute of Water Problems and Land Reclamation, 03022, Kyiv, str. Vasylkivska, 37, Kyiv, vishnev.v@gmail.com

ABSTRACT 

The main hydrological characteristic of the Lower Danube River namely its water runoff and sediment yield are presented. Based on SRTM data the area of the river basin within the territory of Ukraine was determined. It is equal to 6454 km2.  

The basin areas of the largest rivers, flowing into the lakes of the Lower Danube River, were determined too. The features of their hydrochemical regime were studied as well.  

Based on the remote sensing data it was specified the water area of four largest lakes in this territory namely Kahul, Yalpuh-Kuhurluy, Katlabukh and Kytai. This area at normal water level is as follows: Kahul – 90.6, YalpuhKugurluy – 235, Katlabukh – 60.7, Kytai – 52.9 km2.  

Using the regular monitoring and remote sensing data, it was studied the water temperature and the ecological state of the lakes. It was evaluated the spatial and temporal features of algal bloom as well. The highest algal bloom is usually observed in August in sunny and warm weather.  

It was presented the data about the water management and water use on the researched territory. The largest water intake is from the Danube River, much less water intake is from the local lakes. The most of water is used for irrigation needs.  

Keywords: water resources, Lower Danube River, ecological state, remote sensing 

INTRODUCTION

The Danube River is the largest river of Central Europe. The Ukrainian part of the river basin consists of two parts. One of them covers the southern part of the Ukrainian Carpathian Mountains and adjacent territory, the second one covers the territory on the left bank of the river near the river mouth. In the latter case this territory is located within Odes’ka oblast.

In the past the researched territory on the left bank of the river was a large flood plain, which was periodically flooded during high floods. That time there were some large and small lakes there, named as Danube Lakes. In 1960s this territory was protected against flooding by a very long dam, constructed on the left bank of the Danube River. The hydraulic connection between the river and lakes was performed by means of canals with regulative facilities on them.  

The largest lakes, which nowadays can be considered as reservoirs, are as follows: Kahul, Yalpuh-Kugurluy, Katlabukh and Kytai. Sometimes lake Yalpuh-Kugurluy, which is the largest one in this region, is considered as two separated lakes Yalpuh and Kugurluy. The simple name Yalpuh for whole lake is also used. Much less are the lakes Kartal and Safyany. The first one locates to the west from lake Kugurluy, the second one to the south-west from lake Kartal. Actually, there is one more large water body, named lake Sasyk, which locates some apart. In the early 1980s it was transformed into the reservoir from the sea lagoon by means of a dam constructed on the sea shore (Fig. 1).  

There are quite a lot of scientific works devoted to the water bodies in this territory. One of the most popular issue is the water runoff of the Danube River and its distribution along the river branches in the delta. The corresponding data are presented in the fundamental scientific paper [4] and in some articles [10].  

Fig. 1. The Lower Danube River and the lakes on its left bank:

1 – Kahul, 2 – Kartal, 3 – Yalpuh-Kugurluy, 4 – Safyany, 5 – Katlabukh, 6 – Kytai, 7 – Sasyk

Another important point of scientific researches is the water quality both the Lower Danube River and adjacent lakes. This issue is really rather important as water quality in these water bodies is quite poor. First of all, it concerns the water quality in the lakes. As a result of their artificial separation from the Danube River the water exchange in the lakes essentially decreased. The research works [1, 2, 9] showed that the water mineralization in the lakes can reach 3–4 mg/dm3 and even more. The water mineralization in small rivers, flowing into these lakes, can exceed 5–7 mg/dm3. As the result of that, water mineralization in the northern part of the lakes is larger than in the southern parts. This is the essential obstacle for the water use.  

The results of study, as to the size of lake Yalpuh-Kugurluy, were presented in scientific article [3]. It was determined that the water area of this lake decreased compared to the design data – it is 226 km2 at water level 2.43 m. The largest measured depth of this lake is 5.3 m. The results of similar study as to lake Kartal were presented in [11].  

The water balance of lake Yalpuh-Kugurluy was specified in the scientific paper [5]. It was determined that the largest water amount to this lake comes from the Danube River. The main part of this amount is lost due to evaporation and evapotranspiration. 

