The 21st century is often called the century of water. Water is an essential element of the Earths environment and is indispensable for our life and economic activities. Many places in the world now face water problems, such as water shortages and floods, which can cause food shortages, epidemic diseases, and so on. In addition to these problems, global warming and climate change affect the global water cycle and result in abnormal weather, such as frequent heavy rains and droughts. In order to solve these problems, we urgently need to accurately determine the rainfall distribution, which is the input to water resources, and to improve the techniques of predicting and preparing for abnormal weather.