The runoff formed by precipitation falling on the surface of the earth, the excess of which does not have time to evaporate and flows into rivers. The regime of river flows determines the regime of the river as a whole - fluctuations in water levels, the movement of sediments (solid runoff), the formation of river channels. The doctrine of river flow is the main section of the hydrology of land waters.
The territory from which water flows into the river, called. its watershed (basin); the border dividing the watersheds of the rivers, called. watershed. There is a surface runoff entering the rivers through ravines, streams, rivers, and an underground runoff formed by water seeping into the rocks that cover the earth's surface. Surface runoff is divided into snow runoff - meltwater runoff and rainwater runoff. In turn, in the runoff of melt waters, plain snow nutrition is distinguished - in spring in districts with a stable winter when freed from snow cover, mountain snow nutrition - occurring in spring and summer in high-lying parts of the catchment area, and in the same place - glacial nutrition . Surface runoff is characterized by sharp fluctuations in seasons. Underground runoff is stable.
Depending on the types of food in the annual runoff cycle, periods of high and low runoff alternate - floods, a significant increase in the water content of the river, repeating from year to year to a certain extent. time; low water - a period of low flow, when rivers receive predominance. ground (underground) food; floods - irregular, usually short-term, sometimes very sharp increases in runoff caused by Ch. arr. downpours. Recam b. h. terr. The USSR is characterized by a distinct spring flood, the flow of which is close to 50% per annum. On the Far East spring flood runoff drops to 30-40% per annum, and in sowing. and central Kazakhstan and in the foothills of Cf. Asia - rises to 90-95%. Ground (underground) runoff, which maintains stable low water flow rates in rivers, is significant (up to 30% per annum) in areas with a humid and relatively mild climate - in western Europe. parts of the USSR. In the zone permafrost and in arid areas Yu.-V. groundwater runoff is low, and rivers periodically freeze and dry up. Seasonal irregularity R. s. it is significantly smoothed out by flowing lakes (for example, naturally regulated rivers - the Neva, Angara, etc.).
River runoff is one of the articles of land water balance. The equation of the water balance of the catchment for a certain period of time is written in the form: runoff - precipitation - evaporation - increment of moisture reserves. Atmospheric precipitation includes all types of water entering the catchment: rain, snow, condensation on the surface of land and water bodies, as well as in the pores of the soil. Evaporation occurs from the soil surface of the watershed, from all water bodies - lakes, ponds, rivers, etc., as well as through transpiration (respiration) of plants.
Moisture reserves in the catchment are contained in the form of surface water (reservoirs), snow, ice and groundwater. The increment of stocks for the considered period of time can be positive and negative. In the latter case, the consumable moisture reserves increase the runoff. An example of the intensive use of water reserves is the formation of spring floods due to the melting of snow accumulated over the winter. Fluctuations in moisture reserves in the watershed significantly redistribute the runoff throughout the annual cycle. When considering long-term periods, these fluctuations can be neglected, and the water balance equation takes on a simple form: runoff - precipitation - evaporation.
share precipitation, turning into river runoff, called. coefficient runoff. On the territory of the USSR, this coefficient. ranges from values close to 1 in the Far North and high mountains to values close to 0 in deserts and arid steppes. The value of the average long-term runoff entering the rivers from 1 km2 of the catchment area is characterized by the height of the runoff layer, usually expressed in mm! year, and the flow rate (average multi-year runoff module) - the number of liters of water entering per second from 1 km2 of the catchment area. The water content of various parts of the territory of the USSR is characterized by runoff moduli close to 100 l/sec-km2 in high-mountain regions, and to 0 in the Central Asian deserts. In the central part of Europe. terr. USSR, in the Moscow region, the flow rate is approximately 6 l) sec - km2.
River flow fluctuates continuously. In its fluctuations over the seasons of the year, a clearly expressed repeating cycle is observed from year to year, for example. most of the territory of the USSR is characterized by spring floods and relatively low runoff during the rest of the year, interrupted by rain floods. In long-term runoff fluctuations determined. frequency has not been established.
A graph depicting fluctuations in the flow of water in a river over a certain period of time (year, spring flood, etc.), called. hydrograph.