Some results of the study on the bacterioplankton abundance in water are presented in the paper [7]. It was determined that this abundance compared to the conditions in 1949 increased greatly. The largest bacterioplankton abundance is observed in lake Kartal, some less it is in lake Kytai, the smallest one is in lake Kahul.  

In spite of rather large number of studies, devoted to the water bodies of this region, there are some issues, which have not been sufficiently investigated yet. Among these issues there are as follows: modern water area of the lakes, river basin area of the Lower Danube River etc.  

METHODOLOGY AND DATA

The study of water bodies in the observed region was based on the data of regular monitoring and remote sensing data. As to the regular monitoring of water runoff and sediment yield it was the observation data obtained from the Hydrometeorological Service network. The hydrochemical data of the Hydrometeorological Service and relevant data from the State Agency of Water Resources of Ukraine ware used for the research of water quality.  

The main source of remote sensing data was the results of land and water survey, obtained by Landsat 8 and Sentinell-2 satellites. The data of Shuttle Radar Topography Mission (SRTM) were used as well.

On the base of satellite data, it was determined the water area of the largest lakes located on the left bank of the Danube River near its mouth. These data were used for the study of water temperature and water quality as well. The processing of satellite data was carried out using ArcMAP 10 program.  

The SRTM data was used for determining of the Danube River basin area within the territory of Ukraine and small local river basins. It was used the data with the highest resolution of 1 arcsecond or about 30 m. The determination of the researched areas consisted of some stages. At first the water basins were contoured with an excess of area using SAS.Planet program. The obtained kmz-files were transformed by using Global Mapper program into shape-files, processed after that by ArcMAP 10 program. After the comparison of obtained images with high resolution images it was calculated the actual areas of the water basins. The methods of relevant researches are described in more details in [12].  

RESULTS AND DISCUSSION  

The water runoff. The Danube River is considered as the largest river of Europe after the Volga River. The hydrological observation on the Lower Danube River is being carried out at two main hydrological stations: Reni (163 km away from river mouth) and Izmail (94 km). The average water discharge at Reni station during 1981–2017 was 6530 m3/sec or 206 km3, at Izmail station (the period of 1959–2017) – 3810 m3/sec or 120 km3. During the last years (2001–2017) the part of water runoff, which related to Kyliyske river branch, where Izmail station is located, is 50 %. At the beginning of observation this share reached 60 %.  

The sediment yield during 1978–2017 at Reni station was 32 mln t, at Izmail – 17 mln t. The average turbidity of water on both gauging stations is equal to 150 g/m3.  

The water management. About a half of water amount in this region is withdrawn from the Danube River by the canals connecting the river and the lakes. When a water level in the river is high, the gates are open and water goes to the lakes. When a water level in the river is lower than in the lakes, the gates are usually closed. In some cases the water exchange is performed by means of opening the gates on the hydro-technical facilities.   

During the last years the water amount flowed annually into Danube lakes, is 300–400 mln m3. Thus, in 2016 the total water volume, taken from the Danube River for Danube Lakes, was 301.1 mln m3. The volume of water, returned to the river, was 128.9 mln m3. That year 49.3 mln m3 was taken for lake Kahul and 43.8 mln m3 was returned to the river. The correspondent values for other lakes were as follows: Kartal – 12.4 and 13.6 mln m3, Yalpuh-Kugurluy – 182.7 and 62.9 mln m3, Safyany – 3.0 and 3.8 mln m3, Katlabukh – 33.0 and 4.2 mln m3, Kytai – 20.7 and 4.4 mln m3.  

As can be seen, the largest water exchange is observed in lakes Kahul, Kartal and YalpuhKugurluy. The smallest water exchange is observed in lakes Katlabukh and Kytai.  

The water inflow into Sasyk lake is significant as well. In 2016 it was 197.7 mln m3.

The similar situation was in 2017. During this year the water intake for the lakes was as follows: Kahul – 20.4 mln m3, Kartal – 30.4, Yalpuh-Kugurluy – 73.2, Katlabukh – 10.9, Kytai – 15.8. The water discharge was observed only from lake Kahul – 9.5 mln m3. The water inflow into lake Sasyk was 146.4 mln m3.

