This project provides a method to calculate the water balance for small estuaries, known as Intermittently Open or Closed Lakes/Lagoons (ICOLLs) in Australia. Their management requires the prediction of lake water levels and the probability of breaching of the entrance barrier in real time for management and long term for planning. The open water evaporation is a key quantity in the water balance. The project developed and demonstrated a method to determine evaporation for ICOLLs from data routinely available, primarily from government agencies, without using external parameter values. It has been applied in case studies of two ICOLLs on the South Coast of NSW, Australia with very different morphologies: Durras Lake, a drowned stream valley with a largely steep forested catchment, and Lake Wollumboola, a back-dune lagoon ICOLL with a wider, shallow lake. The calculated seasonal curve of evaporation and the correlation of evaporation with incident solar radiation developed in this study are two simple but powerful tools for use in predicting or hindcasting evaporation in an ICOLL. The calculated evaporation agreed with the data intensive formulas of Makkink (1957) and Priestly and Taylor (1972), with calibration of the latter. The results indicate that the use of regional parameter values is quite unreliable for these small catchments.