Surface water is the water of different bodies on the Earth’s surface (rivers, lakes, bogs, reservoirs) as well as underground water of a certain territory that is used or can be used for economic needs. Surface water is replenished annually while underground water storage which is hydraulically connected to the rivers, is recharged very slowly.

In Ukraine’s territory, there are 63 119 rivers and streams with a total length of more than 206 ths km. About 60 ths (93%) of them are very small (less than 10 km long). Their total length is 112 ths km; small rivers with the length of over 10 km number 3 219 and their total length is about 74 ths km. There are 81 medium rivers in Ukraine with the total length of 15 488 km.

The Danube, Tysa, Dnipro, Prypiat’, Desna, Dnister, Sivers’kyi Donets’, Pivdennyi Buh, and Zakhidnyi Buh are big rivers.
Most of the rivers belong to the basins of the Dnipro (27.7%), Danube (26.3%), Dnister (23.7%), and Southern Buh (9.3%).

Drainage basins of big and many medium rivers are situated in several geographical zones and geomorphological regions; while small rivers flow mainly within one geomorphologic region. As a result, natural conditions of some parts of catchment areas differ in big and medium rivers, while the basin of a small river is mainly uniform.

River catchment areas greatly vary. Small waterways (with the length of 10–12 km) have basins of 20–30 to 100–200 km². Sometimes, when these watercourses have a very developed drainage network, the area of their basins exceeds 10 ths km². Catchment areas of big rivers amount to 100 ths km² and more.

The majority of river basins (95.9%) have catchment areas that do not exceed 50 km². Basins of rivers with the area of 50–500 km² account for 3.5%, those with the area of more than 500 km², only 0.6%.

The average density of the drainage network of main river basins is (km/km²): the Dnipro – 0.26, the Dnister – 0.60, the Southern Buh – 0.35, the Sivers’kyi Donets’ – 0.22, the Vistula (within Ukraine) – 0.52, and the Danube (within Ukraine) – 0.68. In rivers of the Azov region, it is 0.36, the Crimea – 0.24, in the Danube – Dnister interfluve – 0.17, and the Dnister – Southern Buh interfluve – 0.009. For the entire territory of Ukraine, the average value of the drainage density is 0.39 km/km².

The characteristic of rivers water regime is presented according to natural zones: mixed forests, broad-leaved forests, forest-steppe, steppe, and mountain countries of the Crimea and the Ukrainian Carpathians.

Mixed forest zone. The water regime of rivers in this zone is characterized by a protracted high water period and occasional summer and autumn floods.

The high water period begins in the first half of March and ends in the late April. The spring peak flow occurs in the late March – early April. Flood lasts 50–100 days. The floodwater depth of runoff is 40–80 mm, or almost 55% of annual flow. The peak runoff (discharge) rate is 50–200 l/s · km².
The low-water period is long, in summer it has low water level, and in winter, medium water level. The monthly mean runoff (discharge) is about 0.8 l/s · km². Through frequent thaws the winter water level is much higher than summer. The volume of summer-autumn streamflow is about 25%, and of winter, about 20% of the annual flow. The duration of the freeze-up reaches 100 days, though during the last 20–25 years, owing to changes of climatic conditions, this value is considerably less.

The over-year water balance of this zone, as compared with others, has higher values of constituents: precipitations amount to 690 mm, the runoff – 110 mm, and the surface evaporation along with infiltration 580 mm.

Broad-leaved forest zone. The water regime of most of the rivers of this zone is characterized by distinct spring high water and low mean water interrupted by summer-autumn and winter floods. Rainfall floods in general do not exceed the spring high-water level.

In the basins of main rivers of the zone (upper reaches of tributaries of the Prypiat’, the Dnister, and the Zakhidnyi Buh) the spring high water begins usually in the first ten days of March, the early one, in the late January – early February (the Dnister) and ends in the second half of April, sometimes in the first ten days of May. The highest levels of spring high water are found in the second – third ten days of March (the Dnister) and early April (tributaries of the Prypiat’). The period lasts 1.5–2.0 months. The rise of water level makes up 20–90 cm per day, the greatest rise of water on small and medium rivers reaches 30–150 cm, and on big rivers – 1.5–3.0 m per day.

The summer-autumn low water period continues from May to October–November and is repeatedly interrupted by rainfall floods.Water runoff of most of the rivers in this zone is 3–4 l/s · km², on the Dnister tributaries it is higher – from 4 to6 l/s · km², while in the upper reaches of some rivers, 9.5–12.6 l/s · km² (the Stryvihor and others).

