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SEASONAL AVERAGE FLOW

IN RÂUL NEGRU HYDROGRAPHIC BASIN

VIGH MELINDA

1

ABSTRACT. Seasonal average flow in Râul Negru hydrographic basin. The

Râul Negru hydrographic basin is a well individualised physical-geographical unit

inside the Braşov Depression. The flow is controlled by six hydrometric stations

placed on the main river and on two important tributaries. The data base for

seasonal flow analysis contains the discharges from 1950-2012. The results of data

analysis show that there significant space-time differences between multiannual

seasonal averages. Some interesting conclusions can be obtained by comparing

abundant and scarce periods. Flow analysis was made using seasonal charts Q =

f(T). The similarities come from the basin’s relative homogeneity, and the

differences from flow’s evolution and trend. Flow variation is analysed using

variation coefficient. In some cases appear significant Cv values differences. Also,

Cv values trends are analysed according to basins’ average altitude.

Keywords: Seasonal flow, evolution and trend, altitude differences.

1. INTRODUCTION

Braşov Depression is the biggest unit from the line of depressions that

separate the volcanic and the crystalline schist mountainous ranges of the Eastern

Carpathians. As a complex physical – geographical entity, it presents a remarkable

diversity of genetic factors and flow influences.

The Râul Negru hydrographic basin expands north – south towards the

eastern part of the depression, forming a rectangle. The basin’s watershed includes

the highest peaks of the Bodoc, Nemira, Vrancea and Întorsurii mountains. The

mountainous and depression footsteps are even developed. The mountainous and

piedmont areas present high slopes in contrast with the almost plain depression.

The basin’s area is 2320 km2, and the hydrographic network’s density is

0.7 km/km2. The main collector, with a length of 88 km, diagonally crosses the

basin’s rectangle. It’s most important tributaries are Caşin, on right, and Covasna,

on left.

The main genetic factors are measured at two meteorological stations:

Lăcăuţ – in the mountainous area and Târgu Secuiesc – in the depression. The

rainfall regime presents maximum values in spring, even though the richest rainfall

month is July (173 mm at Lăcăuț, 85 mm at Târgu Secuiesc). Autumn and winter

are scarce periods. The monthly minimums present differences: October at Lăcăuţ

(40 mm) and December at Târgu Secuiesc (15 mm) (Vigh, 2014). The summer is

1 Babeș-Bolyai University, Faculty of Environmental Science and Engineering, Cluj-Napoca, Romania

E-mail: vmelindap@yahoo.com

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relatively chill. The negative temperatures period lasts five months in the

mountains and three months at Târgu Secuiesc, where temperature inversions are

frequent.

The flow’s space – time repartition is influenced by the basin’s enclosed

character, relief steps development, density and river network characteristic, etc.

(Ujvari, 1972).

2. DATA BASE AND METHOD

The analysed data come from six hydrometric stations, two on Râul Negru

River (Lemnia, Reci), Caşin (Ruseni, Plăieşii de Jos), respectively Covasna

(Covasna, Boroşneul Mare) (fig.1.).

From the six stations, three controls

the mountainous area, and the others are

closing stations, near the main confluences.

The analysed period is 1950 – 2012, with

some data series being extended for data

homogeneity.

The analysis is based on statistical

data processing and discharge variation

charts interpretation at the six hydrometric

stations. The variation coefficients were

calculated and average altitude correlations

were made (Table 1). The evaluations refer

to flow’s time and space evolutions,

expressed in absolute and relative values.

Fig.1. Râul Negru hydrographic basin

Table 1. Morphometric data of basin’s hydrometric station

River Hydrometric

station Area

Average

altitude Total

length Lenght

down

km2 m km km

Râul Negru Lemnia 101 892 57.2 30.8

Râul Negru Reci 1698 760 80.9 7.1

Caşin Plăieşii de Jos 85.0 954 21.6 32.4

Caşin Ruseni 476 830 51.7 2.3

Covasna Covasna 31.0 1150 10.0 18.0

Covasna Boroşneul Mare 232 739 23.9 4.1

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3. MULTINANUAL SEASONAL FLOW EVOLUTION

The seasonal discharges’ average values analysis at the six hydrometric

stations show a clear domination of spring flow. Next is summer flow. Autumn and

winter present scarce flows, with very close discharge values at all stations.

