| Water shortage and soil salinization are the key factors constraining sustainable agriculture in oasis of arid area.Mulched drip irrigation,a highly-efficient water-saving irrigation method,has been widely applied in southern Xinjiang for cotton production.Understanding dynamics of soil water,salt,and main salt ions under mulched drip irrigation is the base for the regulation of soil water and salt in farmland.The experiment of mulched drip irrigation was conducted at Aksu water balance station,southern Xinjiang.And then,soil water and salt transport processes under mulched drip irrigation were simulated using COMSOL.Through the field experiment and numerical simulation,we explored the dynamics of soil water,Ca2+,Na+,Mg2+,SO42-,and Cl-under mulched drip irrigation,evaluated and optimized the flood irrigation schemes,which could provide a scientific foundation for establishing sustainable irrigation schemes to save water resource and prevent secondary salinization of soil.In addition,a quasi-three-dimensional model was presented for simulating the soil water and salt transport in the unsaturated and saturated zones.A case of Aksu station was analyzed to provide the theoretical basis for the regional regulation of soil salt and "dry drainage" technique.The main research results are as follows:(1)The model of soil water and salt transport in a cotton field with mulched drip irrigation system was developed using COMSOL.Soil water flow was described using the Richards equation,and salt transport process was simulated by the advection-dispersion equation(ADE)and the mobile-immobile(MIM)models.The comparison results between the simulated and measured values indicated that the planar 2-D model implemented by COMSOL was able to delineate the dynamics of soil water and salt under mulched drip irrigation.The behavior of salt under mulched drip irrigation was better quantified by the MIM model than by the ADE model,especially at the region with obvious advection process.The irrigation-driven salt leaching mainly occurred at the 0-40 cm layer under the mulch,and salt gradually accumulated on the exposed soil surface due to evaporation.(2)Based on the model of soil water transport in a cotton field with mulched drip irrigation system,we simulated the dynamics of SO42-,Ca2+,Na+,Cl-,and Mg2+ in salt-affected soil by defining partial differential equations in COMSOL.The cation exchange was described by the Gapon equation,and the chemical reaction between Ca2+ and SO42-was described using the second-order equation.The comparison results between the simulated and measured values indicated that the model was capable of describing the dynamics of soil salt ions under mulched drip irrigation.Due to the different appetency of soil colloids to cations and the presence of precipitate in soil,the transport characteristics of salt ions were different.In the mulched drip irrigation system,all the concentrations of salt ions in soil solution decreased in the upper layer(around 40 cm)of the mulched soil after irrigation,and the leaching effects of Cl-and Na+ were more obviously than those of SO42-and Ca2+.However,all the ions gradually accumulated on the exposed soil surface,and the accumulation amount of Cl-was the biggest due to its strong mobility in soil.In addition,the analysis of salt ions of 1:5 soil water extract suggested that the composition proportions of SO42-and Ca2+ increased under the mulch due to drip irrigation and decreased at the exposed soil surface due to evaporation,while the opposite trend was found for the composition proportions of Cl-and Na+.(3)Soil water and salt transports under mulched drip irrigation are typically three-dimensional processes.Under the irrigation conditions(drip emitter spacing of 30 cm and irrigation amount of 46.8 mm)in this study,no significant difference was found between the planar 2-D and 3-D simulations for soil water flow,except for the early stage of an irrigation event.The planar 2-D model was also able to predict the soil salt distribution during an entire mulched drip irrigation event for the bare strip,yet for the mulched area.In addition,the planar 2-D model simulated soil water and salt contents were close to the averaged values between two adjacent emitters,indicating that it was reliable for field irrigation management.For soil water flow,the planar 2-D model was reliable when emitter spacings were less than 15 and 35 cm for sandy and loam soils,respectively.The upper limit value of the emitter spacing suitable for the simplified 2-D simulation increased with the prolonged irrigation duration.For soil salt transport,the emitter spacing suitable for the simplified 2-D simulation reduced when the effect of solute reaction on retarding solute transport was obvious.(4)Salt usually accumulates along the wetting fronts and the bare soil strips under the mulched drip irrigation.Flood irrigations after harvesting and before sowing(i.e.,winter irrigation and spring irrigation)are the effective method to alleviate the salinity stress.Under the current conditions of soil,weather,and groundwater,the average soil salt content of the 0-50 cm layer could keep stability with 100 mm-spring irrigation each year,150 mm-spring irrigation every two years,or 200 mm-spring irrigation every three years combining with the "drip irrigation after sowing" in the none-spring irrigation year.The proper flood irrigation amount could be adjusted modestly based on soil salinity,groundwater buried depth,weather condition,salinity of irrigation water,and the availability of freshwater.Flood irrigation was necessary if the soil salinity of the arable layer was much bigger than the threshold value for cotton germination.When the salinity decreased to the proper level,the flood irrigation could be properly replaced by the "drip irrigation after sowing" to alleviate the water shortage during the spring sowing period.(5)A quasi-three-dimensional model was developed based on COMSOL for simulating water and salt transport in the unsaturated and saturated zones.The model was based on the assumption of essentially horizontal phreatic groundwater flow,which was described using the 2-D horizontal flow model.The 1-D model was adopted to simulate the soil water and salt transport processes in the unsaturated zone,and the flux of the lower boundary was used to account for the vertical recharge or discharge of the phreatic groundwater.In addition,the position of the lower boundary of unsaturated zone was adjusted automatically according to the hydraulic head of the phreatic groundwater.We examined the model with the measured values in Aksu station.The results indicated that the model could describe the dynamics of groundwater table.Due to the lower groundwater level in wasteland,groundwater gradually transport from farmland to wasteland.As a result,the evaporation-induced salt accumulation decreased in farmland,and the wasteland played an important role of "dry drainage". |
PDF Full Text Request