Forest-steppe zone. The water regime is characterized by spring high water and mostly summer, as well as autumn and rare winter floods. High water begins at the end of February or in the first half of March. Its peak falls to the middle of March or early April. On average, the period lasts 50 days. In dry years, there is no flood at all on the rivers of this zone. The depth of runoff varies from 10 to 90 cm. About 65% of the annual runoff is formed in spring.

Summer and autumn floods are observed on the rivers with the catchment area of up to 10 ths km², while winter ones on all the rivers of the zone. The greatest number of floods per year is ten. The maximum summer and autumn flood runoff do not exceed 100–166 l/s · km², only on the Donbas rivers they reach 180 l/s · km². In the western part of the zone the highest winter maximum reachs 300 l/s · km².
The low flow period is rather long – from May to November. Rates of the summer-autumn and winter low flows are approximately the same – 0.3 l/s. In the summer-autumn period, 25–40% of the annual runoff flows out, and in winter, 10–15%. The drying-up and freezing-up occur on rivers with the catchment area of up to 15 ths km².

The long-term water balance of the forest-steppe zone does not differ much from the Polissia balance. Precipitations here make up 635 mm, the runoff is 85 mm, and the surface evaporation along with infiltration, 550 mm. In the Sivers’kyi Donets’ basin, components of the water balance are considerably less: precipitations are 577 mm, the runoff – 57 mm, and the evaporation, 520 mm.

Steppe zone. The water regime of the rivers in this zone is characterized by spring high water and rare autumn-winter floods. Through the unstable winter, the spring high water does not occur every year. According to flood regime, rivers of this zone are typically steppe.In the western part of the zone, the spring high water begins in the second and third ten-day periods of February, and in the eastern part – in the first half of March. On average, it lasts 45 days. The cumulative runoffs make up about 16 mm, which is 65% of the annual runoff. The observed flood maximum reachs 100–150 l/s · km² and can be almost 200 times greater than the average long-term runoff.

During a year 4–6 summer floods are registered. Winter and especially autumn floods are rare. Maximum dischases of winter floods reach 230 l/s · km², summer – 120 l/s · km², and autumn – 25 l/s · km². In some years, summer, autumn and winter floods exceed high water on rivers with catchment areas of up to 5 ths km².

The summer-autumn low-water period lasts from March – April to November. An average rate of summer-autumn runoff is 0.1 l/s · km², of winter 0.3 l/s · km². In the summer-autumn period, 25% of the annual runoff flows out, and in the winter period – 10% of the annual runoff. Many rivers dry up during the summer-autumn low-water period.

In this zone, the long-term values of the water regime components are the lowest. In the steppe part of Ukraine precipitations make up 496 mm; the runoff – 23 mm, and the surface evaporation along with infiltration – 473 mm; in the Azov region these components are a little higher: precipitations reach 540 mm; the runoff – 45 mm, and the surface evaporation along with infiltration – 495 mm. In the steppe Crimea, precipitations amount to 559 mm; the runoff is only 3 mm, and the surface evaporation along with infiltration – 556 mm. On this territory water flow is practically absent.

The Ukrainian Carpathians. Rivers of this mountainous country are typical mountainous streams, and only coming out to the Zakarpattia (Transcarpathian) lowland and Peredkarpattia (Forecarpatian) highland they get features of semi-mountainous and lowland rivers. Through the small height of the mountains, the water regime of the rivers has no substantial territorial differences.

Rivers in the Carpathians have spring high water and floods in other seasons and in the rivers of the Zakarpattia (Transcarpathian) lowland the flood regime is observed during the whole year.

Snow, rain, and, partially, underground water are main sources of replenishment of rivers of the mountain country, and rainfall, of the lower part. Through frequent floods, the water regime of rivers has sharp fluctuations in water levels and discharges.

Spring high water on mountainous rivers through the unstable regime of winter temperatures, frequent thaws, and winter rainfall has several peaks. The spring high-water period begins in the first half of March and culminates in the second half of March. Floodwater layers reach 100–200 mm, which averages 30% of the annual runoff. The maximum flood runoff on the rivers with the catchment area of 300–2 000 km² amount to 500–600 l/s · km².
Due to the mild climate, the water regime of lowland rivers is characterized by spring, summer, and autumn rainfall floods. The number of floods reaches 15 per year. Their average duration is 10–12 days. As distinct from mountainous rivers, where maximum of summer flood runoff reach 1 200 l/s · km², on lowland rivers maximum runoff are about 600–700 l/s · km². Winter floods here are also considerably lower than in the mountainous zone.

