Classification of ground water There are various classifications of groundwater given by different researchers. However, as per the most popular classification given by Meinzer (1923), the groundwater has been divided mainly in two groups: interstitial water and internal water. The interstitial water is again subdivided into two divisions. They are vadose water present in the zone of aeration and groundwater present in the zone of saturation. The vadose water is further subdivided into three zones, i.e., soil water zone, intermediate zone and capillary zone. Fig. 1.2 shows the classification of groundwater. The soil water zone is adjacent to the ground surface. The intermediate zone is between the lower edge of the soil water zone and the upper edge of the capillary zone. The capillary zone extends from the bottom edge of the intermediate zone to the upper edge of the saturated zone. The thickness of the capillary zone depends on the properties of the soil and also on the homogeneity of the soil. The depth of capillary zone is varying from few centimeters to few meters. In capillary zone, all the pores are field up with water. However, we cannot draw water by inserting a well up to that depth. This is because of the negative pressure developed at this zone due to surface tension effect. Groundwater zone starts from the bottom edge of the capillary zone. In this zone, all the pores of the soil matrix are filled with water. This zone is also known as zone of saturation. The top surface of the zone of saturation or groundwater is known as phreatic surface. This phreatic surface is also known as water table.

Fig. 1.2 Classification of groundwater

The degree of saturation for the soil below the water table is equal to 1, i.e. the soil is fully saturated. As a groundwater hydrologist, we are primarily interested for the water below the groundwater table, i.e. the water available in the zone of saturation . For the soil above the water table, the degree of saturation of the soil is varying between 0 and 1 . However, the degree of saturation will never be 0 due of the presence of hygroscopic water. The hygroscopic water is the water that held tightly on the surface of the soil colloidal particle. Hygroscopic water can be removed from the soil by oven drying. Fig. 1.3 shows the moisture distribution in soil column.