Besides the points of gravity water intake for the lakes, there are some other ones which take water from the Danube River by pump stations. Among the water consumers the largest ones are those, which take the water for irrigation, first of all for rice growing. One of the largest water consumers is located 3 km away to the east from Kylia town. There are also some facilities built for drinking water supply to local towns: Reni, Izmail, Kiliya and Vylkove. The first two towns use the underground water. The water intake systems of these towns are located on the flood plain of the Danube River near its left bank. Two points of water intake for drinking water supply are located in Kiliya town, from which one is used for drinking water supply of this town, another one (Kiliya Group Water Supply System) – for Tatarbunary town and some adjacent villages. The last water intake point for drinking water supply on the Danube River is located in Vylkove town.  

The total water intake from the Danube River during the last years was as follows: 2016 – 764, 2017 – 452, 2018 – 571 mln m3.  

Compared to the total withdrawn water amount, the amount actually used for human needs is greatly less. In 2016 this water amount was 96.4 mln m3. From this amount 85.4 mln m3 was used for irrigation (including 75.1 mln m3 for rice growing) and 5.8 mln m3 for drinking water supply.  

In 2017 the total water use was 105.6 mln m3, while the amount for irrigation was 93.7 mln m3 (including 82.0 mln m3 for rice growing). In 2018 the total water use was 136.1, including for irrigation needs – 117.3 mln m3.  

The water intake from Danube Lakes is relatively small – less than 10 mln m3. The largest volume is taken from lake Katlabukh for irrigation needs. Among the water consumers there is Bolgrad town, which intakes water from lake Yalpuh.  

During the last years the water amount used for the irrigation, tends to increase. For example, in 2010 this amount was 62.3 mln m3. This increase was observed after essential decrease in the 1990s. Thus, one of the largest the Danube-Dniester Irrigation System, which was operated using the water from lake Sasyk, stopped its operation in the late 1990s. The main reason of that was the inadequate water quality. Chervonoyarska Irrigation System, which used water from the northern part of lake Kytai, also ceased the operation for the same reason.  

The hydrochemical characteristics.  

The regular monitoring of water quality on water bodies, located in the region, is carried out by Hydrometeorological Service and State Agency of Water Recourses of Ukraine.  

The data show that water quality in different water bodies differs greatly. The smallest mineralization is observed in the Danube River where it is 330–350 mg/dm3. Among the lakes the smallest values are recorded in lake Kahul, the largest ones – in lake Kytai (Table 1).  

Table 1. Hydrochemical characteristics of the Lower Danube River and adjacent lakes
during last years (mg/dm3)

There is essential difference between water mineralization in different places of lake Yalpuh-Kugurluy. The largest mineralization is observed in the northern part of lake Yalpuh, the smallest one – in its southern part (lake Kugurluy), where the water exchange with the Danube River is the largest. In lakes Katlabukh and Kytai, where the amount of the Danube water is rather small, the water mineralization is the largest. In lake Kytai it can reach 5.5–6.0 g/dm3, sulfate concentration – 2.0– 2.3 g/dm3.  

As with water mineralization there is a great difference in biochemical oxygen demand (BOD) and chemical oxygen demand (COD) between researched water bodies. The highest values of these parameters are observed in lake Kytai, more precisely – in its northern part.  

During two last decades the water quality in the lakes has been reduced as a result of the decrease in water exchange with the Danube River. The essential growth of water mineralization, biochemical oxygen demand, chemical oxygen demand, concentration of total phosphorus is observed in lakes Katlabukh and Kytai (Fig. 2).

Fig. 2. The long-term changes in water quality in lake Kytai – near Chervonyi Yar village: a – sulfate concentration; b - biochemical oxygen demand

The comparison these data with the data of the previous study proves the decline in water quality [1]. In fact, the worst water quality is observed in the small rivers, which flow into the lakes. For example, the water mineralization in the Aliyaga River, which flows into the northern part of lake Kytai, can exceed 8.0 g/dm3. A little less water mineralization (6.0–7.0 g/dm3) is observed in the Malyi Katlabukh River, which flows into the northern part of lake Katlabukh. The smallest mineralization (about 3.5 g/dm3) is observed in the Yalpuh River, which is the largest tributary of the Danube River in the researched region.    

