Introduction to Geohydrology

Geohydrology is a study of Groundwater Science.

Physical and Chemical interactions of groundwater (including how it interacts with surface water and associated environments)

Geological characteristics that govern the occurrence, movement and management of groundwater (hydrological properties of rocks/soil)

  • In total 100% water 97.5% of water is salt water and only 2.5% of water is fresh water.
  • In 2.5% of freshwater, there is 68.7% of the water is in glaciers, 30.1% of the water is in groundwater form and 0.8% is in permafrost also 0.4% is in surface and atmospheric water.
  • Now in 0.4% of the surface and atmospheric water, 67.4% of water is in freshwater lakes, 12.2% of water is in soil moisture, 9.5% is in the atmosphere, 8.5% is in wetlands, 1.6% is in rivers and 0.8% is in biota.
  • The relative amount of water in various locations on or near Earth’s surface. More than 97% of the water lies in the ocean. Of all water at Earth’s surface, ice on land contains about 1.7%, groundwater 0.8%, rivers and lakes 0.007%, and the atmosphere 0.001%.
Figure: 1.1
1998 Encyclopaedia Britannica, Inc.

Water, below the surface, at a pressure greater than atmospheric pressure, which thus flows freely into a hole through interconnected void spaces, is groundwater.

Groundwater and soil water together make up approximately 0.5% of all water in the hydrosphere. Figure 1.1 illustrates the various zones of water found beneath the surface. The water beneath the surface can essentially be divided into three zones:

  1. The soil water zone, or vadose zone.
  2. An intermediate zone, or capillary fringe.
  3. The groundwater, or saturated zone.

The top two zones, the vadose zone, and capillary fringe can be grouped into the zone of aeration, where during the year air occupies the pore spaces between earth materials. Sometimes, especially during times of high rainfall, those pores are filled with water.

Groundwater and soil moisture occurs in the cracks, voids, and pore spaces of the otherwise solid earth materials. Porosity, n, is the percentage of the total volume that is void of material, Where Vv is the volume of void space, and V is the total volume. We can write Vv as Vv = V – Vs, where the subscript refers to the volume of the solid phase, Vs = md/rs, the dry weight divided by the density of the soil/rock.


n = 100 x Vv / V

Materialn (%)
Well sorted sand and gravel25-50
Sand and gravel, mixed20-35
Glacial till10-20
Porosity of sediments

Porosity can be measured by drying the sample completely, at 105C, and then submerging it in a known volume of water. What goes into the rock is a measure of the effective void space. Well-rounded, and sorted, sediments have a porosity between 26 – 48%, depending on the packing, but independent of particle size. If there is a mixture of grain sizes, the porosity will be lower, since smaller particles tend to fill in the void spaces between larger ones. The porosity of sedimentary rocks is very variable, from 1% to 30%. Plutonic rocks have a porosity of 1.5% if un-fractured,
fracturing increases the porosity (by 2 – 5%) and weathering can also increase the porosity (+30 –

Conductivity and Permeability

Some rocks are porous, but the voids are not, or are poorly, interconnected. These rocks cannot convey water from one void to another. In the mid-1800s, Henry Darcy, a French engineer made the first systematic study of the movement of water through a porous medium. Intrinsic permeability depends only on the properties of the porous medium.

  • Specific yield, Sy, is the ratio of the volume of water that drains from a rock, owing
    to gravity only, to the total volume.
  • Specific retention, Sr, is the ratio of the volume of water a rock can retain against gravity drainage, to the total volume of the rock. If two rocks have equal porosity, but different grain sizes, more moisture will be left in fine-grained rocks, due to surface tension.

The sum of specific yield and retention is equal to the porosity, n = Sy +Sr.

  • The specific storage, Ss, is the amount of water per unit volume of a saturated formation that is stored or expelled from storage owing to the compressibility of the mineral skeleton and the pore water per unit change in head.
  • Capillary fringe can saturate the soil above the water table, but the fluid pressure will be negative w.r.t. local atmospheric pressure, indicating that the capillary fringe is a part of the vadose zone.

Soil Moisture

The zone between the ground surface and down to the water table is called the vadose zone (unsaturated zone). The water there is held to the soil particles by capillary forces. The vadose zone may contain a three-phase system:

  • Solid – Mineral grains, and organic material.
  • Liquid – Water with dissolved solutes.
  • Gaseous – Water vapor, and other gases.

