# Linking Singular Spectrum Analysis and Machine Learning for Monthly Rainfall Forecasting
## 2 Study Sites and Data Collection  
Taiwan is located in the Western Pacific Ocean. The East Asian Monsoon characterizes its climate. The main sources of rainfall are heavy rainfall systems (Mei-Yu front) and typhoon events. The average rainfall for Taiwan is 2510 mm per year, spatially and temporally unequally distributed. About 70 percent of the annual rainfall occurs in the rainy season (May-October), and the dry season occurs from November to April.  
This present study selected the Shihmen and Deji reservoir watersheds as the study areas, as shown in Figure 1. The study areas were chosen as the research interest for rainfall forecasting because they are critical to water resources in Taiwan. The watershed area of Shihmen reservoir is situated in the Danshuei river basin, in the north of Taiwan. The storage capacity in this reservoir is around 309 million m\({}^{3}\). The watershed's area is 763 km\({}^{2}\) and the reservoir's elevation is 250 m above sea level. The annual average rainfall is about 2250 mm. In 1964, Shihmen reservoir was constructed as a multipurpose water supply storage tank for water irrigation and was used to produce hydroelectricity, prevent flooding, and for recreation [71]. Deji reservoir is situated in the middle of northern Taiwan. It has a capacity of about 266 million m\({}^{3}\). The watershed covers an area of 1411 m above sea level. The annual average rainfall is about 2450 mm. Like Shihmen reservoir, Deji reservoir has been used since 1974 to supply municipal water, to produce hydropower, for recreation, and to prevent flooding.  
Data were collected from Taiwan Water Resource Bureau, which includes the monthly rainfall data during 1958-2018 for each rain gauge in the Shihmen watershed and during 1981-2017 for each rain gauge in the Deji Reservoir watershed. For each reservoir watershed, the first 70% of the entire data collection was used for training, and the remaining 30% for validation. Tables 1 and 2 present geographical information on the two studied watersheds. The areal monthly rainfalls for each reservoir watershed were calculated using the Thiessen polygon method to calculate areas in relationship to  
To understand the characteristics of the monthly rainfall series in Deji and Shihmen Reservoir watersheds, the descriptive statistics were used for exploring their rainfall characteristics. The statistical behavior of any hydrological series can be described based on certain parameters such as mean, standard deviation, kurtosis, skewness, and coefficient of variation. In the present study, the mean, standard deviation, skewness, and kurtosis are used to describe the variability of monthly rainfall. Using the observed data, the descriptive statistics of rainfall data were estimated. Table 3 shows the statistical parameters of the monthly rainfall series for the two reservoir watersheds. The standard deviation value indicates that the precipitation pattern varies greatly. The skewness values show that rainfall patterns do not comply with a normal distribution. The skewness and kurtosis indicate differences in their statistical distribution.