**1. Introduction**

The annual average precipitation in Taiwan is 2500 mm, 2.6 times higher than the global average values. In spite of the high precipitation per unit area, the annual precipitation per person is only 1/7 of the global average, due to both small land area and large population [1]. Analysis of environmental sustainability indicators revealed that Taiwan ranked 18th among 146 countries experiencing water shortages, with the average available water per person in

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Taiwan being 1740 m3 /year [2]. Climate changes and increasing temperature experienced in recent decades caused the precipitation in Taiwan to increase during the rainy season, while decreasing in the dry season, resulting in a drier dry season and a wetter wet season [3]. Consequently, stream flow in both dry and wet seasons varied significantly. Thus, it is urgent to assess and discuss groundwater recharge characteristics, toward effective water resource management in Taiwan.

When analyzing the hydrogeological model and groundwater system of a region, the study on recharging groundwater through precipitation is a very important but complex issue. Meteorological factors (e.g., intensity and delay of precipitation, temperature, humidity, and wind speed), thickness of the soil layer, prevailing groundwater level, topographical condi‐ tions on the surface, vegetation cover, and land uses, all have direct relationships with the system and must be considered [4–10]. It is very easy to measure precipitation and run‐off amounts when analyzing water balance, but it is very difficult to quantify the recharging process. Its evaluation requires not only precipitation data, but also other factors, such as prevailing climatic conditions, soil type, soil moisture status, vegetation cover, and evapo‐ transpiration conditions [11, 12]. The infiltration of precipitation into the soil leads to the recharging process and is an important factor determining circulation of the groundwater system and its recharge volume.

Groundwater recharge can be quantitatively estimated using two methods. The first is the water balance model [13–16], which is applicable for humid regions; the second is for arid regions and involves the use of tensiometers, tracers, infiltrometers, and other instruments on site to observe the movement of water in the unsaturated zone, before estimating the groundwater recharge for that area [17–19]. However, it is generally more difficult to implement the second method because of high costs and the need for long‐term monitoring on site. The existing methods for estimating groundwater recharge at the regional level using the water balance model are further divided into two types: (i) precipitation, infiltration, run‐ off, evapotranspiration, and groundwater recharge are treated as components of an interrelated system, with the soil moisture status being that of an ever‐changing soil water balance model [7, 20, 21]; and (ii) the hydrograph of a stream flow is used to estimate its base flow, with the latter being treated as the groundwater recharge (based on the assumption that the heterogeneous hydrogeological conditions within the catchment area are ignored) [22–26].

In this study, the base flow estimation method was used to determine groundwater recharge and the evapotranspiration from the unsaturated zone was not considered. The calculation method to determine the effective recharge is simple, and it neither requires any complicated hydrogeological models, nor factors such as weather conditions and soil types. In this study, the calculation method was used to assess long‐term changes of groundwater recharge in Northern Taiwan, and the findings can serve as reference for the management of water resources.
