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

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 [ABSTRACT FROM AUTHOR]