River flow calculations performed in connection with the design of hydraulic engineering. structures are based on data on the flow of rivers, registered hydrometric. observations. Patterns traced in runoff fluctuations over the past period serve as the basis for predicting runoff for the future. The composition of the calculated characteristics of the runoff and the order of analysis depend on the water management problem. Key Features- the average long-term water flow, which determines the total water flow of the watercourse, the variability of annual runoff volumes, the seasonal distribution of runoff, the values of min. and max, water flow. In the absence or lack of observations of the runoff of the river in question, geographic methods are used. hydrological interpolation. characteristics, as well as empirical dependencies. The results of such indirect calculations are not highly reliable and accurate.
River flow regime changes over time under the influence of human activities that transform the watershed landscape. The impact on the runoff of directly water management measures carried out through hydraulic engineering is most pronounced. structures. Changes introduced into the runoff regime consist in its redistribution between years and seasons as a result of the operation of regulating reservoirs, in the withdrawal of water from rivers for water supply, watering and irrigation, in the transfer of part of the runoff to other basins, in the loss of water by evaporation from the water surface reservoirs. The regime of river flow is also influenced by agro- and forestry. activities carried out in watersheds - plowing of virgin lands, transverse plowing, snow retention, deforestation, afforestation, etc.
Lit .: Kritsky S. N., Menkel M. F., Hydrological foundations of river hydraulic engineering. M.-L., 1950; Sokolovsky D. L., River runoff, 2nd ed., L., 1959.
Runoff in a broad sense is the main element of the continental link of the global circulation of matter and energy. The runoff includes surface and underground parts. Surface runoff, in turn, consists of river runoff and ice runoff from sheet glaciers.
River runoff includes water runoff, sediment runoff, solute runoff and heat runoff.
water runoff (water runoff) is both a process of water runoff in river systems and a characteristic of the amount of runoff water. Water runoff is one of the most important physical, geographical and geological factors; study of water runoff - the main task land hydrology. Calling water runoff "liquid runoff" is not recommended.
sediment runoff is the process of sediment movement in river systems and characterizes the amount of sediment moving in rivers. The sediment runoff consists of the suspended sediment runoff (sediment carried in the thickness of the river flow in a suspended state) and the traction load runoff (sediment carried by the flow along the river bed in a traction state). The sediment runoff is not recommended to be called "solid runoff".
Dissolved waste - this is the process of transfer in river systems of substances dissolved in water and a characteristic of their quantity. Substances dissolved in river waters are salt ions, biogenic and organic matter, gases, etc. Sometimes the runoff of dissolved substances is called the ionic runoff or the runoff of salts (in this case, only the runoff of dissolved mineral substances is meant).
Heat sink (thermal runoff) is the process of transferring heat together with river waters and its quantitative characteristic.
It is obvious that the main component of river runoff is water runoff, without which other types of runoff are also impossible. Water runoff is a process that determines all other types of movement of matter and energy in river systems, their driving force. The runoff of sediments, dissolved substances and heat depends both on the runoff of water (the carrier of other components of the river runoff) and its quantitative characteristics, and on the content of sediments, dissolved substances and heat per unit of water flow.
Much has already been said about the main natural and anthropogenic factors that determine the runoff of water, in particular, when it came to feeding rivers. These are, first of all, climatic factors, as well as factors of the underlying surface and human economic activity. Let us consider the main quantitative characteristics of the water runoff used in hydrology: water discharge, runoff volume, runoff layer, runoff modulus, runoff coefficient, modulus coefficient.
The main characteristic of the river flow is water consumption, i.e. the volume of water flowing through the cross section of the flow per unit time ( Q, m3/s). Measurements determine only the average water flow in a given hydrometric section during the measurement time (per big rivers ah, it could be an interval of time measured by hours). The process of measuring water flow in rivers is quite laborious, and therefore the number of measurements during the year is usually limited. To calculate the average daily values of water discharge in practical hydrology, graphs of the relationship between water levels and water discharges are usually used. According to such schedules (they are called cost curves, or schedules Q=fiji)) water discharges can be determined from the level data for any day, regardless of whether the water discharge itself was measured on that day or not. Based on the average daily water flow thus obtained, a hydrograph can be constructed. To the number characteristic water flow include the costs of various phases of the water and ice regime of the river, for example, the maximum (peak) water flows of floods and floods, the minimum water flows of low water, the water flows at the beginning of the spring ice drift, etc.
