The project reported here isa subproject within the multi-disciplinary programme "Gulf of Bothnia Year 1991 ".
Runoff from a land area of approximately 490 000 km2 enters the Gulf of Bothnia. This runoff is of essential importance for the flushing of the Gulf. A change in the volume of runoff effects the residence tirre. There are many natural changes in the runoff, both long-rerm changes ovcr many years and such within one year. There are also man-made changes. The most important of these is hydropower regulation. This report describes the effect of the development of the hydropower plants in Sweden and Finland by means of recorded regulated runoff and calculatecl natural runoff. The recent tirne period, 1980-91, and the time before regulation, 1925-36, are simulated. The monthly rnagnitudes of the redistributed flows were on the average 1 700 m3/s, but the maximum redistributed rnonthly tlow in May - June reached 5000 - 6 000 m3/s. The "Gulf of Bothnia Year 1991" is compared with the period 1981-91. The annual mean runoff in 1991 was c. 6 000 m3/s, which is somewhat less than the examined period. This originated from a low spring and summer runoff, e.g. c. 9 000 m3/s in May instead of 13 000 - 14 000 m3/s. Also the effects of a hypothetical climatc change of both a temperature increase and a decrease are simulated.