Old water contribution to streamflow is studied by the means of a simple groundwater model. The principal aim is not to develop a tool for applications but rather to understand some of the dynamics of old and new water after a rainfall event by using a simple model with a minimum set of variables. Event water is traced by solute transport analysis. Adopting a simple rainfall pulse of finite duration, the method identifies two key dimensionless parameters ruling the old water contribution to streamflow: a dimensionless rainfall time δ, ratio between the rain duration and the characteristic response time of the aquifer, and the ratio ρ between rainfall volume and the water volume present in the aquifer at the beginning of rainfall. It was found that the ρ parameter is crucial in the old water contribution to a rainfall event; less so are the temporal dynamics of rainfall, expressed through δ. For typical values of ρ it is expected that old water contributes more than 50% of the total streamflow. These findings suggest that the groundwater dynamics may have a strong influence on the way that the hillslope responds to the rainfall input, and hence the partition between new (precipitation) and old (pre-event) waters.