The flow of river water is subject to continuous changes. In the hydrology of rivers, there are two main approaches to the analysis of their changes. At the first - genetic - they analyze the reasons for the change in runoff, reveal the relationship of runoff fluctuations with the determining, mainly climatic, factors. In the second - probabilistic - estimate the probability of the occurrence of certain water flows on a given river: the more the river water flow differs in this moment more or less than some medium size(“norm”), the less likely it is. In hydrology, developed whole system special methods of statistical and probabilistic assessment of fluctuations in river flow in the presence, lack and absence of observational data. In hydrology, the concept of average water flow over a certain time interval is widely used. At(decade, month, season, year, number of years). Such water costs are calculated according to formulas of the form:
where qi- average daily water consumption; P is the number of days in the considered time interval. So, for example, the average annual water consumption in a normal (non-leap) year is determined by summing up all the average daily water consumption for the year and dividing the sum by 365. Similarly, the average long-term water consumption (it is often called "flow rate" and denoted by Q 0) is determined by the formula:
where qi - medium annual expenses water; N- number of years.
It is assumed that the runoff rate is a stable value, i.e., the arithmetic mean value calculated over a sufficiently long period remains constant regardless of the addition of new terms to the variation series. The concept of stability of the runoff rate is not entirely correct. Climatic factors on large spaces do not remain unchanged for long periods, not only prehistoric, but also historical. These fluctuations are cyclic in nature with a cycle duration of about 1800 years; wet cycles are replaced by dry ones, and the latter are replaced by wet ones again. In addition to cyclic fluctuations in runoff caused by cyclical fluctuations in climatic factors, runoff changes are caused by economic activity person. These changes are usually unilaterally directed. Given the cyclic fluctuations in runoff, it is customary to consider the annual runoff as its arithmetic mean value calculated over a long period, including at least two full cycles of runoff fluctuations. The cycle consists of two phases of water content - high-water and low-water.
Flow volume water is the volume of water that has passed through a given cross section of a river flow in any time interval. The flow of water can therefore be considered the volume of water flow in 1 s.
The volume of water flow is calculated by the formula:
where W - runoff volume, m 3; Q - average water consumption for a time interval At(Q in m 3 / s, At in c). For big rivers W it is often more convenient to express in km 3 (especially if we are talking about annual values).
runoff layer - this is the amount of water flowing from the catchment for any time interval, equal to the thickness of the layer, evenly distributed over the catchment area and expressed in millimeters. It is convenient to represent this value as the amount of water, numerically equal to the thickness of the layer, which will be obtained if the volume of runoff for any period is evenly distributed over the basin area. Depending on the units of measurement, the flow volume is calculated as follows:
where at - runoff layer, mm; F- basin area, km2.
Drain module water is the amount of water flowing from a unit catchment area per unit of time. The water flow module is usually denoted by M, l / (s-km 2), and calculated by the formula:
where Q- any water flow (both instantaneous, for example, maximum, and average over a time interval At)
Runoff coefficient - the ratio of the magnitude (volume or layer) of the runoff to the amount of precipitation that fell on the catchment area, which caused the occurrence of this runoff:
where at and X in mm, 7i1vm 3 or km 3. The runoff coefficient is usually calculated for the average long-term runoff layer and precipitation layer, or for a hydrological year. Sometimes the runoff coefficient is also calculated for the flood; in this case, the runoff layer for the flood is divided into the layer of water, which consists of atmospheric precipitation during the flood period and water reserves in the snow cover accumulated over the previous winter. The runoff coefficient is a dimensionless value that varies from 0 to 1.
Modular factor To can be obtained from relations:
K, \u003d Q, / Q a \u003d M i / M 0 \u003d W l / W 0 \u003d / y 0, (2.19)
where respectively Q h M h W h y t- runoff for any period; Q 0 , M 0 , W0,wow - runoff over a multi-year period, or runoff rate. In dry years To To > 1. Flow characteristics can be calculated separately for underground and surface components, for example, the module surface runoff and an underground runoff module.
In this article, we will consider in detail the question of what is the annual flow of the river. We will also find out what affects this indicator, which determines the fullness of the river. We list the most significant rivers of the planet, leading in annual flow.
river runoff
The most important part of the planetary water cycle - this guarantee of life on Earth - are rivers. The movement of water in their networks occurs under the influence of a gravitational gradient, that is, due to the height difference between two points earth's surface. Water moves from a higher area to a lower area.
