Israel is dependent on rainfall. The connection to Our Creator and the land is direct and clear. During times of shortage of rain it is imperative that we recognize and turn to G-d and ask for the blessing of rain. The following prayer is inserted during the Shemona Esre in the 16th (Sama Kolanu) blessing during the winter months of a year that does not have a sufficient amount of rain.
The year 5771- 2010/2011 has been declared as year that the blessing should be said. G-d willing we should be blessed with rain and can at that time stop saying the prayer. As of Pesech we no longer add this prayer.