干旱区绿洲农业生态系统土壤优先流量化方法可行性综述

Feasible evaluation of quantifying soil preferential flow in an oasis agroecosystem:A review

归因于多孔介质的空间变异性和各向异性,非饱和带中土壤水流运动与溶质运移对优先流响应机制的研究十分具有挑战性。绿洲农业生态系统作为新疆主要经济支柱,如果忽略土壤优先流现象,可能会降低灌溉用水利用效率,减少土壤吸附污染物时间,进而导致地下水污染,最终制约农业发展。因此,该研究以新疆干旱绿洲区121团场的棉花种植区作为研究对象,旨在探明与探究在干旱绿洲区中可能导致优先流发生的潜在因素以及适合该地区观测优先流现象的量化技术。总体而言,在121团场,冻融循环,残膜现象,深耕棉秆还田的耕作方式以及土壤盐渍化会影响并改变土壤耕层孔隙度与结构,形成大孔隙式的优先入渗路径,进而引发大孔隙流。与此同时,因为该地区的主要土壤类型为壤土,其斥水性驱动着水流不稳定运动,导致指流或漏斗流的发生。通过对量化技术的总结与评估,染色示踪成像方法由于其能直观表述土壤水的非均匀运动,在121团场具有能很好地评价土壤优先流的潜力与实用性。其他的量化技术虽然具有各自的优势,但相较于染色示踪成像试验成本高,前期准备工作量大,评价尺度不统一,因此不适用于干旱绿洲区优先流的量化评价。在未来的研究中,需要适当地采用将微尺度观察与大尺度调查相结合的方法来对优先流现象进行全面评估。该研究有望为新疆及世界其他类似地区干旱绿洲生态系统优先流的研究提供理论参考。

Preferential flow in the vadose zone brings challenges in characterizing water and solute transportation,mainly ascribed to the high spatial variability and anisotropy of porous medium.The dynamic instability of the preferential movement in soil lessens the contacting duration between the soil matrix and the solutes,arising the risk of deep leakage of chemicals from the surface to the groundwater.Consequently,in agricultural practice,the preferential transportation will negatively impact the hydraulic activities in fields thus generating low water-fertilization utilization efficiency.Oasis agroecosystems as the economic pump in Xinjiang have been neglected regarding the preferential flow phenomenon,which potentially affects water use efficiency and groundwater pollution,eventually restricting agricultural development.In these regions,the consecutive land-reclamation has made some achievements for the increased irrigated arable land.Nevertheless,pressures coexist due to the desertification,the limited precipitation and poor land fertility;whether the natural conditions or the agronomic management will affect the soil preferential flow is uncertain.Therefore,a specific oasis planting area of Regiment 121(R121)in Xinjiang of China was selected in this study to seek and discuss the underlying factors possibly causing the preferential flow and the proper quantifying techniques.In R121,generally,the potential initiating drivers,including the freeze-thaw cycle,residual film,cultivation modes,and soil salinization will change the soil porosity,and alter the soil structure,thus shaping the preferential flow pathways concerning the macropore flow.During the constant freezing-thawing processes,the soil proportion of solid soil water increases,putting on the soil porosity and soil volume;but the effects of freeze-thaw on the soil depend on the collecting time and conditions of the soil samples.In addition,the accumulating plastic fragments imped the subsurface water movement,retard the horizontal movement,thus accelerating the downward movement of the soil water.Furthermore,the returning cotton straws into the fields will signally have an impact on the water storage during sowing seasons,enhancing the penetration and permeability of the soil;on the other hand,the straws in the soil also elevate the development of soil pores which then are transferred to bigger pores,thus increasing the total porosity;with the extension of reclamation time,these effects will become much more obvious.Additionally,the soil type in this region is loam,whose water-repellency will weaken the influence of the capillary of the soil conduit,force the flow unstably selecting the area with higher water conductivity,and may trigger the finger flow.The techniques of quantifying the preferential flow have also been summarized and evaluated,in which the dye tracing imaging methods can appraise well the preferential flow for its direct expression of non-uniform infiltration process in the soil.Other methods such as micro-tension measurement and acoustic detection may not be applicable in this region.Moreover,future research needs to properly combine the micro-observation with the large-scale investigation for a thorough evaluation.This review is expected to provide a reference for the research on preferential flow in arid oasis ecosystems of Xinjiang and other similar parts around the world.