Fed by melting glaciers, rainfall, and groundwater, which came to the surface, the rivers carry their waters to the mouth - usually to one of the seas.
They differ from each other both in the length, density and branching of the river network, and in the flow of water over a certain period of time - in the amount that passes through the section or alignment of the river per unit of time. In this case, the key parameter will be the water flow in the river section at the mouth, since the saturation or full flow changes upward from the source to the mouth.
The annual flow of a river in geography is an indicator, to determine which it is necessary to take into account the amount of water flowing per second from square meter the territory under consideration, as well as the ratio of water discharge to the volume of precipitation.
annual runoff
So, the annual flow of the river is, first of all, the volume of water that the river throws out when it falls into its mouth. You can also say it a little differently. The amount of water that passes during the named period of time through the section of the river at its confluence is the annual flow of the river.
The definition of this parameter helps to characterize the full flow of a particular river. Accordingly, the rivers with the highest rate of annual flow will be the most full-flowing. The unit of measurement of the latter is the volume, expressed in cubic meters or cubic kilometers per year.
solid stock
When taking into account the magnitude of the annual runoff, it must be taken into account that the river does not carry clean, distilled water. River water, both in dissolved and suspended form, contains a huge amount of solids. Some of them - in the form of insoluble particles - strongly affect the index of its transparency (turbidity).
Solid waste is divided into two types:
- weighted - a suspension of relatively light particles;
- bottom - relatively heavy particles that are drawn along the bottom to the place of confluence.
In addition, solid runoff consists of products of weathering, leaching, erosion, etc. of soils, soils, rocks. The indicator of solid runoff can reach, depending on the fullness and turbidity of the river, tens, and sometimes hundreds of millions of tons (for example, the Yellow River - 1500, the Indus - 450 million tons).
Climatic factors determining the parameter of annual river runoff
The climatic factors that determine the annual flow of the river are, first of all, the annual amount of precipitation, the catchment area of the river system and the evaporation of water from the surface (mirror) of the river. The latter factor directly depends on the number of sunny days, the average annual temperature, the transparency of river water, as well as on numerous other factors. An important role is also played by the time period in which the largest number precipitation. If it is hotter, then this will reduce the annual runoff, and vice versa. Humidity also plays a huge role.
The nature of the relief
Rivers that flow mostly on flat terrain, other things being equal, are less watery than predominantly mountain rivers. In terms of annual runoff, the latter can exceed the flat ones by several times.
There are many reasons for this:
- mountain rivers, which have a much greater slope, flow faster, which means that river water has less time to evaporate;
- in the mountains, the temperature is always much lower, and, therefore, evaporation is weaker;
- in mountainous areas, there is more precipitation and more rivers, which means that the annual flow of the river is higher.
This, running a little ahead, is enhanced by the fact that the nature of soils in mountainous areas has less absorption, respectively, a larger volume of water comes to the mouth.
The nature of soils, soil cover, vegetation
River runoff is largely determined by the nature of the surface over which the river carries its waters. The annual river flow is an indicator that is primarily influenced by the nature of the soil.
Rocks, clay, stony soil, sand are very different throughput in relation to water. Highly absorbent surfaces (e.g. sand, dry soil) will drastically reduce the volume of the annual flow of the river flowing through them, while almost water-impervious surface types (protruding rocks, dense clays) will have practically no effect on river flow parameters. , passing river waters through its territory without any losses.
Extremely an important factor is also the water saturation of the soil. So, abundantly moistened soils will not only not “take away” melt water during spring snowmelt, but are also able to “share” excess water.
The nature of the vegetation cover of the banks of the river under study is also important. For example, those that flow through wooded areas are more watery, all other things being equal, compared with rivers in the steppe or forest-steppe zone. In particular, this is due to the ability of vegetation to reduce the total evaporation of moisture from the earth's surface.
