**2.1 Vertical distribution of groundwater**

The groundwater occurrence is typically categorized into two major zones based on the water table namely unsaturation zone and saturation zone. The zone of unsaturation is also known as the zone of aeration (vadose zone), which is also subclassified into the soil moisture zone, intermediate vadose zone, and capillary zone. The unsaturation zone is comprised of interstices or void spaces that are partially filled with water and air. All interstices are fully saturated with water under hydrostatic pressure in the saturated zone under the water table.

## *2.1.1 Soil-moisture zone*

The soil-moisture zone occurs across the main root zone beneath the earth's surface, but its thickness varies with the types of soil and vegetation. This zone plays a significant role in the recognition of hydrological processes [9] and also important for the interaction of the land-surface atmosphere [10]. The practices of agriculture and irrigation, particularly in arid and semiarid areas, primarily depend on the timely characterization of spatial and temporal soil moisture fluctuations in the root zone as a consequence of the soil moisture effect on health status and production of crops and salinization [11]. Several environmental factors such as physicochemical characteristics of water, surface slope and roughness, soil hydraulic conductivity, the porosity of the soil, and pre-existing soil pore moisture content are controlling the soil matrix's capacity to transfer water of which affecting the infiltration process [12].

### *2.1.2 Intermediate vadose zone*

The intermediate vadose zone is located beneath the soil moisture zone and upper part of the capillary zone. Water that drops into this zone can be either drawn into the capillary interstices of the transition area through the molecular attraction or drawn downwards to the adjacent saturated zone.

### *2.1.3 Capillary zone*

The zone is the lowest part of the aeration zone and directly above the water table where water as a component of the capillary action can be drawn back toward it. For a capillary zone of clay with a 0.0005 mm porous radius, the typical height may be 3 m, contrasted with fine sand of less than 10 cm with a 0.02 mm porous radius. Capillary water is the water stored above a surface of the water table in the capillary openings of unsaturated or saturated substances.

### *2.1.4 Saturation zone*

The saturation zone is above the water table which is often referred to as the phreatic zone or aquifer system. Water that has profoundly infiltrated through the vadose zone enters the saturation zone and filled all pore spaces with water. The thickness of the saturation zone varies from several meters beneath the earth's surface to numerous hundred meters. The factors to determine the thickness of this zone depend upon the local geology, accessibility of openings or pores in the rock formation, and water flow within the zone from recharging to discharge points. This saturation can take place range from several days or weeks to many months in duration. Moreover, groundwater is controlled by quantity and rainfall intensity, temperature, rock porosity, and permeability, dryness of the air, vaporization intensity during the rainy season, land slope, vegetative covering, and water absorption ability for soil. As well, significant volume of water can be contained within fractures and joints structures. The following are typical opening types contained in rock: (1) openings in gravel and sandstone formations with individual particles; (2) vugs, caverns, and solution channels in dolomite and limestone rock; and (3) joints, crevices, gas holes and faults in metamorphic rocks and igneous formations.

#### **2.2 Types of aquifers**

Aquifers are generally categorized into two major classifications, confined and unconfined aquifers; leaky and fractured aquifers are sometimes addressed in some other aquifers (**Figure 1**).

An unconfined aquifer is a layer of water-bearing formations or rocks that do not have a confining bed at the top of the groundwater which is referred to as the groundwater table where the pressure becomes equivalent to the atmospheric pressure. The variation of groundwater levels varies and depends on the pumping from the wells, permeability, area of recharge and discharge, in effect impacting the increasing or declining water rates in wells that are extracted from aquifers. The water table is free to rise or to fall which is often called the free or phreatic surface. Contour graphs and water table profiles of wells that use the water to determine water quantities available as well as water distributing, and movement may be prepared from elevations of wells. The perching water sources, as shown in (**Figure 1**) are a case of unconfined aquifers. Their high susceptibility to contamination is a major problem with nonconfined aquifers. If something dumps on the surface, it will penetrate vertically and go down into the storage of groundwater.

#### *2.2.1 Confined aquifer*

The definition of the confined aquifer as "a formation in which the groundwater is isolated from the atmosphere at the point of discharge by impermeable geologic formations; confined groundwater is generally subject to a pressure greater than atmospheric" [13]. It is also known as "artesian or pressure aquifers" and it occurs mostly just above the base of confined rock bodies or layers which is mostly composed of clay that can protect it from surface pollution. Punctured wells from artesian aquifers are more prone to fluctuate with their depth of water because of changes in pressure than the amount in stored water. When such an aquifer is well penetrated, the water level should increase over the base of the confined layer, as illustrated by the flowing and artesian wells of (**Figure 1**). The water reaches a confined aquifer in a region in which the confining layer reaches the surface. The groundwater flow system into aquifers is frequently affected by gravity and geological formations in such areas either vertically or horizontally. A zone that provides water to a restricted area is considered a recharge area and water may even be leak into a restricted bed. Water ups and downs in confined aquifers penetrating wells mainly result from pressure changes instead of storage volume changes. Confined aquifers thus show only limited variations in storage and are predominantly used as conduits to move water to natural or artificial discharges from recharge areas.
