Chapter 2.2 : Impact of the forest on the hydrological cycle and chemical balance in tropical humid watershed (Mule Hole, India)

Jean-Christophe Maréchal (1) , Jean Riotte (2) , Laurent L. Ruiz (3, 2) , M. Sekhar (4, 2) , Jean-Jacques Braun (2)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 IFCWS - Indo-French Cell for Water Sciences
3 SAS - Sol Agro et hydrosystème Spatialisation
4 Department of Civil Engineering [Bangalore]

It is commonly accepted that forest plays a role in modifying the water cycle at the watershed scale. However, the impact of forest on aquifer recharge is still under discussion: some studies indicate that infiltration is facilitated under forest while other studies suggest a decrease in recharge. This paper presents an estimate of recharge rates to groundwater in a humid forested watershed in 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 a 3-years monitoring period (2003-2006). The direct and localised recharge resulting from rainfall over the entire watershed surface area is estimated at 45 mm/yr while the indirect recharge occurring from infiltration of the stream during flood events is estimated at 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, and corresponds to 1100 mm/yr on average. This unexpectedly high value for a deciduous forest is nevertheless in agreement with the forest worldwide relationship between rainfall and evapotranspiration. The water yield of the watershed is low with an average of 180 mm/yr and does not increase much if rainfall increases, due to the high demand for water from the forest. The high level of evapotranspiration from the forest cover contributes to a decrease in the recharge rate; consequently the water table is low. This is the reason why the stream is highly ephemeral there. Another impact is that the water table is recharge-controlled and not topography-controlled. As a result, the water table is not a subdued replica of the local topography. It results in groundwater flows below the existing weir which contributes for more than 90 % of the total chemical outputs, and for less than 10 % of surface runoff. More generally, the impact of forest cover on the water yield (based on empirical worldwide curves, and compared with grass cover) appears to range between 210 and 340 mm/yr according to the rainfall rate, and other sites specific parametres.

Domaines: Hydrologie

Jean-Christophe Maréchal, Jean Riotte, Laurent L. Ruiz, M. Sekhar, Jean-Jacques Braun. Chapter 2.2 : Impact of the forest on the hydrological cycle and chemical balance in tropical humid watershed (Mule Hole, India). Patrick Lachassagne; Michel Lafforgue. Forest and the Water Cycle: Quantity, Quality, Management, Cambridge Scholars Publishing, pp.72-100, 2016, 978-1-4438-8825-7. ⟨hal-02272551⟩