We can write the equation for porosity as,

n = 100 x (1 – Pb / Pm)

Where Pb = ms/V is the dry bulk density, subscript ‘s‘ for solid, and Pm = ms/Vs is the particle density.

The water that is immediately available to plants is a part of the soil water. Soil
water can be sub-divided into three categories:

  1. Hygroscopic water
  2. Capillary water, and
  3. Gravitational water.

Hygroscopic water is found as a microscopic film of water surrounding soil particles. This water is tightly bound to a soil particle by molecular forces so powerful that it cannot be removed by natural forces. Hygroscopic water is bound to soil particles by adhesive forces that exceed 31 bars and may be as great as 10 000 bars (recall that sea level pressure is equal to 1013.2 millibars which is just about 1 bar).

Capillary water is held by cohesive forces between the films of hygroscopic water.
The binding pressure for capillary water is much less than hygroscopic water.
This water can be removed by air drying, or by plant absorption, but cannot be
removed by gravity. Plants extract this water through their roots until the soil capillary
force (force holding water to the particle) is equal to the extractive force of the plant
root. At this point, the plant cannot pull water from the plant-rooting zone and it
wilts, called the wilting point.

Gravity water is water moved through the soil by the force of gravity. The
amount of water held in the soil at the point where gravity drainage ceases is called
the field capacity of the soil. The amount of water in the soil is controlled by the soil

Field capacity and wilting point for different types of soil

Hydrological Characteristics

  1. Aquifer Materials
  2. Rock Properties
  3. Porosity/Hydraulic Conductivity
  4. Specific Yields & Specific Retention
  5. Water Flow
MaterialSpecific Yield(% Porosity)
Sands (Unconsolidated)10-3025-40
Specific Yield ForSome Rocks
ClassK (m/d)Examples
Extremely Permeable>10Coarse sandstone, limestones, and fissured crystalline rocks, pebbles, and gravel.
Semi-permeable10-0.1Fine-grained sands, loams, and slightly jointed crystalline rocks.
Impermeable<0.1Clays, marks, compact igneous rocks.
Classification of Rocks on Permeability
Disposal of rainwater
Infiltration Curve (Horton)

Rock Properties

Aquifer – Saturated but poorly permeable material. Has the ability to store and transmit.
e.g, Sands, gravel, etc. An aquifer is an underground layer of water-bearing permeable
rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater can
be extracted using a water well.

Aquiclude – Saturated but relatively impermeable. E.g. Clay

Aquifuge – Relatively impermeable. Neither containing nor transmitting water. e.g.,

Aquiterd – A saturated but poorly permeable. Transmit water but less yeilding capability.
e.g., Sandy clay

Types of Aquifer

  1. Confined Aquifer – It is also called an artesian aquifer. It is a type of aquifer overlain as well as underlain by confining layers. The water within the aquifer is therefore held under pressure. It is sometimes called a pressure aquifer also. If the aquifer has a high outcrop laterally than the ground surface there will be positive hydrostatic pressure to create conditions for a flowing well. Water from such a well comes to the surface without pumping. The imaginary level up to which the water will rise is called the piezometric surface.
  2. Un-confined Aquifer – An aquifer that is not overlain by any confining layer but has a confining layer at its bottom is called an unconfined aquifer. It is normally exposed to the atmosphere and its upper portion is partly saturated with water. The upper surface of saturation is called the water table which is under atmospheric pressure, therefore, this aquifer is also called a phreatic aquifer.
  3. Semi-confined Aquifer (Leaky Aquifer) – An aquifer that is overlain or underlain by a semi-pervious layer (aquitard) through which vertical leakage takes place due to head difference is called a leaky aquifer or semi-confined aquifer. The permeability of the semi-confining layer is usually very small as compared to the permeability of the main aquifer. Thus the water which seeps vertically through the semi-confining layer is diverted internally to proceed horizontally in the main aquifer.
  4. Perched Aquifer– It is a special case of an unconfined aquifer. This type of aquifer occurs when an impervious or relatively impervious layer of limited area in the form of a lens is located in the water-bearing unconfined aquifer.
    1. Homogenous Aquifer – hydrological properties everywhere identical
    2. Isotropic Aquifer – properties identical to its direction
    3. Anisotropic Aquifer – possesses directional characteristics

Featured image credit – Image by Freepik

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