Subsurface Movement of Water (Part - 1) - Civil Engineering (CE) PDF Download

Introduction 

In the previous lectures of this module, we talked about the hydrologic cycle, which is a continuous process of transformation of water in the form of water vapour as it evaporates from land and ocean, drifts away to clouds and condenses to fall as rain.  Of the rain falling over the land surface, a part of it infiltrates into the soil and the balance flows down as surface runoff.  From the point of view of water resources engineering, the surface water forms a direct source which is utilized for a variety of purposes.  However, most of the water that infiltrates into the soil travels down to recharge the vast ground water stored at a depth within the earth.   In fact, the ground water reserve is actually a huge source of fresh water and is many times that of surface water.  Such large water reserves remain mostly untapped though locally or regionally, the withdrawal may be high. Actually, as a result of excess withdrawal of ground water in many places of India (and also of the world), a number of problems have arisen.  Unless the water resources engineer is aware of the consequent damages, this type of situation would lead to irreversible change in the quality and quantity of subsurface water which likely to affect our future generations.

In this lecture it is proposed to study how the water that infiltrates into the soil and the physics behind the phenomena.  We have deliberately separated the study of subsurface movement of water from that of surface flow, as discussed in the earlier lectures, because of the fact that the scale of movement of these two types of flows can vary by an order of magnitude 10 to more than 1000!  This would be clear from Figure 1. 

FIGURE 1. Surafce and sub-surface movement of water

Subsurface water and the soil – rock profile 

Figures 2 and 3 show two examples of underground soil–rock profiles and their relations with subsurface water that may exist both as confined and unconfined ground water reserves. 

FIGURE 2. Sub-soil water reserve without any confining layer of soil

FIGURE 3. Sub-soil water reserve as unconfined as well as confined aquifers

In fact, water is present in the pores of soils and fissures of rock up to a depth beyond which there is solid rock with no gaps which can store water.  Although water is present in the pores of the soil and permeable rocks, there is difference between that stored above the water table and below it.  The soil above the water table has only part of the voids filled up with water molecules whereas the soil below is completely saturated. 

If we look more closely at the upper layers of the soil rock system, we find that it is only the change in moisture content that separates the unsaturated portion and the saturated portions of the soil.  Figure 4 shows a section through a soil – rock profile and corresponding graph showing the degree of saturation.  Except the portion of the soil storing groundwater the remaining is unsaturated. 

FIGURE 4. Changes in the degree of saturation for different zones of soil

It may be noted that even in the driest climate, the degree of saturation in the unsaturated zone would not be zero as water clings to the soil particles by surface tension. 

Some of the definitions related to subsurface water are as follows:

• Soil water:  The water stored in the upper layers of the soil from the ground surface up to the extent of roots of plants
• Vadose water:  That stored below in the region between soil water zone and the capillary fringe.  It is a link between water infiltrating from the ground surface and moving down to the saturated layer of ground water
• Capillary water:  That which has risen from the saturated ground water region due to capillary action.  Naturally, the pressure here would be less than atmospheric. 
• Ground water:  This is the water in the fully saturated zone.  Pressure of water here would be more than atmospheric.
• Water table:  An imaginary surface within ground below which all the voids of the soil or permeable rock are completely filled with water.  Below this imaginary surface, the pore water pressure is atmospheric. As one moves downwards from the water table, the pressure increases according to the hydrostatic law.  Above the water table, the voids of soil/porous rock are only partially saturated with water clinging to the surface of the solids by surface tension.  Hence, the pressure here is sub-atmosphere. 

Water pressure in unsaturated zone

In literature, the term ‘ground water flow’ is used generally to describe the flow of water in the saturated portion of soil or fractured bedrock.  No doubt it is important from the point of extraction of water from the zone using wells, etc.  But the unsaturated zone, too, is important because of the following reasons:

• The water in the unsaturated zone is the source of moisture for vegetation (the soil water)
• This zone is the link between the surface and subsurface hydrologic processes as rain water infiltrates through this zone to recharge the ground water.
• Water evaporated or lost by transpiration from the unsaturated zone (mainly from the soil water zone) recharges the atmospheric moisture.

Further, the process of infiltration, quite important in hydrologic modeling catchment, is actually a phenomenon occurring in the unsaturated zone.  Hence, knowledge about unsaturated zone water movement helps to understand infiltration better.

At the water table, the pressure head (conventionally denoted by Ψ) is zero (that is atmospheric), that in the unsaturated zone is (here Ψ is also called the moisture potential) and in the saturated zone, it is positive.  The hydraulic head at a point would, therefore, be defined as  

h = z +ψ   (1) 

Where, z is the elevation head, or the potential head due to gravity.  According to the mechanics of flow, water moves from higher hydraulic head towards lower hydraulic head

It may be noted that we may measure the negative pressure head within the unsaturated zone using a tensiometer.  It consists of a porous ceramic cup connected by a water column to a manometer.  The positive pressure head below water table can be determined using the hydrostatic pressure head formula γD, where γ is the unit weight of water and D is the depth of water below water table.