Assessing diffuse and concentrated recharge in average- and dry-rainfall year in a semiarid carbonate sloping aquifer, a preliminary report

The chloride mass balance (CMB) method was applied in the unsaturated zone to estimate potential recharge (RT) by rainfall in two small catchments of southern mid-to-high slope of Sierra de Gador carbonate aquifer (SE Spain), in the average hydrological year 2003-04 and the unusually dry 2004-05. Unknown fractions of diffuse (RD) and concentrated recharge (RC) into RT were firstly evaluated to fit average and lower RT thresholds for modeling further long-term recharge. Daily rainfall and actual evapotranspiration (AET) from the Eddy Covariance (EC) technique provided yearly RT of 189 mm year-1 in 2003-04 and 8 mm year-1 in 2004-05. Difference on RT among CMB method and EC technique is ~5% in year 2003-04, increasing to ~25% in year 2004-05. RD from soil-moisture field data was 65 mm year-1 (~34% of RT) in 2003-04 and negligible in 2004-05. The use of complementary methods checks low RT rates and identifies RD and RC fractions mixing in the unsaturated zone before RT reaches the regional water-table top. The results can explain long-term variations on groundwater storage and hydrochemistry and validate lower RT ~ RC yearly rates from the EC technique.