IT is becoming less clear when exactly China started closing the taps on its overheating economy. Vice-Premier Zhu Rongji went public with his 16 measures to turn down the heat on July 3, following a barrage of warnings from outside economists that the needle was on danger. Yet as the final figures emerge for Hong Kong's economy in the first six months, evidence is growing that the mainland was moving to turn down the heat before that announcement. Assessing the implications for Hong Kong is a growth industry by itself. The Government last week stood firm by its 5.5 per cent forecast for gross domestic product growth this year, and remained confident that the local economy could absorb any punches that might come across the border. Yet the half-yearly economic report, issued late last week, showed a sharp downward trend in the growth of domestic exports to China. After soaring by 20 per cent in the first quarter, they edged up by two per cent in the second. This reduction in momentum is not confined to domestic exports across the border, and it begins to look less likely that any slack in the China market will be taken up by other major markets for Hong Kong products. Total exports for the first half are expected to have grown by 16 per cent, against 19 per cent in the whole of the previous year. But while the first quarter showed a 20 per cent gain, the estimates for the second quarter are for 13 per cent. In real terms, the rise in the value of re-exports was also slowing - down from 26 per cent in the first quarter to an estimated 19 per cent in the second. The figure for total domestic exports should also set off some early warning alarm bells, for that has not just slowed down, but has turned negative. In real terms domestic exports were stagnant in the first quarter, but fell seven per cent in the last three months, leaving the total value down by four per cent over the half. That compares with last year's annual growth of one per cent. As Hong Kong is giving up its hard export business to the Guangdong hinterland, the figures are not as serious as they would be for other countries. But the report could be another straw in a strengthening wind.