The largest rivers in the world
Consider the rivers with the most abundant flow. To do this, we bring to your attention a table.
| Hemisphere | river name | Annual river runoff, thousand cubic meters km |
||
| South America | R. Amazon | |||
| Northern | ||||
| South America | R. Rio Negro | |||
| Northern | South America | R. Orinoco | ||
| Northern | R. Yenisei | |||
| Northern | Sev. America | R. Mississippi | ||
| South America | R. Paraná | |||
| Northern | ||||
| South America | R. Tocantins | |||
| R. Zambezi | ||||
| Northern | ||||
| Northern |
After analyzing this data, one can understand that the annual flow of Russian rivers, such as the Lena or the Yenisei, is quite large, but it still cannot be compared with the annual flow of such powerful full-flowing rivers as the Amazon or the Congo, located in the southern hemisphere.
28.07.2015
Fluctuations in river runoff and criteria for its assessment. River runoff is the movement of water in the process of its circulation in nature, when it flows down the river channel. River flow is determined by the amount of water flowing through the river channel for a certain period of time.
Numerous factors influence the flow regime: climatic - precipitation, evaporation, humidity and air temperature; topographic - terrain, shape and size of river basins and soil-geological, including vegetation cover.
For any basin, the more precipitation and less evaporation, the greater the flow of the river.
It has been established that with an increase in the catchment area, the duration of the spring flood also increases, while the hydrograph has a more elongated and “calm” shape. In easily permeable soils, there is more filtration and less runoff.
When performing various hydrological calculations related to the design of hydraulic structures, reclamation systems, water supply systems, flood control measures, roads, etc., the following main characteristics of the river flow are determined.
1. Water consumption is the volume of water flowing through the considered section per unit of time. The average water consumption Qcp is calculated as the arithmetic average of the costs for a given period of time T:
2. Flow volume V- this is the volume of water that flows through a given target for the considered period of time T
3. Drain module M is the flow of water per 1 km2 of catchment area F (or flowing from a unit catchment area):
In contrast to the water discharge, the runoff modulus is not associated with a specific section of the river and characterizes the runoff from the basin as a whole. The average multi-year runoff module M0 does not depend on the water content of individual years, but is determined only by geographic location river basin. This made it possible to zonate our country in hydrological terms and to build a map of isolines of average long-term runoff modules. These maps are given in the relevant regulatory literature. Knowing the catchment area of a river and determining the value M0 for it using the isoline map, it is possible to determine the average long-term water discharge Q0 of this river using the formula
For closely spaced river sections, the runoff moduli can be taken constant, i.e.
From here, according to the known water flow in one section Q1 and famous squares watersheds in these sections F1 and F2, the water discharge in another section Q2 can be established by the ratio
4. Drain layer h- this is the height of the water layer, which would be obtained with a uniform distribution over the entire basin area F of the runoff volume V for a certain period of time:
For the average multi-year runoff layer h0 of the spring flood, contour maps were compiled.
5. Modular drain coefficient K is the ratio of any of the above runoff characteristics to its arithmetic mean:
These coefficients can be set for any hydrological characteristics (discharges, levels, precipitation, evaporation, etc.) and for any periods of flow.
6. Runoff coefficient η is the ratio of the runoff layer to the layer of precipitation that fell on the catchment area x:
This coefficient can also be expressed in terms of the ratio of the volume of runoff to the volume of precipitation for the same period of time.
7. Flow rate- the most probable average long-term runoff, expressed by any of the above runoff characteristics over a multi-year period. To establish the runoff norm, a series of observations should be at least 40 ... 60 years.
The annual flow rate Q0 is determined by the formula
Since the number of observation years at most water gauges is usually less than 40, it is necessary to check whether this number of years is sufficient to obtain reliable values of the runoff norm Q0. To do this, calculate the root mean square error of the flow rate according to the dependence
The duration of the observation period is sufficient if the value of the root-mean-square error σQ does not exceed 5%.
The change in annual runoff is predominantly influenced by climatic factors: precipitation, evaporation, air temperature, etc. All of them are interconnected and, in turn, depend on a number of reasons that are random in nature. Therefore, the hydrological parameters characterizing the runoff are determined by a set of random variables. When designing measures for timber rafting, it is necessary to know the values of these parameters with the necessary probability of exceeding them. For example, in the hydraulic calculation of timber rafting dams, it is necessary to set the maximum flow rate of the spring flood, which can be exceeded five times in a hundred years. This problem is solved using the methods of mathematical statistics and probability theory. To characterize the values of hydrological parameters - costs, levels, etc., the following concepts are used: frequency(recurrence) and security (duration).