The water inflow from the tributaries and the rise of salt concentration as a result of evaporation from the water area are main reasons of decline in water quality in the lakes.

The hydrography of rivers and lakes. Till today the data about the Danube River basin within the territory of Odes’ka oblast are not clarified enough. Using the method, described above, it was found out that this area is 6454 km2. This result practically coincides with the data from the State Agency of Water Resources of Ukraine – 6416 km2. It is about 20 % of the oblast territory or some less than 1% of the whole river basin area, which is which is considered some larger than 800,000 km2 (Fig. 3).

As to the largest tributaries of the Danube River within Odeska oblast the Kahul and Yalpuh Rivers, the area of their river basins is 613 and 3289 km2, respectively (Fig. 4).

 The correspondent data on the area of river basins in the reference book [8] are as follows:

the Kahul River – 605 km2, the Yalpuh River – 3280 km2. As it can be seen there is coincide of old and modern results.

Some larger differences were determined as to the length of the rivers. The length of the Yalpuh River, clarified using the satellite images, is 115.4 km. The length of the same river presented in the reference book [8], is much longer – 142 km. This fact can be explained by the significant hydraulic works along the river channel performed in recent decades. Nowadays the river channel in many sections is almost straight.

Fig. 3. The Lower Danube River basin within the boundaries of Ukraine

Fig. 4. The Kahul (on the left) and Yalpuh (on the right) River basins

Another important point of the research is a water area of the lakes. As with the area of river basins it has not been studied enough. According to the design data, established 50 years ago, the areas of the lakes are as follows (Table 2).

These data show that the depths of the lakes are rather small. The mean depths of the lakes are as follows: Kahul – 2.42 m, Kartal – 1.53, Yalpuh-Kugurluy – 2.50, Safiany – 1.63, Katlabukh – 1.91, Kytai – 2.08 m.

Table 2. The design data of the lakes (reservoirs) of the Lower Danube River

Most of the lakes, except Kytai one, is hydraulically connected with the neighboring ones. As a result of that the water level in the lakes is quite similar. It is somewhat larger in the lakes, located upstream, and it is a bit lower in the lakes, located downstream of the Danube River. The main data about water level in the four largest lakes according to the monitoring of the Hydrometeorological Service are presented in Table 3.  

Table 3. Typical water level of the lakes (reservoirs) of the Lower Danube River during a long period of observation up to 2017, above sea level, m

To determine water areas of the lakes, located on the left bank of the Lower Danube River, high-quality satellite images were used, obtained in the cold period of the year, when the vegetation on the banks and on shallow water was the smallest. The most suitable were the images obtained on 04.12.2016, 11.04.2017, 21.11.2017 and on 01.04.2019.  

Using the method described above, the water areas of the researched lakes were determined. As of 04.12.2016 the water areas of the lakes were as follows: Kahul – 90.29, Yalpuh-Kugurluy, Katlabukh – 59.69, Kytai – 50.34 km2. The water areas are shown in blue in Fig. 5.  

The water level in these lakes at the above-mentioned date (04.12.2016) was as follows: Kahul – 3.10 m, Yalpuh-Kugurluy – 2.12, Katlabukh –1.24, Kytai – 0.78 m. Similar calculations were performed based on other satellite images (Table 4).  

Table 4. Water area of the largest lakes on the left bank of the Lower Danube River

These data show that at normal water level the water areas of the lakes are as follows: Kahul – 90.6, Yalpuh-Kugurluy – 235, Katlabukh – 60.7, Kytai – 52.9 km2.  

As it can be seen, the water areas of the lakes are now smaller than design data presented in the reference books. The difference is about 10 %. At the same time the water area of lake Yalpuh-Kugurluy is some larger than in the paper, devoted to the research of lake water area [3]. In the latter case the difference can be explained by the existence of small water areas isolated from the main water body. In our view this area should be considered as a lake area as at high water level this territory is actually covered by water.  

