Look for teltale signs of coolant leakage around the filler cap and overflow, if there is no sign of leakage anywhere else the chances are that the head gasket is leaking and pressurising the system and blowing the water out. You could try leaving the filler cap not screwed down tight. if the leakage is not to bad this could stop water loss but it also lowers the boiling point so don't thrash it.

The gaskets seem to go at around 150,,000 miles on the 1.9 engine the Hdi's usually last longer.