The frequency shows how many cases during the considered period of time the value of the hydrological parameter was in a certain interval. For example, if the average annual water discharge in a given section of the river changed over a number of years of observations from 150 to 350 m3/s, then it is possible to establish how many times the values of this value were in the intervals 150...200, 200...250, 250.. .300 m3/s etc.
security shows in how many cases the value of a hydrological element had values equal to or greater than a certain value. In a broad sense, security is the probability of exceeding a given value. The availability of any hydrological element is equal to the sum of the frequencies of the upstream intervals.
Frequency and availability can be expressed in terms of the number of occurrences, but in hydrological calculations they are most often defined as a percentage of total number members of the hydrological series. For example, in the hydrological series there are twenty values of average annual water discharges, six of them had a value equal to or greater than 200 m3/s, which means that this discharge is provided by 30%. Graphically, changes in frequency and availability are depicted by curves of frequency (Fig. 8a) and availability (Fig. 8b).
In hydrological calculations, the probability curve is more often used. It can be seen from this curve that the greater the value of the hydrological parameter, the lower the percentage of availability, and vice versa. Therefore, it is generally accepted that years for which the runoff availability, that is, the average annual water discharge Qg, is less than 50% are high-water, and years with Qg more than 50% are low-water. A year with a runoff security of 50% is considered a year of average water content.
The availability of water in a year is sometimes characterized by its average frequency. For high-water years, the frequency of occurrence shows how often the years of a given or greater water content occur on average, for low-water years, a given or less water content. For example, the average annual discharge of a high-water year with 10% security has an average frequency of 10 times in 100 years or 1 time in 10 years; the average frequency of a dry year of 90% security also has a frequency of 10 times in 100 years, since in 10% of cases the average annual discharge will have lower values.
Years of a certain water content have a corresponding name. In table. 1 for them the availability and repeatability are given.
The relationship between repeatability y and availability p can be written as follows:
for wet years
for dry years
All hydraulic structures for regulating the channel or flow of rivers are calculated according to the water content of the year of a certain supply, which guarantees the reliability and trouble-free operation of the structures.
The estimated percentage of provision of hydrological indicators is regulated by the "Instruction for the design of timber rafting enterprises".
Provision curves and methods of their calculation. In the practice of hydrological calculations, two methods of constructing supply curves are used: empirical and theoretical.
Reasonable calculation empirical endowment curve can only be performed if the number of observations of the river runoff is more than 30...40 years.
When calculating the availability of members of the hydrological series for annual, seasonal and minimum flows, you can use the formula of N.N. Chegodaeva:
To determine security maximum spending water apply dependence S.N. Kritsky and M.F. Menkel:
The procedure for constructing an empirical endowment curve:
1) all members of the hydrological series are recorded in decreasing order in absolute value;
2) each member of the series is assigned a serial number, starting from one;
3) the security of each member of the decreasing series is determined by formulas (23) or (24).
Based on the results of the calculation, a security curve is built, similar to the one shown in Fig. 8b.
However, empirical endowment curves have a number of disadvantages. Even with a sufficiently long observation period, it cannot be guaranteed that this interval covers all possible maximum and minimum values river runoff. Estimated values of runoff security of 1...2% are not reliable, since sufficiently substantiated results can be obtained only with the number of observations for 50...80 years. In this regard, with a limited period of observation of the hydrological regime of the river, when the number of years is less than thirty, or in their complete absence, they build theoretical security curves.
Studies have shown that the distribution of random hydrological variables most well obeys the type III Pearson curve equation, the integral expression of which is the supply curve. Pearson obtained tables for constructing this curve. The security curve can be constructed with sufficient accuracy for practice in three parameters: the arithmetic mean of the terms of the series, the coefficients of variation and asymmetry.
The arithmetic mean of the terms of the series is calculated by formula (19).
If the number of years of observations is less than ten or no observations were made at all, then the average annual water discharge Qgcp is taken equal to the average long-term Q0, that is, Qgcp = Q0. The value of Q0 can be set using the modulus factor K0 or the sink modulus M0 determined from the contour maps, since Q0 = M0*F.
The coefficient of variation Cv characterizes the runoff variability or the degree of its fluctuation relative to the average value in a given series; it is numerically equal to the ratio of the standard error to the arithmetic mean of the series members. The value of the Cv coefficient is significantly affected climatic conditions, type of river feeding and hydrographic features of its basin.
