The drawdown, mass and heat transport in the Ellidaár geothermal reservoir are studied. A lumped model and a distributed model are discussed, describing the past and present state of the field, its behaviour, and predictions for future exploitation. The lumped model is mainly based upon measured drawdown, flowrate, and mass and heat changes since the start of production in the field. A more complex and accurate distributed model gives a broader and more detailed picture of the reservoir. It also takes into account all physical properties of the fluid and porous media. Although the models are very different in their complexity and approach to the problems discussed, both of them give results which match the measurements very well.