## 2 Materials and Methods
### Methods
#### 2.2.2 Nutrient Loads Estimation  
While reservoir eutrophication processes are significantly influenced by nutrient contents in the water, there is no information on the reservoir water quality. Because of the lack of observation data on nitrogen and phosphorus, the estimation of nutrient inputs to the reservoir was made on the basis of the quantity of NPK fertilizers applied to crops in the watershed because these nutrients come from NPK chemical fertilizers [17; 18]. Data relating to fertilizers, such as the amount, duration, and frequency of application, were integrated into SWAT according to crops by sub-basin and HRU. The mineral fertilizers are of the xN-yP-zK formula, x, y, and z representing, respectively, the proportions in percentage (%) of N, P, and K (Potassium) in 1 kg of fertilizer used. If the fertilizer formula is known, the quantities of N, P, and K are determined (Equations (8) and (9)) [33]:  
\[\text{N}(\text{kg})=\text{\%N}\times\text{1 kg}(\text{of the fertilizer})/100 \tag{8}\]  
\[\text{P}(\text{kg})=\text{\%P}\times\text{1 kg}(\text{of the fertilizer})/100 \tag{9}\]  
For this study, only N and P quantities were estimated. From the soil, the crops take nitrogen in the form of nitrates [33]. The principle of simulating the transfer of nitrogen in SWAT takes into consideration the natural and artificial processes of its introduction into the soil. Figure 3 shows the nitrogen cycle in the soil. Mineral nitrogen from mineral fertilizers is applied to the soil, either in the form of ammoniacal nitrogen or in the form of nitrates. The ammoniacal form is unstable and volatile. Nitrate remains the most stable form in the soil. It is in this form that it is taken up by plants. The organic form of nitrogen contains three pools (fresh, stable and active). \"Fresh\" organic nitrogen is associated with crop residues and microbial biomass, while \"active and stable\" organic nitrogen is associated with humus. The two forms of nitrogen (organic and mineral) are readjusted every day at the URH scale according to the inputs and transformation processes that cause the nitrogen to change form. For a large quantity applied, the remainder not used by plants is transferred by runoff water, especially since it is very soluble in water [53]. Nitrate transportation within the watershed is performed via surface runoff, lateral flow, or percolation. The amount of nitrate contained in runoff is assessed from the nitrate concentration in moving water. This concentration is a proportion of the nitrate mass in the runoff volume [33]. It is calculated according to Equation (10):  
\[\text{Conc}_{\text{NO3,mobile}}=\frac{\text{NO3}_{\text{ly}}\cdot\left(1-\exp \left[\frac{-\text{w}_{\text{mobile}}}{(1-\Theta_{\text{e}})\text{SAT}_{ \text{ly}}}\right]\right)}{\text{w}_{\text{mobile}}}, \tag{10}\]  
where \(\text{Conc}_{\text{NO3,mobile}}\) is the concentration of nitrate in mobile water for a given soil layer (kg mm\({}^{-1}\)), \(\text{NO3}_{\text{ly}}\) expresses the quantity of nitrate (kg ha\({}^{-1}\)), \(\text{w}_{\text{mobile}}\) is the amount of mobile water in the layer (mm), \(\Theta_{\text{e}}\) is the porosity fraction on which the anion exclusion depends, and \(\text{SAT}_{\text{ly}}\) is the water content when the soil layer is saturated (mm). The amount of mobile water in the soil layer is assumed to be equal to that lost by runoff, lateral flow, or percolation and can be estimated by Equations (11) and (12). \[\text{w}_{\text{mobile}}=\text{Q}_{\text{surf}}+\text{Q}_{\text{lat,ly}}+\text {w}_{\text{perc,ly}}\qquad\text{for top 10 mm} \tag{11}\]  
\[\text{w}_{\text{mobile}}=\text{Q}_{\text{lat,ly}}+\text{w}_{\text{perc,ly}} \qquad\text{for lower soil layers}, \tag{12}\]  
\(\text{Q}_{\text{surf}}\) is the depth of surface water runoff on the given day (mm), \(\text{Q}_{\text{lat,ly}}\) is the height of water escaping from the layer laterally (mm), \(\text{w}_{\text{perc,ly}}\) is the amount of percolation water in the underlying soil layer on the given day (mm).