This report describes some work undertaken by GWRC in 2012/13 to better understand the hydrological water balance of Lake Wairarapa. This work helps inform the management of the lake. A water balance for the lake is presented, based on an analysis of rainfall, evaporation, stream inflows and lake outflows. A spreadsheet model to predict water level fluctuations is also presented along with some interim recommendations about the allocation of water from the lake (eg, for irrigation).