Although research on a field investigation about quantification drawdown of ground water wells has hitherto been conducted with emphasis on Sana'a basin which is 3 200 km2 in area characterized by general hazard in quantity and quality of water，there exist uncertainties about the size of the hazardous annual decline in the level of underground water． So the authors are trying to assess reliable hazard data on the depth of groundwater which were obtained by measuring static water level． The data set are twenty six wells from 9 regions which were selected to represent Sana'a basin and collected during the course of the 20 months groundwater monitoring survey from January 2008 to January 2009． The results show that the average drawdown during 20 months to be 3. 22 m with an average 0. 16 m per month and 2 m per year．