The differences are highlighted also by the seasonal discharge ratio

analysis. The spring/summer ratio oscillates between 1.37 (at Boroşneul Mare) and

1.55 (at Ruseni). The spring/autumn and spring/winter ratios are close, with high

values, above 3.0. The highest values appear at Ruseni, and the smallest at

Boroşneul Mare.

The autumn/winter ratio values are very close to the unit (0.99 – 1.10) and

express flow’s regularity in these seasons. As logic, in all cases, the Reci values

places between the other two.

Fig. 2. Seasonal variability of flow in absolute and relative values

in Râul Râul Negru Catchement

From the relative values can be observed that at all stations, spring flow

exceeds 40% of the total annual flow. At Lemnia, it is close to half (48%). The

summer flow is more uniform, with all values very close to 30%. The percentages

at Lemnia Station in autumn and winter are at their minimum, little below 10%.

The flow at the other stations is higher, between 12 – 14% (Fig. 2.).

The multiannual average values differentiation can be made comparative,

between high flow (1970 – 1985) and low flow periods (1986 – 2001), both

including 16 years. For this comparison there have been chosen the stations with

extreme average altitudes: Covasna and Reci (mean altitudes: 1150 m, respectively

760 m).

In Table 2 it can be observed that between 1970 – 1985, the spring flow

percentage at Covasna is smaller than at Reci (37.3 %, respectively 40.8 %). In

summer, the ratio reverses: 36.4 % Covasna, 33.4 % Reci. In autumn and winter,

the flow’s relative values are very similar at the two stations.

The low flow period ratios maintain the same laws in spring (38.7 %

Covasna, 44.0 % Reci) and summer (32.1 % Covasna, 29.1 % Reci). Obvious

differences are observed in the other seasons. The difference in autumn is much

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higher (17.3 % at Covasna, 14.4 % at Reci) than that in the high flow period. The

ratio is even reversed in winter time: the percentage at Covasna is smaller (11.8%)

than that at Reci (12.5%).

Table 2. Comparison of reach and low flow periods

Covasna Spring Summer Autumn Winter

1970-85 Q 1,5 1,5 0,6 0,5

% 37,3 36,4 14,5 11,7

1986-01 Q 0,8 0,7 0,4 0,2

% 38,7 32,1 17,3 11,8

Reci Spring Summer Autumn Winter

1970-85 Q 19,1 15,7 6,7 5,4

% 40,8 33,4 14,3 11,4

1986-01 Q 11,7 7,7 3,8 3,3

% 44,0 29,1 14,4 12,5

4. SEASONAL FLOW EVOLUTION BETWEEN 1950 AND 2012

For seasonal flow’s annual values analysis has been chosen two

hydrometric stations with extreme average altitude – Covasna and Reci. There can be observed some evident logical similarities, due to stations’

placement in the same hydrographic basin. The differences are caused by the different altitudes, conditioning flow genesis, and also by the basin’s influences on Reci Station.

4.1. Spring The two charts present similar allures, excepting for 1995 – 2005, when the data evolution at Covasna Station is more uniform than that at Reci Station.

Fig. 3. Spring discharge’s evolution (linear and polynomial trend)

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The linear trends are contrary – decrease at Covasna, increase at Reci. The

3th

degree polynomial oscillations are similar, but the inflexions are more powerful

at Covasna Station (Fig. 3).

4.2. Summer

Charts allure presents more evident differences than those in spring,

especially in the second part of the analysed period. In this period, the oscillations

at Covasna Station are smaller. The linear trend presents stability at Covasna and

increase at Reci. The 3th

degree polynomial oscillations are much closer than in

spring time (Fig. 4).

