沟灌肥液入渗硝态氮运移特性数值模拟及影响因素分析

Numerical simulation and influence factors analysis for infiltration characteristics of nitrate nitrogen under furrow irrigation with fertilizer solution

为探明沟灌肥液(硝酸钾,KNO3)入渗特性,选取杨凌区砂壤土和黏壤土作为供试土壤,以室内试验资料为基础,对不同肥液浓度条件下的土壤水力特性参数与溶质运移参数进行了反演,并模拟分析了不同因素对沟灌肥液入渗过程中硝态氮(NO3^--N)的运移规律。结果表明:肥液浓度变化对土壤水力特性参数有一定的影响,其中土壤饱和含水率θs、形状系数n、饱和导水率Ks随肥液浓度的增大呈增加趋势,但进气吸力倒数a值则呈现减小趋势;采用反演所得参数对沟灌肥液入渗过程进行了模拟,其中不同位置处土壤水分和NO3^--N模拟值与实测值相对误差绝对值均值最高分别为6.52%和11.49%,说明反演所得土壤水力特性参数和溶质运移参数是可靠的;土壤初始含水率和肥液浓度对NO3^--N分布的影响较显著,其中NO3^--N分布范围随着土壤初始含水率和肥液浓度的增大而增大;沟中水深和沟底宽对NO3^--N竖直向分布影响较小,但对水平向分布影响较大,其水平向分布范围随着沟中水深和沟底宽的增大而增大。研究结果可为农田沟灌施肥系统的设计与管理提供理论依据。

This paper aims to study solute transport in soil under furrow irrigation and fertilization.Sandy loam and clay loam taken from Yangling district in Shaanxi province were repacked into laboratory tanks;2D furrow irrigation and fertigation with KNO3 as fertilizer was conducted in attempts to investigate the movement of water and nitrate in the soils under different conditions.For each treatment,we inversely calculated the hydraulic and solute transport parameters of the soils by calibrating the simulated results against the measurements,and we then analyzed nitrate transport in the soils under different initial soil moisture and fertilizer concentration.The results showed that change in fertilizer concentration could alter soil hydraulic properties.In particular,we found that increasing fertilizer concentration led to an increase in saturated moisture content,the shape parameter n in the van Genuchten formula and saturated hydraulic conductivity Ks,while reducing the air-entry parameterαin the van Genuchten formula.The parameters obtained from the inverse method were used to simulate fertilizer movement under furrow irrigation and fertigation.The simulated cumulative infiltration and advancement of the wetting front were both consistent with the measurements,with their mean relative errors being less than 6.52%.The mean relative errors of the simulated solute transport were higher than that of the water but were less than 11.49%.Given other uncertainties involved in water flow and solute transport in the soils,the errors of the parameters and the model were tolerable and were representative of the soils.The simulated water flow and solute transport under different conditions revealed that NO3 distribution in the sandy loam and the clay loam were geometrically similar,and that the initial soil moisture and fertilizer concentration significantly impacted NO3 movement in which the spatial spreading of NO3 increased with initial soil moisture and fertilizer concentration.The depth of water in the furrow and the width of the furrow bottom were found to affect NO3 movement in horizontal direction more significantly than in vertical direction,and that the spreading of NO3 in horizontal direction increased with the water depth in the furrow and the width of the furrow bottom.These suggest that under furrow irrigation and fertigation,increasing water depth in the furrow and/or the width of the furrow bottom could ameliorate the risk of NO3 leaching by enhancing its horizontal movement;and NO3 leaching can be further reduced by not fertilizing when initial soil moisture is high and reducing the concentration of the fertilizer solution.Our results have a wide implication and they provide a general guidance for scheduling furrow irrigation and fertilization.