engleski

Low Streamflow Analysis of the Lower Drava River

History shows that lower Drava is an important factor of social progress. The birth and growth of human settlements are related to the river. The Drava River is a source of water for many purposes, but it is also a natural wastewater recipient. Knowing the regime and characteristics of low streamflows are very important to different water management systems and structures, in view of planning, designing, construction, maintenance, exploitation and overall management. The low streamflows are specially important when different aspects of the water quality are discussed. The characteristic of low streamflow is to have the quantities of water in a streamflow, i.e. recipient, at the minimum and therefore the possibilities for dilution of pollution brought in are also minimal. Analysis of low streamflow of the lower Drava River is necessary as a starting point for consideration of future development trends, particularly those relating to protection of natural values of Drava and Danube rivers and Kopački Rit ; stream capacity for receiving wastewater, and water quality protection. The paper presents results of analysis of the Drava River low streamflows from the gauging station at the Lower Drava River – Donji Miholjac  77+700/ Drava rkm in the period from 1980 to 2007. Frequency curves of minimum annual flows are widely used in the classification of streams according to their capacity for waste substance discharage and planning the development of surface-water resources. A low streamflow frequency curve for gauging stations, which shows magnitude for the various numbers of consecutive days against recurrence interval, is presented in the article. Also, analysis of stochastic processes of stream low flows is presented by the paper. The calculating method takes into account all essential components of the aforementioned process like deficit, duration, time of low-flows appearance, in addition to number, maximum deficit and maximum duration of low streamflows in the determined time interval. In analysed period, mean annual flow is Qsr = 500 m3/s, maximum observed flow is Qmax=1891 m3/s, and minimum is Qmin = 157 m3/s. Only the deficit less than accepted truncation discharge has been stochastically analysed so the attention has been concentrated only to the lower extreme zone, that is to the partial duration series. As to the truncation discharge, the flow of 95 % of low-flow has been accepted, from the flow duration curve of the mean daily flows, Qr =243 m3/s. Number of 38 low-flows were recoreded within 28 years. There was a period of 10 years without low-flows. Low-flows were not recoreded only in May and June, whereas there were 6 low-flow days in April, and 1 low-flow day in July. Days with maximum recorded low-flows were in January - a total of 112, in February – 95, and in December – 66. The highest probability is to have only one low-flow per year, i.e. P(k=1) = 0, 349. Maximum deficit in analysed period was Dmax = 186.019.200 m3, with maximal observed duration 94 days, and average deficit was Davg = 19.828.800 m3. Ten-year recurrence interval of deficit is D10 = 82*106 m3, i.e. 100-year recurrence interval D100 = 158 *106 m3, during one year interval. The extreme low-flow duration of 10-year recurrence interval T10= 45 days, i.e. 100-year recurrence interval T100= 86 days, for one year interval.

Lower Drava River; hydrology; discharge; low streamflow; truncation level

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