Fig. 4. Summer discharge’s evolution (linear and polynomial trend)

4.3. Autumn

The variations are less similar due to local rainfalls’ nature that influences

autumn minimum flow. Oscillations amplitude is higher at Covasna. The linear

trends are also increasing and with similar slopes. Also, the 3th

degree polynomial

oscillations present high similarities (Fig. 5).

Fig.5. Autumn discharge’s evolution (linear and polynomial trend)

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4.4. Winter

The two charts highlight, with a strong resembling, the stability of flow

genetic factors during winter time. The two stations’ variation curves almost

overlap. The inflexions are more powerful at Covasna station (Fig. 6).

Fig. 6. Winter discharge’s evolution (linear and polynomial trend)

5. SEASONAL FLOW VARIATION

The variation coefficient (Cv) expresses how the physical – geographical

factors influences flow forming and evolution. Even though the hydrometric

stations from Râul Negru basin present relative evident factors’ homogeneity, the

Cv values are sometimes with well differences.

The spatial analysis highlights the hydrographic basin’s homogeneity. The

Cv values are relatively uniform at all hydrometric stations. But during autumn

time the differences are greater. The autumn Cv is much greater than those from

the other seasons at Plăieşii de Jos (0.78), Ruseni (0.71) and Boroşneul Mare

stations (0.66). At Reci station, the value is the smallest (Cv = 0.42) due to the

diminishing effect caused by basin’s high area. The seasonal values at Lemnia

station are higher than at other stations, excepting during spring time. The

maximum Cv difference between Lemnia (0.53) and other stations (0.30 – 0.35)

can be observed during winter time.

Cv values are the highest in autumn. During this season, the highest Cv

values can be observed at different stations (0.42 at Reci, 0.78 at Plăieşii de Jos).

The Cv values at all stations in the other seasons vary between 0.3 – 0.4. The

exception is Lemnia Station during summer and winter time. The most obvious

variation coefficients uniformity can be observed during spring time, with a

maximum difference of only 0.08 (Fig. 7).

It can be highlighted that the Cv values of Râul Negru, Caşin and Covasna

rivers upstream stations are higher than those from downstream in all seasons,

representing an obvious law for flow characteristics. There is also an exception:

Covasna River during autumn time.

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Fig. 7. Cv’s spatial and temporal variability in Râul Râul Negru Catchement

5.1. Cv’s variation in altitude

The Cv values present obvious differences also in the relation with basins’

average altitude. The values increase – law like – with the altitude, excepting in

autumn time. The increase slopes are relatively similar. The strongest correlation

appears during summer (R2 = 0.85).

Fig. 8. Correlations between altitude (Hmed) and Cv in spring and summer

in Râul Râul Negru Catchement

Fig. 9. Correlations between altitude (Hmed) and Cv in autumn and winter

in Râul Râul Negru Catchement

00.10.20.30.40.50.60.70.80.9 Spring

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During autumn time, the exceptions previous presented are highlighted in

correlation with altitude. If analysing separately each river we can be observed that

variation law is not respected on Covasna River.

The relationship between Cv and mail altitude of catchment areas facilitate

the territorial generalization of the flow variations (Sorocovschi, Pandi, 2002).

6. CONCLUSIONS

The Râu Negru hydrographic basin presents an obvious homogeneity of

seasonal flow. The local influence factors – firstly basins’ altitude and size - cause

differences of flow characteristics. This is highlighted by the seasonal average

values evaluation at multiannual level, but also by evolutions’ interpretation in the

analysed period. These conclusions are highlighted by the flow variation

coefficients. The most affected hydrographic basin by the local factors is Covasna

River.

REFERENCES

1. Sorocovschi V., Pandi G. (2002), Characteristics of river flow in the

Transylvanian Basin, in Development and Application of Computer Techniques to

Environmental Studies, IX, WIT Press, Southampton, UK, 489-498.

2. Ujvari I. (1972), Geografia apelor României, Edit. Academiei, București

3. Vigh Melinda (2014), Monthly average flow in Negru River hydrographic basin,

in Pandi G., Moldovan F. “Air and water Components of the environment”, 21-22

March 2014, Cluj-Napoca, 286-292.