The main reason of decreasing water area is the accumulation of sediments coming along with the water from the Danube River. There are three more factors of the water area decrease: growth of vegetation on shallow water, sediments coming along with the water of the tributaries and bank erosion.  

Water temperature. There are some hydrological stations where the water temperature is measured. Some of them are located on the Danube River and its main river branches, some – on the lakes. All measurements are performed near the banks at rather small depths.  

The result of the measurements shows that water temperature in lakes is almost the same. At the same time this temperature differs from the temperature in the Danube River and its main river branches. For example, the water temperature in July of 2017 was as follows: in the Danube River at Reni station – 26.8 °C, in the Danube River at Izmail station – 26.9 °C, in lake Kahul at Nahirne station – 25.7 °C, in Yalpuh at Kosa station – 25.8 °C, in Katlabukh – at Kyslitsa station – 25.4 °C.  

The limited number of monitoring sites does not enable to determine the water temperature of the river and lakes in full. Under these circumstances it is desirable to use remote sensing data. Based on these data it can be stated that water temperature depends not only on the location of lakes but also on water depth, water turbidity and algal bloom rate. Moreover, it also depends on the wind intensity (Fig. 6).

As it can be seen in the fig. 6, the researched lakes differ from each other by water temperature. As of 13.07.2016 the highest temperature was observed in lake Kytai, as of 29.07.2016 it was in lake Katlabukh, as of 30.06.2017 it was in lake Kahul, as of 07.08.2019 it was in lake Kugurluy.  

It should be mentioned that the highest mean air temperature in these four cases was observed before satellite imagery as of 29.07.2016, when during 5 days it exceeded 25 °C. The lowest air temperature was observed before satellite imagery as of 07.08.2019, when during 5 days it was about 22 °C.

Algal bloom. The study of algal bloom was based on the satellite images obtained in the summer time of last years. We used the combination the images of visible part of spectrum B2, B3 and B4 of Landsat 8 satellite. The highest algal bloom in the lakes is usually observed in August in sunny and warm weather (Fig. 7).

Fig. 6. Temperature of water surface in the lakes located on the Lower Danube River:

a – 13.07.2016, b – 29.07.2016, c – 30.06.2017, d – 07.08.2019

Fig. 7. Image of the lakes located near the Lower Danube River at 07.08.2019,

made in the colors similar to the natural ones

The obtained images show that algae bloom varies in space and time. The lowest algal bloom is observed usually in the northern part of lake Kytai. This water body is characterized by not only high water mineralization, but also a rather small pH value and the concentration of dissolved oxygen. In fact, there is another important reason of that – the large spread of red clay on the banks of the lake. It is not a coincidence that the village located on the bank of the lake is named as Chervony Yar.

CONCLUSIONS

The basin of the Lower Danube River within the territory of Ukraine is a unique region. This region is rich in heat and large water reservoirs at the same time. Besides the Danube River, there are some large lakes, partly transformed into reservoirs. They are Kahul, Yalpuh-Kugurluy, Katlabukh, Kytai and Sasyk. Based on the remote sensing data it was clarified the water areas of Danube Lakes. The determined areas are about 10 % less than design data.  

The water quality in these lakes is not high – much worse than in the Danube River. There are two main reasons of that: small water exchange with the river and poor water quality of local rivers, which flow into the lakes. As a result, the water mineralization in the lakes usually exceeds 2 g/dm3 and can even reach 4.5–5.0 g/dm3 as in case of lake Kytai.

Using the data of regular monitoring and the remote sensing data, it was studied the water temperature and ecological state of the lakes. The water temperature depends on not only the location of the lakes but also on water depth, water turbidity and algal bloom. Moreover, it depends on the wind intensity. The highest algal bloom in the lakes is usually observed in August in sunny and warm weather.  

The water amount annually taken from the Danube River is 500–700 mln m3. About half of this amount goes for feeding the lakes. The water amount, actually used for human needs, is comparably small. The largest water amount water is used for irrigation needs, first of all for rice growing.  

The inadequate water quality in the lakes, mainly distant from the Danube River, is the important factor, which restricts the water use in the region. It leads to the concentration of main water consumers near the Danube River.

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