Summer and winter low-water period is small and is often interrupted by floods. Average summer low-water runoff range from 2 to 3 l/s · km², and winter – from 1 to 2 l/s · km².

The Ukrainian Carpathians are characterized by the highest values of water balance components for a long-term period. In the mountainous part, precipitations reach 1 140 mm, the runoff – 540 mm, and the surface evaporation along with infiltration – 600 mm. Here 47% of the precipitation runs off. On the plain, precipitations amount to 990 mm, the runoff – 438 mm, and the evaporation – 552 mm. Here 44% of the precipitation runs off.

The Crimean Mountains. Rivers belong to the category of mountain streams that have mixed replenishment. The water regime is characterized by sharp fluctuations in water levels and discharge, very high-level flood with its peak in the second half of March, summer-autumn and winter floods that in some years exceed high water, an interrupted low-water period, and short-term ice events.

High water on the mountainous rivers, like on Carpathian rivers, is low. This is connected with small snow storage and unstable winters with frequent thaws. In the years of small amount of snow, high water does not occur at all.

High water begins in the first ten days of March, and its peak on rivers of the western slope of the mountains is in the middle of March, and of the northern, at the end of this month. About 30% of the annual runoff flows during the spring high water. On the rivers of the Southern Coast of the Crimea, the part of the spring runoff reaches sometimes 70%.

Floods on the mountainous rivers of the Crimea are a rather often occurrence. Their greatest number is observed on the rivers of the western and northern slopes of the mountains – to 15 annually, in the eastern part the number of floods reduces to 10.

The highest rainfall maximums are observed on the rivers of the western slope in winter and spring months and on the rivers of the northern slope – in summer and autumn. The greatest of high water runoff typical of streams with the catchment area of 1 ths km², reach: in summer 300 l/s · km², in winter 150 l/s · km², and in autumn 75 l/s · km².

On very small rivers summer maximums runoff of rainfall floods in the central and southern parts of the territory can reach 3 500 l/s · km², and in the eastern part, 1 500 l/s · km².

The summer-autumn mean water period is unstable. The highest values for low-water flow are observed in the upper courses of rivers in the western and northern regions where minimal runoff range from 2 to 5 l/s · km².

In the middle course of rivers of this region, the runoff decreases to 0.40–0.30 l/s · km², and to the issue, to 0.30–0.00 l/s · km². High values of minimal runoff are found also in the upper reaches of the western part of the Southern Coast. On other rivers of this part of the region, runoff in the upper reaches are 1–2 l/s · km², and to the issue their value decreases to 0.20–0.00 l/s · km².

Winter minimums, as a rule, are higher than summer-autumn. In the Crimea, the drying-up of small rivers is a widespread and frequent occurrence. Drying-up is observed mainly on the plain sections of rivers, where besides climatic factors that contribute to it (decrease in precipitations, increase in evaporation) losses of the runoff occur through its seepage into the underchannel deposits and withdrawal for economic needs.

The Crimean Mountains are characterized by such values of components of over-year water balance: precipitations – 563 mm, runoff – 98 mm, surface evaporation with infiltration – 465 mm. The runoff percentage is 0.17. The runoff of rivers is a major component of water resources in the Mountainous Crimea.

Water resources have a significant place among other natural resources. Their availability determines normal conditions for the operation of existing industrial and agricultural objects and in many respects – the prospects of their development.

As far as water resources available for use are concerned, Ukraine is one of the poorest countries in Europe. In the average water years, 1.09 ths m³ of internal river runoff falls at a person, in dry years – 0.52 ths m³ (according to the data of the UN European Ecological Commission, the state whose water resources do not exceed 1 500 m³ per capita, is considered water-deficient). Moreover, the uneven distribution of Ukraine’s water resources by territories complicates the issue even greater. Of 68% water needs of the population are satisfied by surface water sources and 32% – by underground water.

The cumulative river runoff in Ukraine (without the Danube) is 95.2 km³ in the average water year. The surface-water inflow into Ukraine amounts to 40.5 km³/year. Directly in the territory of Ukraine 54.7 km³/year form. Besides, account must be taken of the Danube water resources, along whose Kiliia branch 123 km³/year runs on average.

Total water resources of Ukraine in average water years (probability P=50%) comprise 90.1 km³, in low water years (P=75%), 71.6 km³, and in dry years (P=95%), 50.3 km³/year. Water resources of Ukraine’s main rivers (km³/year) are presented in Table 1.

The underground water volume included in the resource part of water-management balance is 7.0 km³. Besides, about 1.0 km³ of sea water is used in economy.

Table 1. Water resources of Ukraine (km³/year)