计算SPI标准化降雨指数，按月为单位，最少30年数据(Calculate SPI standardized rainfall index, monthly, minimum 30 years data)

MATLAB绘制干旱指数面积图，标准化降水指数（Standardized Precipitation Index，简称SPI）采用加利福尼亚大学欧文分校的code计算。(Drawing drought index area map with MATLAB. The standardized precipitation index (SPI) is calculated using the code of the University of California, Irvine.)