Cautions in Using Chloride Mass Balance Method for Groundwater Recharge Estimation in Coastal Areas with Vegetation Changes

Understanding and quantification of changes in groundwater recharge after surface vegetation alteration in coastal areas are important not only for water resource management, but also for land-use and land-cover management. Application of simple chloride mass balance (CMB) method faces three major difficulties in especially mountainous coastal areas. First, to identify the status of chloride equilibrium is premise to apply CMB in the catchments with vegetation change. Second, large spatial variation in the bulk chloride deposition commonly occurs in these areas. Third, the chloride deposition can be further modified by canopies and may experience significant change in total amount of deposition after vegetation clearance. The study is based at the Mount Lofty Ranges of South Australia where historical vegetation clearance has dramatically altered surface conditions. A residual kriging method (ASOADeK) was developed to construct 1 km × 1 km resolution bulk chloride deposition (BCD) of the study area. With this map, catchment chloride O/I value (the ratios of stream chloride output (O) over atmospheric chloride input (I)) was examined for 11 selected catchments of various sizes and climatic and hydrologic conditions. The results indicated that chloride O/I values was well correlated to annual precipitation of the catchment. Based on this correlation, it was estimated that a catchment of annual precipitation around and above 900 mm has reached new post-clearance chloride equilibrium condition. Another study on throughfall was carried out in Kuitpo forest, Mount Lofty Ranges to examine the canopy's role in modifying chloride atmospheric deposition to the ground. It showed that throughfall chloride flux was much higher than the bulk deposition in the adjacent open field, implying that underestimation of groundwater recharge is likely to occur if only use the traditional chloride deposition measurement in the open field and not considering the canopy’s role in enhancing chloride deposition.