Quantifying preferential flow occurrence in dependence of land cover on the southern slopes of Mount Kilimanjaro, Tanzania

This study examined preferential flow occurrence and patterns in eight ecosystem types on the southern slopes of Mount Kilimanjaro in Tanzania. We analyzed continuous soil moisture content measured in three soil depths and rainfall data at 1-h resolution from January 2022 to August 2023. After a separation and clustering of rainfall events we assessed the dynamics of soil moisture responses. The sequence and timing of soil moisture changes informed about the occurrence of preferential versus sequential (matrix) flow. The results showed that the frequency of occurrence of preferential flow events were notably high in Erica forest (81.6 %), montane forest (26.4 %), and Ocotea forest (30.1 %), highlighting rapid subsurface water movement and potential for groundwater recharge. Other land covers such as disturbed Ocotea forest, grassland and maize indicated more uniform flow dynamics, which may favor surface runoff and increased soil erosion. We found that rainfall depth, duration and intensity are important factors that influence preferential flow. Additionally, the level of initial soil moisture also played a role in preferential flow occurrence, with optimal initial soil moisture levels between 35 % and 45 %. Both, very dry and very wet soil conditions were less favorable for preferential flow occurrence. These findings have important implications for understanding subsurface water movements and managing water resources in both natural and disturbed ecosystems. • Preferential flow occurrence was assessed using soil moisture response times. • Preferential flow occurred more frequently in the lower montane and montane forests. • Sequential flow dominated in grassland, disturbed Ocotea forest, and Helichrysum. • Rainfall characteristics influenced preferential flow occurrence. • Very dry and wet soil conditions are less favorable for preferential flow occurrence.