Based on geographical characteristics, Thailand can be divided into 25 river basins. The total volume of water from rainfall in all river basins is estimated at more than 800,000 million m3, of which 75% is lost through evaporation, evapotranspiration and infiltration, and the remaining 25% constitutes the runoff that flows into the rivers and streams. Of these water resources, groundwater is important as it supplies 20% of public water supply and 75% of domestic water. The groundwater system is mainly recharged by rainfall of about 40,000 million m3 and seepage from rivers. It was estimated from previous hydrological balance studies that about 12.5 to 18% of rainfall reaches aquifers.