BASIC CHARACTERISTICS OF THE PLUVIOMETRIC REGIME IN THE UNA RIVER BASIN

Intensity of action and frequency of high and low pressure barometric fields of large and medium scale have great influence on dynamics of pluviometric regime. On their action depend characteristics of thermic regime, relative humidity, cloudiness and windiness, which directly affect the precipitation. Concerning that the water balance is difference between the inflow and evaporation, it comes that precipitations do not have just special role, but they are also the most important factor of the Una river regime. Disposition and precipitation structure are the river regime's essential assumptions, so at the same annual height of precipitation, we have more water in rivers in the year with more precipitations during the colder period. Then we have less evaporation so the inflow from the basin is bigger. The amount and disposition of the precipitation in the Una River basin is analyzed in this paper in order to define the pluviometric regime of this area. Pluviometric regime greatly influences the Una river regime and water balance, what was the reason for this analysis and further hydrological research in the Una River basin.


Introduction
The Una River basin feeds the Sava River with water, and it is a part of the Black Sea Basin. The Una basin, located in the northwest of Bosnia and Herzegovina, partly crosses into the neighboring Croatia. It borders with the following basins: the Vrbas and the Pliva on the east and southeast, the Cetina, Krka and Zrmanja on the south and southwest, underground waters on the west flow towards Gacko polje, while with basins of the Korana and Glina it borders on the northwest and with direct Sava basin on the north. Various data on the area covered by the Una basin can be found in the professional papers. The data are mainly stated that the basin covers in total 9640 km 2 (Spahić, 1991), on the territory of Bosnia and Herzegovina 9368 km 2 , of which on the territory of Federation of Bosnia and Herzegovina 5020 km 2 (Žigić, et al., 2010). Majority of data is linked to orographic, relatively topographic watershed which, due to prevail terrains with aquifers of fracture-cavernous porosity, does not make a real watershed between neighboring basins and the Una basin, so it should be taken with the reserve. It is very hard to determine the concrete and accurate watershed, so as the basin surface, until directly hydrogeological researches are done in the whole basin's area (Spahić, Korjenić, Hrelja, 2014). Recently, such researches were conducted in the area of the western and southwest part of the watershed. Since there were no large-scale hydrogeological charts for this area, as a basis for this paper, hydrogeological chart of the former Yugoslavia was used, Zagreb, Sarajevo and Dubrovnik papers with scale 1:500 000, with certain changes due to recent research [8]. Based on the mentioned map and supplement, the Una basin was segregated (according to the own measurements from charts in the program ArcGIS 9, ArcMap) with surface area of 9979.5 km 2 . General climate characteristics in the Una River basin are consequences of physical-geographic determinants and predominantly zonal and azonal position of this area. In zonal view, the Una River basin's area is situated inside two large climate belt zone that are: the southern parts of the northern moderate belt and the northern parts of the northern hemisphere subtropical belt. The southern parts of the basin approached the Adriatic facade, from that reason significant Considering the global distribution of air masses, belts (zones) and sectors, it can be distinguished zonal layout of isotherms and monthly isotherms, but also isohyets in the Una basin area. However, hypsometric position and exposition to the general global air circulation brought to uneven precipitation and temperature disposition in certain areas inside these zones. The analysis on climatic elements and appearances led to the conclusion that a moderately continental type or a moderately warm and a humid climate type (Cfb), that is beech climate, were formed in this area in lower hypsometric levels and generally in the basin's larger part.

