The insatiable thirst of the world's burgeoning billions has caused a spurt of dam building in temperate regions in the last 40 years, and a scientist with the space agency has found that the reservoirs are affecting Earth's orbital rotation.

Although Earth's rate of spin is gradually slowing because of the tidal drag of the moon, the slowing would have been measurably greater if it were not for the influence of 88 reservoirs built since the early 1950's, said the scientist, Dr. Benjamin Fong Chao, a geophysicist at the Goddard Space Flight Center, an arm of the National Aeronautics and Space Administration in Greenbelt, Md. Each of the reservoirs contains at least 2.4 cubic miles of water weighing 10 billion metric tons. The reservoirs contain the bulk of the world's impounded water.

The shift in the distribution of Earth's water caused by the reservoirs has tended to speed the planet's spin. Without lunar tidal drag, the reservoir effect would have reduced the length of a day by 0.2 millionths of a second a day for the last 40 years, Dr. Chao calculated.

The reason for this, he said, is that the shifting of water to mid-latitude reservoirs in the Northern and Southern Hemispheres has increased the amount of the world's water in those latitudes in relation to the Equator. In effect, more water is closer to Earth's axis. Moreover, Earth's axis is being slightly tilted by the weight of water that has collected in the 88 reservoirs, Dr. Chao found, and the shape of the planet's gravitational field has been altered.