Quantifying preferential flows on two farmlands in the North China plain using dual infiltration and dye tracer methods

Preferential flow, as a significant part of soil water infiltration, affects crop water use efficiency and pollutant transport in soil. Although it is important to properly assess the effect of preferential flow on soil water infiltration, there is a lack of widely accepted methods to quantify preferential flow. In this study, a new dual infiltration method (DI method) was used to quantify the contribution of preferential flow at two farmlands in the North China plain. The experiments were performed at two field sites (LY and SZ). To apply the new method, a double-ring infiltrometer was used to measure the steady-state infiltration rate (i.e., when infiltration rate becomes constant or stable with time, SIR) at the field sites, while a constant-head infiltrometer was used to measure the saturated hydraulic conductivity of packed soil columns consisting of disturbed soil obtained from each field site. Water flow in the disturbed soil columns was used to approximate the matrix infiltration rate (MIR), and the difference between SIR and MIR represented the preferential flow infiltration rate (PFIR). A dye tracer was also applied to the infiltrometers to quantify the proportion of preferential flow. For the LY site, the DI method found that the PFIR was 68 times larger than the MIR, while, based on the dye tracer method, the PFIR was only 2 times larger than the MIR. The DI and dye tracer method derived values of PFIR were similar for the SZ site, with MIR accounting for 61% (DI method) and 67% (dye tracer method) of the SIR. Wormholes were an important factor in the preferential flow generated in the LY site. The total areas of wormholes visible at the surface and the PFIR values were significantly positively correlated, r = 0.79 (P < 0.05). The DI method reasonably quantified preferential flow at the field sites, while the dye tracer method might underestimate preferential flow in soil with abundant macropores.

Automated summary

Preferential flow is an important factor in soil water infiltration, but there is a lack of widely accepted methods to quantify it. This study used a new dual infiltration method to quantify preferential flow and found that wormholes were an important factor in preferential flow generation.