MONTHLY AVERAGE FLOW IN RÂUL NEGRU HYDROGRAPHIC BASIN

Abstract

Monthly average flow in Râul Negru hydrographic basin Râul Negru hydrographic basin represents a well individualised and relatively homogenous physical-geographical unity from Brasov Depression. The flow is controlled by six hydrometric stations placed on the main collector and on two of the most powerful tributaries. Our analysis period is represented by the last 25 years (1988 - 2012) and it's acceptable for make pertinent conclusions. The maximum discharge month is April, that it's placed in the high flow period: March - June. Minimum discharges appear in November - because of the lack of pluvial precipitations; in January because of high solid precipitations and because of water volume retention in ice. Extreme discharge frequencies vary according to their position: in the mountain area - small basin surface; into a depression - high basin surface. Variation coefficients point out very similar variation principles, showing a relative homogeneity of flow processes.

Headnote

ABSTRACT. Monthly average flow in Râul Negru hydrographic basin Râul Negru hydrographic basin represents a well individualised and relatively homogenous physical-geographical unity from Brasov Depression. The flow is controlled by six hydrometric stations placed on the main collector and on two of the most powerful tributaries. Our analysis period is represented by the last 25 years (1988 - 2012) and it's acceptable for make pertinent conclusions. The maximum discharge month is April, that it's placed in the high flow period: March - June. Minimum discharges appear in November - because of the lack of pluvial precipitations; in January because of high solid precipitations and because of water volume retention in ice. Extreme discharge frequencies vary according to their position: in the mountain area - small basin surface; into a depression - high basin surface. Variation coefficients point out very similar variation principles, showing a relative homogeneity of flow processes.

Keywords: monthly average flow, space-time variation, flow principles, occurrence frequency, variation coefficient

1. INTRODUCTION

Râul Negru collects its tributaries from the eastern sector of Brasov Depression and from its mountainous slopes: Bodoc, Nemira, Vrancea and Întorsurii Mountains. The hydrographic basin presents two representative, almost equal relief levels. The mountainous margin has relatively steep slopes and well developed piedmonts. The alluvial compartment from the depression has an almost horizontal relief.

The hydrographic network has a similar development on both river sides, which has a length of 88 km. Its longitudinal slope decreases from 55 m/km on upper river part, to below 1 m/km before its confluence with Olt River. It's most important tributaries are Casin River, on right, and Covasna River, on left.

Flow genesis is determined by climatic conditions, which evolution can be monitored by Lacaut (1770 m) and Târgu Secuiesc (568 m) meteorological stations. Precipitations regime presents maximum values in June (173 mm at Lacaut Station, 85 mm at Târgu Secuiesc Station), but some differences appear for minimum values: October at Lacaut Station (40 mm) and December at Târgu Secuiesc Station. The highest monthly average temperatures appear in July (over 17°C at Lacauti and 10°C at Târgu Secuiesc). The lowest months are January in the mountain area, and February in the depression. Negative temperatures period has 5 months in the mountain area and 3 at Târgu Secuiesc.

These genetic factors determine water flow regime for all rivers in the hydrographic basin and also the space-time repartition for water resources.

2. DATA BASE AND METHOD

To analyse monthly average flow parameters, we took into consideration the discharge from six different hydrometric stations - Lemnia and Reci on Negru River, Plaiesii de Jos si Ruseni on Casin River, and Covasna and Borosneul Mare on Covasna River. The time period use for this analysis is 1988 - 2012. This time period was chosen because some hydrometric stations had been liquidated in the last years, so diminishing hydrometric control over river basin. Some stations needed prolonged time periods for data homogenization.

Our analysis is based on variation graphics for multiannual monthly average discharges, frequency and cumulative frequency graphics, but also on calculating variation coefficients.

3. RESULTS

3.1.Space-time variation

Monthly flow principles were analysed on all three main rivers that are controlled by two hydrometric stations.

Their location, the relative homogenous physical-geographical conditions and a not too big hydrographic basin surface (2320 km2) determine similar parameters for river's flow.

The highest river discharges appear on all rivers in April. This phenomenon is determined by early snow layer melting together with spring rainfalls. High flow appears also in March, May and June. Usually, the discharges from March are equal or lower than those from May or June. There can be also observed that, with the exception of Borosneul Mare Station, all discharges from May are lower than those from June. This thing is determined by early summer rainfalls quantities that are higher than those from late spring.