If there are observational data for at least ten years, the annual runoff variation coefficient is calculated by the formula
The value of Cv varies widely: from 0.05 to 1.50; for timber-rafting rivers Cv = 0.15...0.40.
With a short period of observations of the river runoff or in their complete absence the coefficient of variation can be established by the formula D.L. Sokolovsky:
In hydrological calculations for basins with F > 1000 km2, the isoline map of the Cv coefficient is also used if the total area of lakes does not exceed 3% of the catchment area.
In the normative document SNiP 2.01.14-83, a generalized formula K.P. is recommended for determining the coefficient of variation of unstudied rivers. Resurrection:
Skewness coefficient Cs characterizes the asymmetry of the series under consideration random variable about its average value. The smaller part of the members of the series exceeds the value of the runoff norm, the greater the value of the asymmetry coefficient.
The asymmetry coefficient can be calculated by the formula
However, this dependence gives satisfactory results only for the number of observation years n > 100.
The asymmetry coefficient of unstudied rivers is set according to the Cs/Cv ratio for analogue rivers, and in the absence of sufficiently good analogues, the average Cs/Cv ratios for the rivers of the given region are taken.
If it is impossible to establish the Cs/Cv ratio for a group of analogous rivers, then the values of the Cs coefficient for unstudied rivers are accepted for regulatory reasons: for river basins with a lake coefficient of more than 40%
for zones of excessive and variable moisture - arctic, tundra, forest, forest-steppe, steppe
To build a theoretical endowment curve for the above three parameters - Q0, Cv and Cs - use the method proposed by Foster - Rybkin.
From the above relation for the modular coefficient (17) it follows that the average long-term value of the runoff of a given recurrence - Qp%, Мр%, Vp%, hp% - can be calculated by the formula
The modulus runoff coefficient of the year of a given probability is determined by the dependence
Having determined a number of any runoff characteristics for a long-term period of different availability, it is possible to construct a supply curve based on these data. In this case, it is advisable to carry out all calculations in tabular form (Tables 3 and 4).
Methods for calculating modular coefficients. To solve many water management problems, it is necessary to know the distribution of runoff by seasons or months of the year. Intra-annual distribution runoff is expressed in the form of modular coefficients of monthly runoff, representing the ratio of the average monthly flow Qm.av to the average annual Qg.av:
The intra-annual distribution of runoff is different for years of different water content, therefore, in practical calculations, the modular coefficients of monthly runoff are determined for three characteristic years: a high-water year with 10% supply, an average year with 50% supply, and a low-water year with 90% supply.
Monthly runoff modulus coefficients can be established based on actual knowledge of average monthly water discharges in the presence of observational data for at least 30 years, on an analogue river or on standard tables of monthly runoff distribution, which are compiled for different river basins.
The average monthly water consumption is determined based on the formula
(33): Qm.cp = KmQg.sr
Maximum water consumption. When designing dams, bridges, lagoons, measures to strengthen the banks, it is necessary to know the maximum water flow. Depending on the type of river feeding, the maximum flow rate of spring floods or autumn floods can be taken as the calculated maximum discharge. The estimated security of these costs is determined by the capital class of hydraulic structures and is regulated by the relevant normative documents. For example, timber rafting dams of class Ill of capitality are calculated for the passage of a maximum water flow of 2% security, and class IV - of 5% security, bank protection structures should not collapse at flow rates corresponding to the maximum water flow of 10% security.
The method for determining the value of Qmax depends on the degree of knowledge of the river and on the difference between the maximum discharges of the spring flood and the flood.
If there are observational data for a period of more than 30 ... 40 years, then an empirical security curve Qmax is built, and with a shorter period - a theoretical curve. The calculations take: for spring floods Cs = 2Сv, and for rain floods Cs = (3...4)CV.
Since river regimes are monitored at water-measuring stations, the supply curve is usually plotted for these sites, and the maximum water discharges at the sites where structures are located are calculated by the ratio
For lowland rivers maximum flow of spring flood water given security p% is calculated by the formula
The values of the parameters n and K0 are determined depending on the natural zone and relief category according to Table. 5.
