Water balance modelling in a tropical watershed under deciduous forest (Mule Hole, India): Regolith matric storage buffers the groundwater recharge process

Abstract

International audienceAccurate estimations of water balance are needed in semi-arid and sub-humid tropical regions, where water resources are scarce compared to water demand. Evapotranspiration plays a major role in this context, and the difficulty to quantify it precisely leads to major uncertainties in the groundwater recharge assessment, especially in forested catchments. In this paper, we propose to assess the importance of deep unsaturated regolith and water uptake by deep tree roots on the groundwater recharge process by using a lumped conceptual model (COMFORT). The model is calibrated using a 5 year hydrological monitoring of an experimental watershed under dry deciduous forest in South India (Mule Hole watershed). The model was able to simulate the stream discharge as well as the contrasted behaviour of groundwater table along the hillslope. Water balance simulated for a 32 year climatic time series displayed a large year-to-year variability, with alternance of dry and wet phases with a time period of approximately 14 years. On an average, input by the rainfall was 1090 mm/year and the evapotranspiration was about 900 mm/year out of which 100 mm/year was uptake from the deep saprolite horizons. The stream flow was 100 mm/year while the groundwater underflow was 80 mm/year. The simulation results suggest that (i) deciduous trees can uptake a significant amount of water from the deep regolith, (ii) this uptake, combined with the spatial variability of regolith depth, can account for the variable lag time between drainage events and groundwater rise observed for the different piezometers and (iii) water table response to recharge is buffered due to the long vertical travel time through the deep vadose zone, which constitutes a major water reservoir. This study stresses the importance of long term observations for the understanding of hydrological processes in tropical forested ecosystems

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.…”

Formation and preservation of pedogenic carbonates in South India, links with paleo-monsoon and pedological conditions: Clues from Sr isotopes, U–Th series and REEs

“…The pristine watershed of Mule Hole (11°72 0 N-76°42 0 E, 4.1 km 2 ) with a MAR of 1280 mm and an estimated potential evapotranspiration (PET) of 1030 mm (water years from 2004 to 2006, Maréchal et al, 2009;Ruiz et al, 2010) is covered by a dry deciduous forest. The partially cultivated watershed of Maddur (11°78 0 N-77°56 0 E, 7.2 km 2 ) with a MAR of 830 mm and an estimated PET of 1250 mm (water years from 2005 to 2007) is covered by rainfed crops and coconut groves in the valley and scrubs and patches of deciduous forest on hilltops.…”

Ferrolysis induced soil transformation by natural drainage in Vertisols of sub-humid South India

“…The study involved the assessment of chemical weathering and regolith thickness ), determination of surface and groundwater balance, recharge and modelling, (Legchenko et al, 2006;Descloitres et al, 2008, Maréchal et al, 2009Ruiz et al, 2010;Parate et al, submitted), physical erosion calculations (Barbiero et al, 2007) and, the appraisal of soil cover dynamics. At the watershed scale, estimates of element exportation are only rarely consistent with long term weathering rate assessments.…”