Infiltrometer ring-size effects on infiltration and macropore hydraulic activation

Climate and global changes will force cities to adapt to new drastic meteorological and hydrological conditions. Within this context, urban planning has pointed to the need to restore the natural water cycle in urban cities. Restoring the natural water cycle means promoting water infiltration in urban areas to facilitate groundwater recharge and minimize runoff at the soil surface. Several techniques were developed with this goal, including those aimed at infiltrating water in specific drainage works like sustainable urban drainage systems (SUDS). However, the management of SUDS requires monitoring their capability to infiltrate water and its permanence with time. Indeed, several processes may impact the hydraulic characteristics of soils and, consequently, the infiltration capacity of bio-retention. Among others, clogging may reduce the soil's hydraulic conductivity and decrease infiltration. Conversely, plant growth and related development of root systems may promote macropore networks and increase the bulk hydraulic conductivity of soil, resulting in an increase in infiltration.

Infiltration techniques, including single-ring water infiltration experiments, were developed to monitor the soil's hydraulic properties and investigate their evolution with time. Infiltration techniques are based on infiltration tests with rings with radii in the order of 5-10 cm. If the question of the type of condition imposed at the soil surface was already posed (e.g., the question of the value of the water pressure head to impose, the presence of a sand layer for tension infiltrometers, etc.), the question of the ring size has not been investigated in depth.

In this study, we investigate the impact of the ring size on the results of water infiltration experiments, particularly regarding the activation of the soil macropore network and the hydraulic characterization of soils. We then performed infiltration experiments with rings of two contrasting sizes (7.5 cm versus 25 cm for the radius). Water infiltrations were carried out, involving the same total cumulative infiltration depth of 300 mm. BEST methods were then applied to derive the soil hydraulic parameters (Angulo-Jaramillo et al., 2019). The results were then compared between the large and the regular (small) rings. Differences in estimate means and standard deviations were discussed for each hydraulic parameter. Numerical modeling was also performed using HYDRUS (Radcliffe and Simunek, 2018) with synthetic soils to explain the difference in results between ring sizes with the concept of partial activation of the macropore network depending on the ring size. Our results constitute the first step toward understanding the ring effect on soil hydraulic characterization and its optimization with regard to the activation of all types of porosities.

References

Angulo-Jaramillo, R., Bagarello, V., Di Prima, S., Gosset, A., Iovino, M., Lassabatere, L., 2019. Beerkan Estimation of Soil Transfer parameters (BEST) across soils and scales. J. Hydrol. 576, 239–261. https://doi.org/10.1016/j.jhydrol.2019.06.007

Radcliffe, D.E., Simunek, J., 2018. Soil physics with HYDRUS: Modeling and applications. CRC press.