Simulation soil water–salt dynamics in salinewasteland of Yongji Irrigation Area in Hetao Irrigation District of China

In order to explore the regional water–salt balance mechanism in Hetao Irrigation District, field experiments were conducted in 2018 and 2019 in the Heji canal study area. The SWAP model was calibrated and validated based on field experiments’ observed data. The SWAP model was used to simulate soil water–salt dynamics in saline wasteland after calibration and validation. The results showed that model simulation results of soil water content and soil salt concentration agreed well with the measured values. Soil water content and soil salt concentration changed obviously under the effect of farmland irrigation in the crop growing period. Soil salt was accumulated in saline wasteland. The soil salt accumulation of each soil layer in saline wasteland was 0.164, 0.092, 0.890 and 1.261 mg/cm, respectively. Soil water content gradually increased and soil salt concentration gradually decreased in the autumn irrigation period. Soil salt was leached in the saline wasteland. The soil salt accumulation of each soil layer in the saline wasteland was 1.011, 1.242, 1.218 and 1.335 mg/cm, respectively. The saline wasteland became a drainage and salt drainage region for cultivated land. The saline wastelands had an obvious role in adjusting salt balance and maintaining salt dynamic balance in Hetao Irrigation District.


INTRODUCTION
Soil salinization has been one of main factors impacting agricultural production all around the world (Wang et al. ; Muyen et al. ). Global salinization land area is 9.5 × 10 8 hm 2 , which approximately accounts for 10% of the Earth's land area, and these salinity lands are mainly distributed in arid and semi-arid areas (Ghassemi et al. ; Abbas et al. ). In China, with a total area of about 3.6 × 10 7 hm 2 , salinity land accounts for 4.9% of the available land area. Especially, there is much salinity land in northwest China, with the arid climate. (Yang ; Wang et al. ). The Hetao Irrigation District (Hetao) located in the upper reach of Yellow River Basin, is a typical salinization irrigated district because of the dry climate and shallow groundwater. The area affected by salinization in the whole irrigated district has reached to 3.9 × 10 6 hm 2 , accounting for 69% of the total land area (Huang et al. ). The soil salinity in the crop root zone should be maintained within the reasonable range to avoid harming crop growth. Engineering drainage and irrigation leaching were two major ways to control soil salinization (Tedeschi & Menenti ; Wu et al. ). The average buried depth of groundwater level in the study area was 1.6-2.2 m, and the groundwater level was shallow. The terrain of the study area was relatively gentle.
The terrain was high in the southeast and low in the northwest in the study area. The elevation was between 1,040 and 1,042 m, and the total area was about 507 hm 2 . It was mainly controlled by two land-use types: cultivated land and salt wasteland. It was controlled by one main canal (Heji branch canal) and two branch canals (Xinli branch canal and Xinzhang branch canal). The distribution of salt wasteland in the study area was relatively concentrated, mainly concentrated in the central and northwest regions of the study area. The east, south and west sides were surrounded by cultivated land and the saline wasteland was lower than the cultivated land. Saline wasteland had the function of drainage and salt discharge, and was a typical 'dry drainage' area in Hetao Irrigation Area.  Table 1. According to the soil physical properties of each soil layer, the soil hydraulic characteristic parameters of each soil layer were generated  flow in the SWAP model, which is given by:

Experiment layout and observation items
where C(θ) is differential water capacity (/cm); θ is soil water content (cm 3 /cm 3 ); h is soil water pressure head (cm); t is time (d); z is the vertical coordinate (cm, positive uptake); K(h) is the hydraulic conductivity (cm/d); S(h) is the soil water extraction rate by plant roots (cm 3 /(cm 3 /d)).
Solute transport is based on the convective-dispersive equation in the SWAP model, which is given by: where J is total solute flux density (g/(cm 2 /d)); q is vertical flow at the bottom (cm/d); c is solute concentration in the The SWAP model has two crop growth modules to simulate the crop growth process, a simple one and another detailed one based on the WOFOST model (   Because there was no crop in the salt wasteland, it was not necessary to input irrigation data and crop growth information. Soil water content was transformed into soil water pressure head by the soil water retention curve. The soil water pressure head and soil salinity before crop sowing were regarded as the initial condition. The measured dispersion length and molecular diffusion coefficient were The root mean square error (RMSE), mean relative error (MRE) and the coefficient of determination (R 2 ) were used to quantify the deviation of the simulated and observed data in calibration and validation. These indicators were defined as follows: where N is the total number of observations in the exper-

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C. Yuan | Soil water-salt dynamics in saline wasteland Water Supply | in press | 2021 Corrected Proof Figure 7 shows the dynamic changes of soil water content in different soil layers of saline wasteland in the crop growth period. In the study area, the soil began to melt in the middle of April, and soil water content of each soil layer gradually increased. In early May, the spring irrigation was carried out in the study area. Affected by the spring irrigation of cultivated land, the soil water content of each soil layer in saline wasteland was higher, which was mainly because the saline wasteland was located in the low-lying area. Due to the 'dry drainage' effect, soil water content of each soil layer in the saline wasteland was relatively high.
In the irrigation season, the soil water content of the saline wasteland changed obviously. After irrigation, the soil water content of the saline wasteland gradually increased, and the soil water content of the deep layer soil was the highest, and the surface water content was relatively low from May to August. From the middle of August to the end of September, the soil water content of the saline wasteland gradually decreased due to no irrigation in the cultivated land, and the soil water content of each soil layer reached the lowest values before autumn irrigation. Simulation of soil water-salt dynamics in autumn irrigation period Figure 9 shows the dynamic changes of soil water content in different soil layers of saline wasteland in the autumn irrigation period. The autumn irrigation period was from early October to mid-November in the study area. Autumn irrigation was a period of large irrigation amount in the year. The

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.