Comparison of water table fluctuation and chloride mass balance methods for recharge estimation in a tropical rainforest climate: a case study from Kelantan River catchment, Malaysia

Estimated groundwater recharge for selected locations in northeast Peninsular Malaysia (North Kelantan River catchment) was determined using the water table fluctuation (WTF) and chloride mass balance (CMB) methods. The WTF method recharge estimates were compared to the CMB method estimates to see if results are similar and the methods can be applied in a humid, tropical region, such as Malaysia. Effective specific yields of 0.18 and 0.16 were obtained for sites which are much lower than values in the literature for use with the WTF method. The WTF gives mean recharge values of 447 and 319 mm/year at PC61 and Wakaf Bharu (WB), respectively. The difference between sites may be attributed to the difference in lithology, whereby less water table fluctuation is observed for finer-grained lower permeability soil or sediment and higher water table fluctuation observed for more granular larger grain soil or sediment. For the site with lower permeability, the time lag between the storm events and water level rise in a well is longer and water does not reach the water table for small storms. Recharge from chloride profile study was estimated through measurement of enrichment of chloride concentration in soil. Rain-fed groundwater direct recharge ranged from 263.3 to 627.7 mm/year in WB and averaged 691.8  mm/year for Pengkalan Chepa. Recharge estimates from unsaturated zone chloride strongly correlate with soil texture, generally and is greater in the coastal area due to a higher percentage of sand content. Parts of the study area located toward the interior of the peninsula showed lower recharge due to the presence of fine-grained sediment. Comparison of methods from different sites indicates that lithologic setting has a significant role in controlling natural recharge rate. The inconsistency observed between recharge value estimated by the WTF and CMB method may in part be due to adsorption/desorption process and ion exchange in the unsaturated soil, which affects the initial assumption that chloride is a conservative tracer.