2.5. Calibration process of the base model

The proportion of this land use type in the hydrological model area (11,308 ha) is: AGRL (Agricultural Area) (64.56%), ORAN (Citrus) (21.49%), ORCD (Orchards) (1.74%), WPAS (Winter Pastures) (9.20%), URMD (Settlement area (Medium Density)) (1.64%), and URLD (Settlement area (Low Density) (1.36%)). The agricultural areas in the study area contain various annual crops such as first crop corn, second crop corn, winter wheat, first crop

Inorganic nitrogen fertilizer (kg elemental N ha−<sup>1</sup>

1 20th June–30th Oct. C2<sup>7</sup> 330 858.33 2 16th Mar.–16th Sep. C1<sup>1</sup> 385 1168.33 2/3 20th Nov.–07th June WW<sup>2</sup> 230 383.33 3 20th June–30th Oct C2<sup>7</sup> 330 858.33 4 15th Mar.–15th Oct. Co<sup>4</sup> 290 1535 4/1 20th Nov.–07th June WW<sup>2</sup> 230 383.33

Perennial 15th Mar.–8th Oct. Orchards 250 1238.33 Perennial 1st Oct.–27th Sep. Citrus 335 1040

)

Irrigation water (mm)

The soil and water assessment tool is one of the recent models, known as a catchment area or watershed scale model, developed by Arnold et al. [31] and improved in the last 30 years [41]. It is a semidistributed hydrological model, which is a physically based, long period of simulation, lumped parameter, and derived from agriculture management systems models such as CREAMS, EPIC, and GLEAMS [41, 42]. The model separates selected basin to subbasins and hydrologic response units (HRU) comprised of identical hydrological properties such as land use, soil, and slope [43]. SWAT is an efficient tool to predict the impact of nitrogen cycle and land management practices on water, sediment, nutrient, and pesticide with the ArcSWAT module [44]. The nitrogen cycle can be represented by the SWAT model in the soil profile and

soybean, second crop soybean, peanuts, and cotton.

<sup>+</sup> All kinds of operations done to orchards and citrus between these dates.

Table 3. Agricultural land management crop rotations used in the model.

2.4. SWAT model description

Year

142 Water Quality

Rotation 5

Orchards and citrus+

<sup>1</sup> C1, first crop corn. <sup>2</sup> WW, winter wheat. <sup>3</sup> S2, second crop soybean.

 Co, cotton. P1, first crop peanut. P2, second crop peanut. C2, second crop corn.

Soil tillage and crop

growing period Crops

Calibration and validation are key processes in reducing the uncertainty and increasing user experience in its predictive results, making the software a practical model and leading to user competence.

The adjustment of model parameters is described as calibration. These parameters are associated with checking results toward observations to assure the same response in time [55]. A number of calibration techniques, comprising manual calibration method and automated method, improved for the SWAT model [39]. The model calibration is done manually and finalized by SWAT-CUP (Calibration and Uncertainty Programs). SWAT-CUP is an interface known as "automated model calibration method" that was improved for SWAT to connect with a link between the input and output of a calibration program and the model [56]. The SUFI-2 algorithm was used for sensitivity analysis, model calibration, and validation process. The warm up period was set for 1 year.

The calibration of SWAT is completed in three phases [39]. The first phase is the determination of most sensitive parameters (such as Alpha\_Bf, Canmx, Ch\_K2, Ch\_N, Cn2, Esco, Gw\_Delay, Gw\_Revap, Gwqmn Surlag for flow, and Nperco, Al1 CMN, Hlife\_Ngw for nitrate-nitrogen) [57]. The second phase is model calibration with use of statistical methods such as Pearson coefficient of correlation (R<sup>2</sup> ), Nash-Sutcliffe efficiency (ENS), and percent bias (PBIAS). The final phase is validation process for hydrological calibration and nitrogen calibration of the model.

Validation, known as the part of simulation, can be done without modifying any parameter values adjusted during calibration for a different time series to input data and also for the same time period at a different spatial location [58]. In this study, daily measured values of irrigation and irrigation return flows, and also nitrate loads for the year of 2008 were used for the warm up period. SWAT was calibrated with daily values over a 4-year period from 2009 to 2012 for hydrological years and used daily values for nitrogen. The 2-year time period from 2013 to 2014 was used for validation of hydrology and nitrogen.