Category I - rivers located within hilly and plateau-like uplands - Central Russian, Strugo-Krasnenskaya, Sudoma uplands, Central Siberian plateau, etc .;
II category - rivers, in the basins of which hilly uplands alternate with depressions between them;
Category III - rivers, most of the basins of which are located within the flat lowlands - Mologo-Sheksninskaya, Meshcherskaya, Belarusian woodland, Pridnestrovskaya, Vasyuganskaya, etc.
The value of the coefficient μ is set depending on the natural zone and the percentage of security according to Table. 6.
The hp% parameter is calculated from the dependency
The coefficient δ1 is calculated (for h0 > 100 mm) by the formula
The coefficient δ2 is determined by the relation
The calculation of the maximum water discharges during the spring flood is carried out in tabular form (Table 7).
The levels of high waters (HWL) of the calculated supply are established according to the curves of water discharges for the corresponding values of Qmaxp% and calculated sections.
With approximate calculations, the maximum water flow of a rain flood can be set according to the dependence
In responsible calculations, the determination of the maximum water flow should be carried out in accordance with the instructions of regulatory documents.
Water resources constitute the national wealth of our country. According to the total annual runoff, Russia occupies one of the leading places in the world.
To perform calculations of water consumption, distribution water resources between various branches of the national economy and in solving other practical problems in hydrology, the following quantitative characteristics of runoff are used.
Flow volume W, m 3 - this is the amount of water flowing through the considered section of the watercourse for any period of time T. Of greatest interest is the volume of annual runoff, for which Т=31.56 10 6 With.
Drain layer y, mm- the amount of water flowing from the catchment for any period of time, expressed as a layer evenly distributed over the area of the basin.
Water consumption Q, m 
/c is the volume of water flowing through the cross section of the flow ( clear section) per unit of time. Average water consumption over time T is defined by the expression
(2.7)
Drain module M, l/s/km 
is the quotient of water discharge divided by the catchment area. The runoff module shows how much water flows from a unit catchment area per unit of time
(2.8)
Runoff coefficient 
is the ratio of the runoff layer to the precipitation layer
(2.9)
Of these characteristics, in the practice of engineering calculations, the most widely used water consumption Q and flow rateW about is the long-term average annual runoff volume. Runoff measurements carried out on rivers for a long time ( over 100 years), show that its value is subject to significant fluctuations. At the same time, the flow of water in the river changes as during the calendar year - i.e. there is an intra-annual distribution of runoff, and from year to year. The first type of runoff fluctuation is mainly due to the feeding of the river and will be discussed below.
The flow regime is determined by the climate and a group of physical and geographical factors. These include relief, soil and vegetation cover, the presence of lakes and swamps in the basin. Recently, the runoff has been increasingly influenced by human activities.
The main factor in the formation of runoff is climatic conditions. The size of the runoff and its changes during the year and over a long period are mainly determined by the amount of precipitation, evaporation, air humidity, etc. In areas of excessive moisture, precipitation plays a decisive role in the formation of annual runoff (the Neva River has a runoff coefficient 0.70 ). In areas with significant evaporation, the dependence of runoff on precipitation is less pronounced (the Don River is the runoff coefficient 0.16 ).
Every year, river runoff goes through the same cycle of changes. Meanwhile, the dates of the onset of the phases of fluctuations and the values of water discharges change in a long-term series. The annual volume of runoff changes with them. These fluctuations are due to a significant number of factors and the runoff can be considered as a random process. To determine the characteristic water consumption - maximum, minimum and average annual, the apparatus of mathematical statistics is used.
The impact of human activity - anthropogenic the impact on nature leads to a violation of the natural processes of runoff formation. Reservoirs make strong changes in the intra-annual flow distribution. However, this also reduces the average annual runoff due to evaporation from the water surface. The change in annual runoff is most noticeable after the creation of reservoirs in arid regions. Even more loss of runoff in areas of irrigated agriculture. The runoff is reduced as a result of municipal and industrial water supply, as well as due to agrotechnical and forest reclamation measures.
Currently, it is not the limited water resources in the country as a whole, but the sharp deterioration in water quality that causes particular concern. Almost all water users are responsible for this: industry, transport, agriculture and other sectors. Therefore, the problem of providing a person with clean water and the problem of preserving the fauna of rivers, lakes and seas have now acquired a global character. Protection of water resources is one of the most important tasks in environmental protection.
