Stability of rain-rate distribution: how many years to measure?

A relative root mean square error (RMSE) between the cumulative distribution of rain-rates derived from several years´ (x years) measurement and a long-term (N years, N>x) measurement is given. Long-term (63 years) data from the Czech Republic are used. It is concluded that a period of 10-20 years´ measurement is required to reduce this relative rain-rate RMSE to under 7%. An applicable example of a relative error computation (caused by the microwave link rain attenuation estimation from only a few years´ measurement) is also given.