Hydrology and hydrogeology

Hydrology

The outlet of Mule Hole watershed is located at an elevation of 827 m. At Mule Hole, streams are temporary flowing for a few hours to a few days after the stormy events of the rainy season. Rainfall and runoff measured at the outlet of the Mule Hole watershed were respectively 431 mm and 1 mm in 2003 (measurements only from July 2003), 1216 mm and 59 mm in 2004, and 1434 mm and 181 mm in 2005.
During almost 3.5 hydrological years of monitoring from 7 July 2003 to 21 November 2006, the stream has flown only an equivalent period of 147 days, i.e. 12 % of the monitoring period or about 44 days a year. The stream is therefore highly ephemeral and flows only in response to specific heavy rainfall events.

Hydrologie bassin Mule Hole

Figure 8. Hydrology of the Mule Hole watershed

The outlet of Maddur watershed is located at an elevation of 878 m. The stream of Maddur is flowing the whole year at a very low discharge rate. This is probably due to human activities and water ponds which reduces the floods peaks during precipitation and contributes to baseflow during dry season.

Hydrologie Maddur F9

Figure 9. Hydrology of the Maddur watershed

Groundwater

Both experimental watersheds are located in gneissic hard-rocks composed of a shallow porous regolith reservoir and a deeper fractured reservoir.

For the Mule Hole watershed, the following ranges of thickness are estimated from borewell loggings and geophysical studies:

  • 0.2 to 6 meters for the soil, with a major part of the watershed (54 %) with less than 1.5 m thick (BOST 2005, BARBIERO ET AL 2007);
  • Few meters to more than 35 meters for the saprolite;
  • 5 to 25 meters for the weathered-fissured gneiss (DESCLOITRES ET AL 2007).

At Mule Hole, the water table is deep and quite below the stream level. This means that the aquifer is disconnected from the stream. Therefore, there is no contribution of groundwater to baseflow during dry season. This explains why the stream is ephemeral.
The high depth of the water table is probably due to low natural recharge in this watershed. The forest cover consumes high amount of water by evapotranspiration, which reduces the infiltration of water through the unsaturated zone.
The water table at Maddur is shallow in many parts of the watershed. The piezometry is locally perturbed by water pumping in borewells, dugwells, openwells and handpumps.