Materials and Methods
For the analysis of pluviometric regime it is necessary to know disposition and amount of the precipitation in the basin. Precipitation quantitative value determination in the basin was difficult also due to a relatively small number of observation stations, which, almost as a rule, did not have complete sets of observations. Because of that reason, for determination of precipitations falling on a basin's surface, three different methods were used in this paper. Based on finding the weight coefficient for precipitation measured at a given point, those are: arithmetic mean method, the Thiessen polygon method and isohyet method.
The arithmetic mean method is used mainly at basins which do not have a pronounced vertical separation of the relief, nor do they have greater differences in precipitation amount. Thus, this method gives a rough estimation of the average precipitation value. According to Hrelja H. (2007), if it is assumed that every pluviometer on, or in the vicinity of the analyzed basin has equal weight w i = 1/n, where nnumber of pluviometers taken into consideration, then the definition term for the average precipitation quantity in the basin is: In this case P1, P2, ..., Pn are precipitation heights taken from the pluviometer monitoring in the basin.
Applying the Thiessen polygon method, all meteorological monitoring in and around the basin have come into consideration with the assumption of linear change in precipitation heights between the two observation points. In order to determine polygonal surface on the basin, it is necessary to connect all measuring stations to make up a network of triangles. The symmetries of all sides of the triangle are the sides of the polygon for individual meteorological stations (Hrelja, 2007). Thus, the Thiessen's procedure is based on the basin division on areas for which the data of each meteorological station on and around the basin are approximately valid. Although this method is also the most suitable for flatland basins without more expressive change of topographic and meteorological factors and at uneven distribution of pluviometer monitoring spaceward, yet it is the most commonly used method in hydrological practice. According to this method, the average precipitation amount P, which is excreted on the basin surface is determined by the term: Where: P -Annual precipitation amount in the basin area in mm; Fnsurface of polygons that belong to individual monitoring points; Pnprecipitation height.
The isohyet method is based on an average precipitation amount between the two isohyets multiplied with surface between them (Ducić, Anđelković, 2004). It is calculated for all isohyets on the basin surface, and a sum of obtained multipliers gives the average precipitation amount in the basin.

Precipitation in the Una Basin
Spatial coverage of the Una basin, considering its position and size, gets also various precipitation amounts. From the data from Table 1, an increase in annual precipitation amount is observed along with increase in the altitude of the station, which however is not continuous, and there are exceptions. The reason for this fact comes from the position of individual meteorological stations, relief characteristics, but also from atmospheric processes. Although the precipitation increases with altitude, during the work a conclusion has been reached that the precipitation increase along with altitude is not uniform and even. The position of meteorological station dominantly affects these precipitation relationships, as well as cyclonic activities or degree of the area continentality where the measurement of this climatic element is performed.
In order to try to show the relationships between annual precipitation amounts with altitude more faithfully, it has been analyzed linear correlation of meteorological monitoring data by hypsometric zones. The largest number of stations is located within the hypsometric zone of up to 200 m. Within them there are evident differences when it comes to the annual precipitation amount. Concerning the small altitude difference and large oscillations of precipitation amounts on individual monitoring within the zone, linear correlation is extremely large and it comes from the relation: P = 2.2293xh + 706.31.
Negative linear trend appears within the zone of 200 up to 500 m, due to a large precipitation amount occurrence in Bihać and Lušci Palanka, which affects normal and homogenized precipitation increase with altitude. Extremely large precipitation amount registered in this area can be explained by their excretion in warmer part of a year, due to vertical thermic convection from cumulonimbus clouds. Larger areas of valleys and fields at lower altitudes will receive larger precipitation amount comparing to the surrounding, hypsometrically higher zones in which comes to cumulonimbus cloud degradation and by that also to the cessation of precipitation. Linear trend established in this zone amounts P = -0.0408xh + 1204.
The zone above 500 m altitude is characterized by homogenized increase with altitude, though the largest changes are seen at the altitude of 600 up to 700 m. On the base of conducted linear correlation, the following indicator was obtained: P = 0.8195xh + 656.73. The rate of the annual precipitation increases along with altitude, for the Una basin as a whole, we should take with reserve, because the allocation of meteorological stations all over the basin is unequable on horizontal and vertical profile. The same rates of precipitation growth with altitude cannot be expected in all parts of the basin also due to the relief dynamics. Besides, precipitation measurements at the meteorological stations as individual points, does not bring enough information on precipitation falls on the basin area that, in fact, represents the basic interest for the water flow process.
There are a number of different methods for determining precipitations falling on the basin area, and they are all based on finding the weight coefficient for precipitation measured at a given point. By the method of arithmetic mean, summation the value of total precipitation amount taken from all analyzed stations in the Una basin area, and then division of the obtained value with the number of observation points, the average precipitation amount in the basin is 1147 mm, see the table 1.
On the base of Thiessen polygons and calculation from the table 2, obtained average annual precipitation amount in the Una basin sums 1177.6 mm. The average obtained Thiessen polygon method differs for about 30 mm from the data obtained by the method of arithmetic meanings.  The isohyet method gives more accurate results than the previous ones. At this method, during the isohyet construction, their position is determined by interpolation that can and cannot be linear, what depends on conditions arouse from topographic position, distance from sea and average annual precipitation characteristics.  For the purpose of this paper a map was used with average annual precipitation amounts according to the SFRJ Climate Atlas.
The results of the analysis of the isohyet map, given in the table 3, show that the average annual precipitation amount in the Una basin sums 1273.8 mm. According to these values, the annual precipitation amount is about 96 mm larger than the value obtained by the Thiessen polygon method, and about 125 mm than data obtained from the analysis by the method of arithmetic means.
Using different methods for assessing the average precipitation in the Una basin, approximately the same result is obtained in given conditions. The lowest average value is obtained by the method of arithmetic means. However, it can be concluded that precipitation increases in the Una basin area along with the altitude growth, though there are certain specifics when data obtained from individual meteorological monitoring is concerned. Dynamic relief also contributes to this, with significant morphological variety in the Una basin area, which affects forming of specific precipitation occasions and differences in pluviometric regime on local level. Precipitations decrease going from the west towards the east, but also going from the south towards the north.
On the base of average trend in rising precipitation along with altitude in the Una basin of 47.7 mm/100 m, and by analysis of the data obtained by the isohyet method, a conclusion was reached on an average annual precipitation amount by hypsometric zones, which in average for the basin sums 1190.9 mm.  structures over 500 m altitude have also larger precipitation amount, in average over 1200 mm, while areas above 1000 m altitude receive more than 1400 mm annually. Knowledge on total precipitation amount by hypsometric zones has great importance and apply when analyzing total flow and inflow in the Una basin.

