Since then, international co-operation within the Danube river basin has been co-ordinated by the German National Committee for UNESCO International Hydrological Programme (IHP). This initiative signalled a new era of co-operation, and a lot of interesting research works and publications were prepared. The main achievements between 1987 and 1992 are briefly described by K. Hofius². Since 1993, specific tasks in the framework of the UNESCO IHP are managed by the Austrian National IHP Committee, headed, at present, by Oskar Behr from the Technical University of Vienna.

The Danube river has been the focus of many other organisations lately; after the political and economic changes in Europe, the PHARE Programme, the Bucharest Convention and the Sofia Agreement have played an important role.

For example, in 1990, the Environmental Programme of the Danube River Basin began. Here, hydrological data are again the basis for various more detailed studies - evaluation of river quality, total flux of pollutants, environ-mental risk analysis and early warning systems. It was found that there is a very urgent need to process new data. It is useful for all ongoing projects to extend their time frames and to evaluate hydrological support for higher level decision-making processes in the river basin, by processing, as much as possible, all the available data for the time period of 1931-1995. This is, at present, the task of the Programme on Regional Co-operation in the Danube River Basin, in the frame-work of the UNESCO IHP, at the expert level.

The first step involved the preparation of supplementary volumes on special hydrological and hydraulics questions. In 1995, and following a certain period of negotiations, common work on the new edition of the Hydrological Monograph of the Danube River Basin began. Participating institutions are - in downstream order: Bayerisches Landesamt für Wasser, München, Germany; Technical University of Vienna, Austria; Czech Hydrometeorological Institute, Prague and Brno, Czech Republic, Water Research Institute Bratislava, Slovak Republic; University of Ljubljana, Slovenia; VITUKI Budapest, Hungary; State Hydrome-teorological Institute and Split University, Split, Croatia; School of Mining and Geology, Belgrade, Yugoslavia; National Institute for Meteorology and Hydrology, Romania; Bulgarian National UNESCO IHP Committee; Hydrome-teorological Service of Moldova; Ukrainian State Committee for UNESCO IHP. In the Slovak Republic Croatia, Bulgaria and Ukraine, there are also other institutions working on the project, but only the co-ordinator is mentioned here.

The role of the Water Research Institute is to take part in the pre-processing of runoff data and in the evaluation of data for evapotranspiration estimation. The water balance refers to an internationally co-ordinated scheme of sub-basins, not all of which are gauged up to the mouth of the confluence of the Danube river. This causes some theoretical and practical problems, which also include problems of runoff data availability and the assessment of the runoff contribution of ungauged interbasins. Some preliminary results are discussed here and some of the possible solutions are shown. Specific questions and runoff data compila-tion related to the complex water balance of particular sub-basins are demonstrated.

Figure 1.  Scheme for the evaluation of runoff contribution from selected areas

Figure 2.  Real scheme of node between Bratislava and Komárno
(The total area of this lowland landscape is 16,082 sq.km)

Figure 3. The river node between Bratislava and Komárno

Runoff differences (Figure 4) for the test territory range from 389 m3/s to -935 m3/s, in absolute units. Expressed as a percentage of the runoff in Komárno, for the period of 1962-1973, figures range from 8.2 to -14 only. A detailed study of the data shows that the runoff of the Arpas on the Lajta river seems to be signifi-cantly influenced by the discharge in the Danube, especially during flooding.

Figure 4. Runoff differences between the discharge in Komárno and the sum of discharges at the nearest upstream profiles

It seems that probably all of the considered rivers in their lower sections are draining the surrounding area. The water infiltrating the inundation area south of Bratislava enriches groundwater reserves and it is therefore possible that this infiltrated and drained water could be measured twice. The discharge in Komárno is lower than sum of measured discharges above the downstream profile.

Significant differences can be seen in June and July 1965 when flooding caused the breakage of flood protection dams and, therefore, discharge in Komárno is partially shifted. However, Figure 5 shows that cumulative monthly mean discharge in Komárno and the cumulative sum of monthly mean discharge in upstream profiles, differs by less than 3%.

This result is not favourable for data error analysis, but the study of daily discharges, which are not available for all the profiles, can be somewhat helpful in this situation. It is also possible to study extreme values and plot all the standardised Qs discharges by applying the Qs = (Q-ì) ó –1 transformation, where m is the mean discharge for a given period and ó is the standard deviation of the sample.

Figure 5. Comparison of the cumulative discharge for Komárno and the cumulative sum of discharges at the nearest upstream profiles

Data collection and related problems are discussed for one particular node lying in the first Danube delta area. It is evident that the hydrological situation is influenced by so many factors, that it is difficult to set up a standard procedure for data verification. Hopefully, the data set standardisation method presented above can be used for obtaining a more complete picture of the nature of some of the extreme differences in data assessment preparation.

Table 1. Monthly hydrometeorological elements for the Nitra river basin (1951-1980)

Further steps have to focus on the areal distribution of specific runoff and actual evapotranspiration. Knowing the areal distribution of precipitation, it may be possible to apply a model similar to the one used for the whole watershed and to use a do-loop for areal layers, or direct for pixels, considering land cover and elevation/slope influence on distributed parameters for runoff coefficient values. Tuning of the model, with the aim of obtaining an integral runoff value as close as possible to the one measured in the balance profile, will be the next step. Such chosen processing is leading to the need for obtaining terrain data for the whole Danube basin, and this is the second important problem, which has to be solved. Local data is supposed to be used where available and, for the rest of the watershed, some more general input has to be chosen.

The hydrological monograph of the Danube river basin has to create a common basis for all water management and environmental studies. The regional experts group provided step by step methodological approaches. Some particular problems - i.e. terrain data and/or common GIS approach - are, at present, still not fully solved and any contribution from the international level or from the Danube Environmental Programme would significantly help to accelerate the progress of this study. Co-operation in this field falls under the umbrella of the UNESCO International Hydrological Programme and constitutes a contribution not only to the environmental study of the largest European river, but also to the improvement of human and scientific relationships in this historical area.