We present and analyze experimental results on the dynamics of hydrothermal
waves occurring in a laterally heated fluid layer. We argue that the large
-scale modulations of the waves are governed by a one-dimensional complex G
inzburg-Landau equation (CGLE). We determine quantitatively all the coeffic
ients of this amplitude equation using the localized amplitude holes observ
ed in the experiment, which we show to be well described as Bekki-Nozaki ho
le solutions of the CGLE. [S0031-9007(99)08940-1].