Indirect and direct recharges in a tropical forested watershed: Mule Hole, India

Abstract

International audienceIt is commonly accepted that forest plays role to modify the water cycle at the watershed scale. However, the impact of forest on aquifer recharge is still discussed: some studies indicate that infiltration is facilitated under forest while other studies suggest a decrease of recharge. This paper presents an estimate of recharge rates to groundwater in a humid forested watershed of India. Recharge estimates are based on the joint use of several methods: chloride mass balance, water table fluctuation, geophysics, groundwater chemistry and flow analysis. Two components of the recharge (direct and indirect) are estimated over 3 years of monitoring (2003-2006). The direct and localized recharges resulting from rainfall over the entire watershed surface area is estimated to 45 mm/yr while the indirect recharge occurring from the stream during flood events is estimated to 30 mm/yr for a two km-long stream. Calculated recharge rates, rainfall and runoff measurements are then combined in a water budget to estimate yearly evapotranspiration which ranges from 80 to 90% of the rainfall, i.e. 1050 mm/y as an average. This unexpected high value for a deciduous forest is nevertheless in agreement with the forest worldwide relationship between rainfall and evapotranspiration. The large evapotranspiration form the forest cover contributes to decrease the recharge rate which leads to a lowering of the water table. This is the reason why the stream is highly ephemeral

Elemental weathering fluxes and saprolite production rate in a Central African lateritic terrain (Nsimi, South Cameroon)

“…Thanks to the thorough geochemical, mineralogical and hydrological studies and using the CMB approach (see previous section), we were able to assess the water recharge (R hill ) and the chemical weathering fluxes in both ridge top profiles (Soumya et al, 2009;Maréchal et al, 2011a,b). The Mule Hole SEW presents the contemporary weathering conditions for an immature weathering cover under sub-humid climate with one rainy season of limited extension (three months) per hydrological year Marechal et al, 2009;Soumya et al, 2009;Ruiz et al, 2010). Over a period of 40 years, the MAR is 1280 ± 380 mm/yr with a coefficient of variation of 30% (Javeed et al, 2009;Javeed, 2010).…”

“…Over a period of 40 years, the MAR is 1280 ± 380 mm/yr with a coefficient of variation of 30% (Javeed et al, 2009;Javeed, 2010). The dry deciduous forest vegetation induces an elevated AET of 1100 mm/yr (85% of MAR) (Marechal et al, 2009;Ruiz et al, 2010). The recharge (R hill ) is 45 mm/yr out of which 100% of the solutes are discharged through the groundwater as underflow from the watershed.…”

“…In order to refine the elemental transfers in lateritic covers developed on silicate basement and to get deeper insight into (i) the contemporary saprolite production rates and (ii) the combined effect of precipitation and evapotranspiration on the aggressiveness of the draining solutions, a first attempt has been carried out both on the studied mature profile of the Nsimi SEW and with the immature ridge top weathering profile from the Mule Hole SEW (South India), developed on a similar granodioritic basement Braun et al, 2009;Marechal et al, 2009;Ruiz et al, 2010;Violette et al, 2010;Soumya et al, 2011;Maré-chal et al, 2011b). Finally, taking into account the results of the present study and the known history of the South Cameroon landscape, we infer the average saprolite production rate on a long term basis.…”

Aquifer Recharge Estimation through Atmospheric Chloride Mass Balance at Las Cañadas Caldera, Tenerife, Canary Islands, Spain

“…For a long enough period (e.g., one hydrological year) under steady-state conditions, in which the temporal storage of chloride in the soil and vadose zones can be assumed negligible, the CMB method is based on the steady balance of chloride mass fluxes from (1) atmospheric bulk deposition, which includes both chloride dissolved in precipitation and from atmospheric dust and marine aerosols [18]; (2) surface runoff leaving the area [19]; and (3) recharge water just arriving to the water-table [15,17]. The CMB method can be expressed as:…”

“…The atmospheric chloride mass balance (CMB) method is of special interest for average net aquifer recharge at different spatiotemporal scales because it does not include E in the formulae thus reducing the overall uncertainty of estimates [8,9,14,17]. For average net aquifer recharge, the CMB method is based on the steady balance of chloride mass fluxes from (1) atmospheric bulk deposition [18]; (2) surface runoff leaving the area [19]; and (3) recharge water just arriving to the water-table [15,17]. In active volcanic islands having steep topography some additional processes must be considered, such as (1) the possibility of mixing chloride flow mass rates produced by recharge rates at different elevation [20]; (2) the variable rainfall-runoff partitioning and recharge mechanisms [14]; (3) the potential contribution of non-atmospheric chloride to groundwater [21]; and (4) the potential storage of chloride in the soil and vadose zones [8,17].…”