Changes in the annual mean runoff

The overall evolution of the flow in a waterd-drained area of the Tarnava hydro-graphic basin highlights a right connection among K values, DQ correction discharges, and the water volumes consumed by water users. During the years when the K values were most reduced, the percentage of the correction discharges exceeded 5% and the volumes of consumed water were some of the highest; there are years when there was no compensation of minimum discharges because of discharges lower than natural discharges and average multiannual natural discharges.

In Tarnava hydrographic basin, the effects of water training and water users were particularly obvious between 1986 and 1990. Significant changes in the annual average flow were registered in 1986 in Tarnava Mica basin at Balaujeri, Tarnaveni, and Blaj. The average annual discharges registered at Mihalt hydro-metric station show high values in 1984 and 1985, of 32.2 m3/s and 43.2 m3/s at the beginning of the period analyzed, followed by 10 years of discharges below the average of 23.6 m3/s. Since 1996 there has been an increase in the discharges under observation, reaching the highest values of 50.9 in 1998. The reduced values of the average annual discharges measured in the years 1986, 1987, 1989-1995 and 2001-2004, lower than the reconstructed average annual natural discharges of the same years and inferior to the average multiannual values of natural discharges emphasize the lack of compensation on reduced discharges. The average monthly discharges registered and reconstructed were below the average multiannual values, but water users in the basin consumed 31.6 Mm3 of water in 1986, almost three times as much as during the previous year, 1985, which registered a high flow capable of compensating the quantities stored and consumed by the water users.

The modifying factors of the natural flow imposed the use of reconstruction modifications in 9 out of the 22 years under analysis. The modifying factors exceeded 5% in 1986-1989, 1990, 1991-1993, 1995, and 2003. The years when the value K coefficients are most reduced coincide with the years when it was necessary to apply the reconstruction correction such as 1986, 1987, 1989, 1990, and 1995. In 1990, the value of the coefficient is 87.46%, which represents the share of the measured discharge out of the reconstructed natural discharge. In the same year 1990 they registered the highest degree of average flow modification from the entire period analyzed.

Although in 1990 registrations did not show the highest annual water storage consumed in the period analyzed, the K values are the lowest because of extremely reduced average annual discharges. In 1995, when they registered the highest net water storage of 34.7 Mm3, the K value was 91.79% but the values of the measured and natural average annual discharges almost doubled in comparison with those of the year 1990.

On the Tarnava Mica River, at Blaj hydrometric station, the K value is the most reduced in 1986, the year with the most insignificant modifications on the average flow due to the large volumes of water consumed by the water users and also to the low-average discharges measured and reconstructed. At Tarnaveni, Tarnava Mica hydrometric station, they registered significant influences of different water users on the average flow conditions in 1986, the highest value being DQ = 6.28%. In all these years reconstruction measurements (calculations) were needed. Since 1995, they have registered an increase of the discharges observed in the whole of the Tarnava basin, reaching a peak in 1998 with 16.6 m3/s. The reduced average annual discharges measured in 1986, 1987, 1989-1994, 2000, 2001, 2003 and 2004, lower than the natural average annual discharges reconstructed in the same years and also lower than the multiannual average values of the natural discharges, emphasize the lack of compensation effects of the minimum discharges.

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