Application of Groundwater Fluctuation Method for Estimating Recharge in the Choushui River Alluvial Fan

The Choushui River alluvial fan is one of the most important groundwater zones in Taiwan. For decades, the groundwater observation network in the fan has collected a large amount of data on groundwater. In this study, researchers integrated Thiessen’s Polygon Method with 30 observed wells installed on the top aquifer to estimate the quantities of recharge, pumping and horizontal exchange. Jiang et. al. (2006) indicated that the level of groundwater level decreases during periods of drought due to artificial pumping. Therefore, the average rate of pumping was calculated by multiplying the gradient of the decreasing groundwater level and the storage coefficient (for a confined aquifer) or specific yield (for an unconfined aquifer). The levels of groundwater observed from two neighboring wells and Darcy’s Law were used to estimate the quantity of horizontal exchange. The increase of water within the same period was calculated by multiplying the increased level of groundwater after a rainy period by the storage coefficient or specific yield. The quantity of recharge brought about by the rainy period was equal to the sum of the increased level and the quantity of pumping within the same period. In this study, the 30 wells included 16 confined wells and 14 unconfined wells. Because field testing of storage coefficient or specific yield is more complex and costly compared to testing of transmissibility, four tests of storage coefficient and six tests of specific yield were conducted in the study area. Storage coefficients for the remaining 12 confined wells were assigned the average value of the four storage coefficients produced by the test. Likewise, values of specific yield for the remaining unconfined wells were assigned the value averaged from the six tests. Dongguang Station is one of the unconfined wells whose value was derived from testing. The value of its specific yield, 0.289, was the highest among unconfined wells. Because the well is located at a distal area of the fan, its high porosity might be a characteristic of locale and might have been overestimated. Therefore, the values for Dongguang Station weres set as 0.289 and 0.2 for Cases 1 and 2, respectively. In Case 1, the annual quantity of recharge for the entire fan ranged from 1.3 to 1.9 billion tons and the cumulative quantity of recharge from 1998 to 2003 was approximately 10 billion tons. In Case 2, the annual quantity of recharge ranged from 1.0 to 1.6 billion tons and the cumulative quantity of recharge was approximately 8.09 billion tons. Comparing the results the quantities of recharge in Case 2 were determined to be more similar to Jiang’s results, so further analysis of Case 2 was conducted. After ranking the recharge quantities of each well, researchers calculated that the Dongguang Station, with a cumulative recharge quantity of 4.3 billion tons, provided 53.44% of the quantity of recharge in the entire fan. Calculations using Thiessen’s Polygon Method showed that the control area of Dongguang Station was significantly larger than the control area of other unconfined stations. Due to a large control area and high specific yield, the quantity of recharge for Dongguang station was significantly greater.