Soil Water And Salt Movement Under Different Soil Textures With Negative-Pressure Irrigation

Negative-pressure irrigation (NPI) has been proposed as a new water saving irrigation method to counter the shortage of water resources and low water use efficiency. In dryland farming and agricultural facilities, NPI relies on soil matric suction to irrigate crops under unsaturated soil conditions, which can achieve precise and continuous control of the soil moisture and improve water use efficiency. However, uncertainty remains regarding the mechanisms of water movement and solute transport through different soil texture configurations under NPI, as well as the effect of irrigation management parameters such as initial soil conditions, conductance of the emitter wall, and the irrigation pressure of the system on water movement and salt transport. In order to answer these questions, a soil tank laboratory test was combined with numerical modeling in this study. Firstly, the soil tank test was used to determine the water and salt distribution in different textures of homogeneous soil. Secondly, the applicability of the numerical model was verified for NPI. Thirdly, different soil texture configurations were simulated, and we determined the water and salt distribution in the soil profile, the cumulative water infiltration, the water infiltration rate, and evaporation. Finally, a model was implemented to quantify total water supply and the volume of soil being desalted under NPI. The main results and conclusions are as follows:(1) As negative pressure increases in NPI, the degree of salt leaching and soil water infiltration (measured by water infiltration rate, wetted zone extent, volumetric water content, and total amount of water infiltration) both decrease. The greater the soil clay content, the faster the soil water infiltration rate, and the soil can also retain more water after irrigation. The stable water infiltration rate and soil moisture in the wetted zone is highest in loam, followed by silty loam, sandy loam, then loamy sand, and the order for total water infiltration in 24 h is loam> sandy loam> silty loam> loamy sand. The degree of salt leaching are highest in silty loam, then sandy loam, loam, and loamy sand.(2) Numerical model can accurately simulate water movement and salt transport during NPI. The results show very good agreement between the measured and simulated values of soil water content, the concentration of NaCl, cumulative water infiltration, and the soil moisture horizontal and vertical distance under NPI. The slopes of the linear relationships are close to 1 and the model efficiency coefficients are in the range 0.94～0.98. Setting the conductance of the emitter wall effectively improves the model accuracy.(3) Analysis of water movement and salt transport in different soil texture configurations shows that:1) With the emitter position in the loam layer, a coarse texture of loamy sand below the loam layer, the water infiltration into the subsoil is impeded in this soil texture configuration.97% of irrigation water accumulates in the topsoil at 168 h, leading to increased evaporation by 22% compared with homogeneous loam. The fine texture of silty loam or silty clay loam layers beneath the loam layer are more conducive to water infiltration into the lower layer, and increases the amount of water infiltration, whilst at the same time effectively reducing the surface evaporation. The maximum soil salt accumulation that occurred above the soil interface was found with sand-loamy sand (21.8 g kg-1); however, for loam-silty clay loam (23.8 g kg-1) and sand-silty loam (20.08g kg-1), the maximum salt accumulation occurred below the soil interface. Thus salt accumulation easily occurs in the clay-rich soil layer at the interface.2) With the emitter position at interlayer under NPI, the water infiltration characteristics for loam soil profile with silty loam interlayer is better than soil profile with silty clay loam interlayer. Soil profile with loamy sand interlayer has the lowest water infiltration, resulting in reduced salt-leaching intensity. Negative-pressure irrigation is not suitable for loamy sand soil.(4) Analysis of the total water supply, the volume of soil desalinated in different initial soil conditions, the conductance of the emitter wall, and the irrigation pressure of the system shows that:1) The total water supply and the extent of desalination increases with an increase in soil matric suction or irrigation pressure. The relationships between the total water supply and the emitter wall conductance, or the extent of desalination and the conductance of the emitter are logarithmic. As conductance of the emitter increases, the total water supply and the degree of soil desalting increases rapidly at first and then more slowly. The effect of initial soil matric suction and emitter conductance on total water supply and degree of soil desalting decreases significantly when irrigation pressure is decreased. The influence of initial soil matric suction and emitter conductance on silty loam is greater than on loam. The effect of initial soil matric suction on desalination is significant between 200 and 700 cm.2) A numerical model is set up to quantify total supply water and the volume of soil desalination under NPI, using irrigation pressure, initial soil matrix suction, conductance of the emitter, and time as variables. The slope of the linear relationship is close to 1 and the normalized root mean square error (NRMSE) is in the range 0.13～0.15. The results show very good agreement between measured and estimated values; therefore, the model can be applied to calculate water infiltration and the volume of soil desalination during NPI.