*2.3.1 Porosity*

Porosity (*n*) is the intrinsic characteristic of a substance and refers to the amount of void or empty space in each material. The porosity (void space) occurs between

**Figure 2.** *The figure is showing the aquifer system in fractured rock formations.*

the fragments of soil or rock. It is defined by the ratio between the volume of the void space and the volume of rocks/soils.

$$n = \frac{V\_v}{V} \ast \mathbf{100\%} \tag{1}$$

where*Vv* is the volume of void space in a unit volume of earth material; and*V* is the unit volume of earth material (solids and voids).

#### *2.3.2 Hydraulic conductivity and permeability*

Permeability is defined as the ability of water movement through rock or soil which is directly related to porosity and it applies to the interconnected of pore spaces in rock or soil. Considering the relationship between driving and resisting forces on a microscopic scale during flow to porous media, hence, the permeability, k, is a function only of the area where the hydraulic conductivity *K* is defined:

$$k = \frac{K\mu}{\rho g} \tag{2}$$

where *k* is the permeability, *K* is the hydraulic conductivity, *g* is the acceleration due to gravity, ρ is the fluid density, and µis the viscosity.

Hydraulic conductivity (*K*) is a physical characteristic that calculates the capacity of substance in the context of an applied hydraulic gradient to transfer water across the pore spaces and fractures of rock/soil [14]. It depends on various physical variables including porosity, the structure of the soil matrix, grain size distribution, type of soil fluid, particle arrangement, water contents, void ratio, and other factors [15, 16].

#### *2.3.3 Transmissivity*

The transmission (*T*) is the rate of discharge where the water is transferred under a hydraulic gradient over a unit width of an aquifer. It is calculated by a formula and expressed in m<sup>2</sup> /s, or m3 /day/m or l/day/m.

$$IT = Kb \left( confirmed aquifer\right) \tag{3}$$

$$T = Kh(unconfine da quifer)\tag{4}$$

where *K* is the hydraulic conductivity, *b* is the aquifer thickness, and *h* is equivalent to the depth of confined aquifers.

#### *2.3.4 Specific yield*

Specific yield ( *Sy* ) as defined by Freeze and Cheery [14] is the storage term for unconfined aquifer where *the amount of water from the unconfined aquifer releases from the storage per unit surface area of aquifer per unit decline in the water table.* It is also known as unconfined storativity.

In other view, specific yield can be defined as the ratio of the volume of water that a saturated rock or soil will yield by gravity to the total volume of the rock or soil [15]. It is expressed in percentage.

*Characteristics and Assessment of Groundwater DOI: http://dx.doi.org/10.5772/intechopen.93800*

$$S\_{\gamma} = \frac{V\_{\omega}}{V} \ast \mathbf{100\%} \tag{5}$$

where*Vw* denotes the volume of water in a unit volume of earth materials; and*V* indicates the unit volume of earth material, including both voids and solids.

#### *2.3.5 Specific storage*

Specific storage (*S*s) is the volume per unit amount of a saturated formation that is a deposit from the storage because of the compressibility of the mineral skeleton and the pore water per unit change in head. The specific storage is given by Jacob (1940) and is typically represented in cm−1 or m−1.

$$\mathbf{S}\_s = \rho\_o \mathbf{g} \left(\boldsymbol{\alpha} + \boldsymbol{n}\boldsymbol{\beta}\right) \tag{6}$$

where ρω denotes water density, *g* is the acceleration of gravity,α shows compressibility of the aquifer skeleton, *n* indicate porosity, and β is the compressibility of water.

#### **3. Groundwater and surface water interaction**

Groundwater moves across flow paths arranged in space and develop a flow system. GW flow system is classified into local, intermediate, and regional flow systems (**Figure 3**) [17]. Water travels to the adjacent discharge area in a local flow system. One or more topographical low and high located between their discharge and recharge regions describe an intermediate flow system; however, contrary to the regional flux system, it does not occupy both the bottom of the basin and the major topographic high [18]. Water flows at a longer distance than the local flow system in a regional flow system and often discharge into large streams and lakes.

#### **3.1 Characteristics of SW-GW interactions at the local scale**

The range of groundwater at the local flow system is from 10 m to 10 km between the adjacent aquifers system and the stream reach. The recharge and discharge zones are associated with high and low areas respectively, associated with sub-watershed boundaries and local streams, respectively. The local GW flow system depends on the slope of topography and hydrogeology of the region (subsurface rock, streambed-sediment characteristics, and climatic conditions). At this scale, the seasonal effect on the hydrological response to recharge is high due to local flow systems, high water flux, and unsteady flow conditions. The fluctuation of the water table in local GW flow systems varies in different climatic conditions. For instance, the low water level in arid and semi-arid climate due to the low amount of precipitation and infiltration while the higher water level in a tropical environment is due to higher rainfall and infiltration. Therefore, SW-GW interaction is found more in a tropical and humid climate.

Generally, SW-GW can be introduced for homogeneous interaction of a stream and adjacent shallow aquifers system with hydrological processes, which is controlled by a SW-GW head and a streambed leakage coefficient. In hydrological processes, water moves with huge quantities of nutrients and streambed sediment and modifies the earth's surface through deposition and erosion. The hydrological processes give

information about the drainage basin, small watershed, stream basin, evaporation, transpiration, evapotranspiration, runoff, and infiltrations rate (**Figure 4**). The hydrological exchange between GW and SW is through the downwelling and upwelling processes (**Figure 5**). Upwelling processes are those in which local GW flow moves toward SW and on the other hand, the situation is referred to as downwelling processes. During these processes, if the shape of the longitudinal streambed profile

**Figure 3.** *GW flow system at the local and regional scale.*

*Characteristics and Assessment of Groundwater DOI: http://dx.doi.org/10.5772/intechopen.93800*

#### **Figure 5.**

*Downwelling, upwelling, and hyporheic exchange processes.*

is convex then the SW movement is through downwelling processes in the hyporheic zone whereas, if the shape of the longitudinal streambed profile of SW is concave then SW movement is through upwelling processes in the hyporheic zone [19]. The shapes of longitudinal streambed profiles are related to pool-riffle sequence and sediment bars, dunes, and ripples. The movement of stream water from riffles to pools is showing in (**Figure 5**) which is affected by the channel's sinuosity and bed load materials.
