Module 1: Introduction

Lecture 1: Introduction to groundwater hydrology

A major component of precipitation that falls on the earth surface eventually enters into the ground by the process of infiltration. The infiltrated water is stored in the pores of the underground soil strata. The water which is stored in the pores of the soil strata is known as groundwater. Therefore, the groundwater may be defined as all the water present below the earth surface and the groundwater hydrology is defined as the science of occurrence, distribution and movement of water below the earth surface. In this section of hydrology, we generally deal with the water that is stored in the voids of the soil below the earth surface and also their interaction with the water that are present above the earth surface. When all the pores of a soil matrix are filled with water, we call that the soil is in the state of saturation. We used the term porosity to quantify the amount of voids space available in a soil matrix. Porosity is defined as the ratio of volume of voids to the total volume of the soil matrix. The porosity is expressed as,

(1.1)

Where Vv is the volume of void and VT is the total volume of soil solid.

Water entered into the earth surface also moves from one place to another through the pores of the underground strata. This is known as subsurface flow. The subsurface flow is three dimensional and can be estimated using Darcy's law. Detail discussion about Darcy's law is presented in Module-2 of this course. The subsurface water also comes out to the earth surface as spring, river base flow, etc . and also goes back to the atmosphere by the process of evapo-

Fig.1.1 Hydro-geological cycle

transpiration. Thus this is a continuous process of recycling of water from the atmosphere down to the soil below the earth surface and back to the atmosphere again. This cycle is called hydro-geological cycle (Fig. 1.1). In this process of recycling, the water molecules spent some time under the earth surface. The average length of time spent by the water molecules under the earth surface is known as residence time of groundwater . The residence time can be calculated as,

 

(1.2)

Where, tr is the residence time for groundwater, Vgr is the volume of groundwater and qav is the inflow or outflow at steady rate.