Groundwater resources management for public water supply in Changhua County, Taiwan - 1. Model analysis

In order to mitigate the subsidence problem in Changhua County, Taiwan, it is important to seek water resources for substituting the groundwater pumping. Niaozueitan Artificial Lake is mainly planned to supply the surface water for public use in Changhua County in 2031 to decrease the groundwater use and mitigate the subsidence problem. Under the premise of stable supply for public water use of the Taiwan Water Corporation, the pumping potential of the water purification stations was estimated to sort the stop of pumping for the stations. The results show that the system is the best in Changhua pumping system and the worst in Erlin pumping system. Based on different water supply scenarios with the planned stops and decreases of groundwater pumping, groundwater level is simulated by MODFLOW to recognize the situations of groundwater level uplifted. Under the situation of water supply of 300,000 and 250,000 tons/day by Niaozueitan Artificial Lake, the 2061 cumulative uplift of four aquifers for two scenarios were 15.00 and 11.45 meters, respectively, and the average uplift amount for each aquifer is similar. The large uplift areas present around the foothills of Pagua Mountain and declines from those areas to the fantail. This study develops a groundwater management system by considering the managed groundwater level for the water purification stations of Taiwan Water Corporation, in order to evaluate the groundwater level position index (P) and groundwater management index (GMI) in Changhua County, Taiwan. For the unchanged pumping condition, the GMI value shows a restriction condition after 2041. Under the scenarios of supplying water resources of 300,000 and 250,000 tons/day from Niaozueitan Artificial Lake, the GMI values show a restriction condition after 2052 and 2061, respectively. Groundwater can still be used but some strategies of groundwater management should be involved to avoid the lack of groundwater resources in the future. The management principles for public groundwater resources are proposed. The items and steps should include: (a) developing the monitoring system, (b) constructing the managed groundwater level, (c) evaluating the groundwater pumping potential, (d) investigating the management scenarios, (e) calculating the groundwater position index, (f) estimating the groundwater management index, and (g) proposing the management strategies. The study results can be used for the future water resources management and plan in Changhua County, Taiwan.