**2.2 Description of the SWAT model**

The SWAT was advanced in the 1990s by the United States Department of Agriculture (USDA). It is a mechanism-based and spatially semi-scattered hydrological model or flexible tool in different parts of the world, designed to calculate and route water, sediments, management practices, and nutrient-point sources of pollution from individual sub-basins through the mainstream watersheds towards its outlet resulting from changes in land use/cover in the river basins [21]. In general, SWAT simulates the hydrological cycle and water balance in the catchment using equation (1).

*Evaluation of Climate Change-Induced Impact on Streamflow and Sediment Yield of Genale… DOI: http://dx.doi.org/10.5772/intechopen.98515*

$$\text{SW}\_{t} = \text{SW}\_{o} + \sum\_{i=1}^{t} \left( R\_{\text{day}} - Q\_{\text{surface}} - E\_{a} - W\_{\text{sep}} - Q\_{\text{gw}} \right) \tag{1}$$

Where; SWt = Final soil water content on a day i (mm/day), SWo = Initial soil water content on day i (mm/day), t = time in days, Rday = amount of precipitation on day i (mm/day),Qsurface = amount of surface runoff on day i (mm/day), Ea = amount of evapotranspiration on day i (mm/day), Wseep = amount of water entering the vadose zone from the soil profile on day i (mm/day), Qgw = amount of return flow on day i (mm/day). The SWAT uses the soil conservation service curve number (SCS-CN) approach to evaluate surface runoff, illustrating runoff to soil type, land use/cover, slope classes, and management practices, and is computationally effective [22]. The model estimates the streamflow in the sub-basins as a result of the total daily rainfall SCS- using the Soil Conservation Service curve number (CN) method as follows:

$$Q\_{Surface} = \frac{\{R\_{Day-0.2S}\}^2}{\{R\_{Day} + \text{0.8S}\}} \tag{2}$$

The retention parameter(S) and prediction of lateral flow by SWAT model expressed as;

$$\mathbf{S} = 25.4(1000/\text{CN} - 10)\tag{3}$$

Where; S = drainable volume of soil water per unit area of a saturated thickness (mm/day), CN = curve number.

The model's water yield within a watershed has been evaluated based on the equation; (Negewo & Sarma, 2021).

$$W\_{\rm YLD} = Q\_{\rm Surfac} + BF - T\_{\rm Laser} = Q\_{\rm Surfac} + Q\_{GW} + Q\_{\rm LAT} - T\_{\rm Laser} \tag{4}$$

Where; *WYLD* = water yield (mm), ¼ *QSurface* = surface runoff (mm), þ*QLAT* = lateral flow contribution to stream(mm), þ*QGW*= groundwater contribution to streamflow (mm), and *TLoss* = the transmission losses (mm) from tributary in the HRU through the bed.

For individual HRU, the sediment losses attributed to the surface runoff were evaluated based on the Modified Universal Soil Loss Equation (MUSLE) [23]. The MUSLE formula of sediment yield in the sub-basin roughly estimates the gross soil erosion caused by sheet, rill, and rain splash but does not include the erosion caused by landslides and gullies.

$$\mathbf{Q} \mathbf{SED} = \mathbf{11.8} \ast \left( Q\_{\text{Peak}} \ast Q\_{\text{Surfac}} \ast \mathbf{A}\_{\text{hru}} \right) \mathbf{0.56} \ast \mathbf{K} \ast \mathbf{C} \ast \mathbf{P} \ast \mathbf{LS} \ast \mathbf{CFRG} \tag{5}$$

Where; QSED = Sediment loss/Sediment yield(ton/ha/day) from individual HRU, *QSurface* = surface runoff associated to HRU (mmH2O/ha/day), Ahru = Area of HRU in(ha), *QPeak* = peak flow rate(m<sup>3</sup> /s), KUSLE = soil erodibility factor, CUSLE = Cover and management practice factor, PUSLE = Conservation support practice factors of land use, LSUSLE = Topographic factor, hill slope steepness factor/ the length slope factor, CFRG = coarse fragment factor.

Typically, the application of the SWAT contained five mains: (a) watershed delineation and streams network generation, (b) combination of DEM, soil data, and land use/cover data and create slopes classes, (c) creating HRU

(Hydrological response unit) definition, (d) combination of climate data (e) run the simulation (**Figure 2**).