From June till November, monthly average discharges constantly decrease. With the exception of Lemnia and Plaiesii de Jos stations, which are placed only in the mountain area, annual monthly minimums appear in November. Sometimes, depletion reserves curve has a very homogenous form. The lowest discharge values or the two mountainous stations appear in January. The genetic factors of this phenomenon are strong water freeze, retention of high water quantities in ice formations, and solid precipitations. The discharges from February are very close to those from January. We can observe that water flow in December is slightly higher than that in November at all stations. This phenomenon can be determined by the slightly rainfalls increase in the last part of the year, and because of late snowfall occurrence.

Because of river's high basin surface, and because of its strong tributaries between the two stations, Negru River has monthly discharges with low relation. The relation between the discharges from the two stations (Lemnia and Reci) presents big variations. The highest values appear in January and September (20 and 19, respectively). In March, April and June, the values are low (11, 10 and 11, respectively). For the other two rivers - Casin and Covasna, the values are almost constant. For Ruseni/Plaiesii de Jos stations, and also for Borosneul Mare/ Covasna stations, the monthly values are 3 and 4. This represents a remarkable flow conditioning on these rivers, even though there are high surface hydrographic basins.

3.2.Frequency

To analyse discharge frequency, we took into account the months with minimum and maximum multiannual monthly discharge (April and November). The maximum flow month on the main river presents some discharge variations from 0,30 to 4,65 at m3/s Lemnia Station, and 4,00 to 44,1 m3/s at Reci Station, respectively.

Maximum frequencies at Lemnia Station appear in the third inferior part of the variation gap, and at Reci Station they appear also for average discharges. 6th degree polynomial trends indicate two peaks for the upstream station and three for the downstream station. The cumulative frequencies graphics are similar for the two stations, with a more pronounced increasing trend for Reci Station.

The extreme discharge values from November at both main stream stations are as follow: 0,054 m3/s and 0,766 m3/s at Lemnia Station, 1,39 m3/s and 7,48 m3/s at Reci Station. The upstream station presents a high frequency peak for discharges of 0,1 - 0,2 m3/s. High frequencies at Reci Station appear in the lower half of variation gap. 6th degree polynomial trends are similar. Cumulative frequency graphics present a high increase of small values and a high inflection in the upper variation gap.

Similar is also the individual and cumulative frequencies distribution on main tributaries, and the graphic and polinomial trends rate, because of the hydrographic basins and of the small distances.

An interesting comparation can be made between monthly flow percentages in the years 1950 - 1967 (Ujvari, 1972) and the present period: 1988 - 2012. High values increase of over 1% appears on June (1,06%), July (1,41%), September (1,07%) and October (1,28%). Values of less than 1% appear in January, November and December. The highest percentual decrease appears in May (- 2,95%), followed by April (- 1,49%). Values of less than 1% appear also in February, March and August. We can observe that the increase period is September - January, and the decrease - February - May. It can be sure that these changes have a direct connection firstly with precipitation regime modification.

3.3.Variation coefficient

Variation coefficients analysis has been made on all three rivers in the maximum (April) and minimum flow (November) months. We can observe that variation principles are the same, which shows flow genesis homogeneity in the entire Râul Negru hydrographic basin.

CV smaller values for high flow appear at upstream stations (for Negru and Covasna streams). CV value of 0,65 appears all year on Casin Stream. From these we can conclude that flow genesis in April isn't uniform even in the mountain area. This fact underlines the importance of snow melting in flow formation. For poor drainage, CV values are higher for upstream stations than those from downstream for Râul Negru and Casin streams. An opposite situation appears on Covasna Stream. At Borosneul Mare Station, flow variation is higher than that from other streams (1,02 in April and 0,70 in November).

CV values comparison for the same hydrometric station presents more obvious variation principles. CV values for upstream stations are always smaller in April than November. For the downstream stations, the highest flow values month presents a higher CV value than the with the lowest month.

4. CONCLUSIONS

We can observe in the Râul Negru hydrographic basin a remarkable homogeneity of flow genesis and control. Physical-geographical conditions maintain a constant space-time flow process course. The small differences that appear are determined by the local topoclimate, the slope and bed morphology, the vegetation characteristics, etc.

References

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