# Assessing Groundwater Level with a Unified Seasonal Outlook and Hydrological Modeling Projection
## 4 Results and Discussion
### Groundwater Table According to Seasonal Rainfall Outlooks
The above results show insufficient rainfall during the spring and Mei-yu season of 2018. Mei-yu is a weather and climate phenomenon in subtropical Asia. It is caused by the Mei-yu front and continuous rainfall for nearly 2 months during late spring and early summer in Taiwan (May and June). In fact, on average, the historical spring rainfall values from February to April and Mei-yu rainfall from May to June are approximately 527.0 mm and 559.0 mm, respectively, for our study site. However, the amounts of spring rainfall at the Guanxi and Xinpu stations were 290.0 mm and 206.0 mm, respectively, in 2018, which were much lower than the average. In addition, the amounts of Mei-yu rainfall were 262.0 mm and 162.0 mm for the Guanxi and Xinpu stations, respectively, which were also much lower than the average. According to the above results, the forecasted groundwater level was overestimated before the high-water-level period, as shown in Figure 6(a)-e. Then, when the typhoon season started, the total annual rainfall amounts in 2018 at the Guanxi and Xinpu stations were 2031 and 1295 mm, respectively. In 2018, the amount of rainfall from July to September at the Guanxi station was approximately 1040 mm, which was more than half of the total annual rainfall amount. At the Xinpu station, the rainfall amount from July to September in year 2018 was approximately 522 mm, which was near the average of historical records. However, with the lower accuracy of rainfall forecast after October, the lack of precipitation was the reason for the decreasing groundwater levels from September to December, as shown in Figure 6(j). It can be seen that the predicted rainfall is indeed the crucial factor affecting simulated groundwater levels. When rainfall predictions agreed with rain gauge observation, the hydrograph patterns in the groundwater simulations were similar to the measurement of seasonal groundwater outlooks. In the 2018 seasonal forecast, the groundwater level was overestimated because the amount of monthly rainfall was far lower than the average of the historical record in some dry season months, as shown in Figure 6(c)-j. However, the amount of rainfall returned to normal values during the wet seasons, which contributed to the agreement between the seasonal forecast and observation, as shown in Figure 6(f)-h. It can be clearly seen that the precipitation overestimation of seasonal forecasting during the dry seasons was the major cause for the above conditions. Although assessing groundwater level with a unified seasonal outlook and hydrological modelling projection is feasible, it should be noted that the phenomenon of groundwater level tends to be overestimated during the dry season. In cases of a low groundwater table, unregistered extraction by private wells is an issue that must be considered, as this might affect the accuracy of the results. In fact, most industrial users need to register for water use in Taiwan. Unregistered water use, mainly for agricultural or aqua-culture purposes, can result in over-extraction from wells during the dry periods. This is also a major cause of land subsidence along the southwest coast of Taiwan. Over-extraction coupled with unregistered water use can cause significant discrepancies when the groundwater level is low during dry seasons. In fact, in terms of consequentialism, the simulation result for 2018 in this study was not quite adequate enough. However, this established approach may be improved in the future through many other experiments. These may involve addressing the acceptable error range for our approach, e.g., by widening the range of 25 th-75 th percentiles, utilizing safety factors, and conducting groundwater level management.