Assessing heterogeneity of preferential soil water fluxes in situ with zero-tension microlysimeters in a temperate forest

Macropore flow in structured soils constitutes an important component of soil water fluxes. This is especially true for unmanaged ecosystems. Preferential flow substantially affects ecohydrological separation, the partitioning of precipitation water into green (soil matrix, vegetation) and blue (recharge) water. Event characteristics, which are affected by changing climate, impact preferential flow; this, in turn, has an impact on ecohydrological separation and water resource availability. Though its importance is widely acknowledged since decades, it remains a challenge to measure preferential flow in soils. Additionally, small-scale heterogeneity of soil and environmental properties triggers spatial heterogeneity of preferential flow. In this study, we test the potential of zero-tension microlysimeters to measure preferential flow in a high spatial resolution. Zero-tension lysimeters have been used to sample soil water solution for chemical analysis. Methodical studies have shown that soil matric fluxes flow around zero-tension lysimeters and only gravity-driven water fluxes are captured. Using this to our advantage, we aim to develop a low-cost and simple method to sample preferential (gravity-driven) soil water fluxes in point measurements. This enables the implementation of a statistical design due to a high possible number of repetitions and the comparison with standard soil water status sensors due to similar scales. We are testing our lysimeters in a temperate mixed deciduous forest at Apelern, Lower Saxony, Central Germany. The soils are shallow and consist of weathered limestone intermingled with loess. By implementing transects starting from tree stems, we aim to cover a range of input fluxes and soil properties. We are combining lysimeters with measurements of soil water content, stand precipitation and soil properties. With our setup, we will be able to gain insight into the heterogeneity of preferential fluxes in situ and compare soil, stand and event impact factors to get a better understanding of the role of macropore flow in ecohydrological separation.