Pluviometric Regime
According to analyzed data given in the fig. 3, it comes that precipitations are equally distributed during the year. Although monthly precipitation amounts are uneven, it should be noted that during the year there are no dryness, both spatially and temporally. As it was already stated, the precipitation distribution during the year is under the influence of air fronts penetrations from the Mediterranean, Atlantic, or from the north during the winter. The basic feature that is noticed in an annual flow of the precipitation height is the existence of two extremely periods with plenty of precipitation. The first one is linked to the second part of the autumn and beginning of the winter season (October-December). During this period 324 mm of precipitation falls, or 108 mm in average by month, what is about 112% from the average annual precipitation amount. The second precipitation period is characterized with extremely reduced precipitation average, during which 82 mm is extracted in average, what makes 85% of average monthly value. This period of precipitation minimum binds to the second half of the winter and beginning of the spring period (January-March). Between these characteristic precipitation periods, pluviometric regime has transitional characteristics, means that two secondary precipitation periods appear. Secondary precipitation maximum binds to the period of the second half of the spring and beginning of the summer climatic season. In this period (April-July), monthly precipitation average sums 102 mm, what makes 106% of total monthly average. Secondary minimum corresponds to the period of the end of summer and beginning of autumn season (July-September), when in average 91 mm of precipitation is extracted in one month or 94%. In general, it can be concluded that in the Una basin area, pluviometric regime is characterized with extreme two maximum, in June, and in November, and two minimum, in February, and September. Months, which show the maximum precipitations differ in individual basin's area, depends on number of factors. Maximums can appear also during May or April, while other, secondary maximum, except for November, appears also in December. Precipitation minimums as well, vary in certain parts of the Una basin. They are noticed mostly during January or March, and then also October.
Nonlinearity of the annual precipitation distribution by months is also stated by the analysis of relative annual precipitation oscillations. (Table 5). Table 5: Relative annual precipitation oscillation in the Una basin not continuous and there are exceptions as well. The reason for this fact comes from the positions of individual meteorological monitoring, characteristics of relief, but also atmospheric processes. The analysis found that annual precipitation amount in the basin increases by 47.7 mm per every 100 m. The results of applying different methods for the evaluation of average precipitations in the Una basin show that in given conditions approximate result is obtained, regardless on applied method. The lowest average value is obtained using the method of arithmetic meanings, 1147 mm. The results of the analysis of the isohyet chart show that the largest value of average annual precipitation amount in the Una basin is obtained by this method and it sums 1273.8 mm. That is for about 96 mm larger value than the one obtained by the Thiessen polygon method (1177.6 mm). Average value of the total precipitation amount using vertical gradient of precipitations by hypsometric zones sums 1190.9 mm. All these differences are product of different physical-geographical position of stations for monitoring as well as absence of continuous monitoring in all hypsometric zones in the basin area.
It can be concluded that in the Una basin area, pluviometric regime is characterized by two extreme maximums, in June and November, and two minimums, in February and in September. However, months with precipitation maximums differ by individual meteorological stations, depending on a number of factors. Thus, maximums can appear also during May or April. Second or secondary maximum appears not only in November, but also in December. Minimums as well, vary by individual stations. They are noticed mainly during January or March and then also in October. Generally looking the Una basin, it is concluded that this area belongs to continental, and that extremely and transitional continental pluviometric regime.