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

Summary

It 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 2 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 from 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.

Introduction

The correct estimate of natural recharge is a key element for the good management of groundwater resources. This important component of the hydrological cycle is dependant on various physical characteristics of the watershed: soil properties, topography, land-use and vegetation cover among the most significant. The impact of forests on the water yield (runoff + recharge) of a watershed is a question of interest and is still debated. Vast majority of studies indicate decreased runoff from areas under forests as compared with areas under shorter land covers (Jewitt, 2005). References on effects of forestation on natural recharge are less abundant specially in fractured crystalline aquifers. Preliminary studies in Australia have shown that clearing of natural woodland or forest has resulted in the increase of recharge and rise of the water tables (Williamson, 1990). Sharma et al. (1987) arrived to the same conclusion after forest clearing in lateritic environment. On the contrary, some studies show there is usually less soil compaction in forests and, depending on soil type, soil structure may improve causing more rainfall to infiltrate (Bruijnzeel, 2004, Ilstedt et al., 2007).

The total recharge to groundwater is constituted by three main components. Direct recharge refers to diffuse recharge, such as from precipitation, whereas indirect recharge results from the percolation of a part of runoff water through the stream beds and localized recharge refers to concentrated recharge by preferential flow through cracks, joints and fissures (Lerner et al., 1990). The relative proportions of these components fluctuate according to climatic conditions, geomorphology and geology. In arid climatic regions, the most important mechanism of groundwater recharge is considered to be infiltration from floods through the alluvial beds of ephemeral streams in wadi channels (Sorman et al., 1997). Except in karst hydrology, up to now, there are only few examples in the literature of the assessment of indirect recharge from a stream (Sorman et al., 1997). The assessment and regional consequences of indirect recharge constitute two challenges for the future research on recharge (de Vries and Simmers, 2002). This is specially challenging when this component highly fluctuates with time and space in the case of an ephemeral stream on a heterogeneous aquifer.

The aims of this study are (i) to identify significant recharge processes at the watershed scale in a forested area and (ii) to assess the direct, localized and indirect transient recharge rates. In order to reach these objectives, a combination of several approaches focusing on the saturated zone – chloride mass balance, water table fluctuation, geophysical investigations, pumping tests and groundwater flow modeling – is used. This study has been conducted on an experimental watershed in humid conditions located on the Indian crystalline basement. The watershed is the object of an integrated study including soil dynamics and erosion process (Barbiero et al., 2007), geophysical investigations (Descloitres et al., 2008) and hydrological and biogeochemical cycles (Braun et al., in press).