# Rainfall Standard of Disaster Prediction for Agricultural Droughs in S. Korea
## 2 Materials and Methods
### Local Rainfall Observatories and Rainfall Status  
Not only South Korean local governments but also other multiple departments run approximately 68 rainfall observatories and retain rainfall DB (database). The Korean Meteorological Administration (KMA) operates rainfall observatories, where they keep over a three-decade observational data and provide diverse reference points for designing or planning disaster reduction initiatives. 68 rainfall observatories operated by the Korea Meteorological Administration have more than 30 years of proven observation data. Local rainfall observatories are shown in Figure 3.  
Sixty-eight rainfall observatories are distributed throughout 17 municipalities of interest to this study. The KMA-led rainfall observatories collected the rainfall data, on a daily basis, from 1965 to 2018. The branch numbers and names of the 68 observatories are outlined in Appendix A Table A1. Depending on the size of the municipalities, 1-19 rainfall observatories are in operation, where they calculate the rainfall weighted based on the area ratios of the [PERSON]'s polygon, to get the average rainfall. The weight of the [PERSON]'s polygon was calculated as the ratio of the area of rainfall observation stations to municipality in the land area. Appendix A Table A2 shows the weight of each municipality by rainfall observation station. From 1965 to 2018, the amount of rainfall in South Korea averaged 1271 mm (min 882 mm-max 1860 mm). The non-linear average annual rainfall curve repeatedly rises above and falls below the average with minimum and maximum rainfall recorded in 1988 and 2003, respectively (Figure 4). The average annual rainfall in South Korea has been increasing and decreasing over a period of about 5 years. The average annual rainfall is small, and about 1000 mm, and in most cases, about 1600 mm. Figure 3: Local rainfall observatories and [PERSON]’s polygon. With the impact of North Pacific and Okhotsk sea air masses, monsoonal fronts are formed in South Korea from June to September, when 65% of the country's total rainfall occurs (Table 3). Due to such rainfall patterns, the country's October-March rainfall averaged 5% or less of its total annual rainfall. Therefore, the agricultural drought damage in South Korea occurs during the spring drought, from April to May, and during the fall drought, from October to November, based on the monsoon season. Setting Up Reference Points for Disaster-Prediction Rainfall Relevant to Agricultural Drought Damage  
In this study, we examined the damage from agricultural droughts that occurred in South Korea, from 1965 to 2018. The period or the time of year when the drought damage occurred and the affected area are described under Section 2.1. A total of 22 agricultural drought events occurred from May to September (Figure 5). Due to the effects of monsoonal fronts, 65% of Korea's average monthly rainfall is concentrated in June to September, on which agricultural drought season largely falls. The type of damage caused by agricultural drought has been repeated continuously for a certain period of time on an annual basis. Despite the monsoon lasting from June to September, the monsoonal season partially overlaps with the agricultural droughts. Therefore, short-term storm events fail to end the droughts, but above-average rainfall has the capacity to end the droughts. Thus, in this study, we attempted to set up the reference points for disaster-prevention rainfall relevant to the beginning of agricultural droughts and for disaster-reduction rainfall relevant to the end of droughts. Figure 4: Rainfall trend by year (1965–2018).