Tropical Montane Forest Conversion Is a Critical Driver for Sediment Supply in East African Catchments

Abstract

Land use change is known to affect suspended sediment fluxes in headwater catchments. There is however limited empirical evidence of the magnitude of these effects for montane catchments in East Africa. We collected a unique 4-year high-frequency data set and assessed seasonal sediment variation, water pathways, and sediment response to hydrology in three catchments under contrasting land use in the Mau Forest Complex, Kenya's largest tropical montane forest. Annual suspended sediment yield was significantly higher in a smallholder agriculture-dominated catchment (131.5 ± 90.6 t km −2 yr −1) than in a tea-tree plantation catchment (42.0 ± 21.0 t km −2 yr −1) and a natural forest catchment (21.5 ± 11.1 t km −2 yr −1) (p < 0.05). Transfer function models showed that in the natural forest and the tea-tree plantations subsurface flow pathways delivered water to the stream, while in the smallholder agriculture shallow subsurface and surface runoff were dominant. There was a delayed sediment response to rainfall for the smallholder agriculture and the tea-tree plantations. A slow depletion in sediment supply suggests that the wider catchment area supplies sediment, especially in the catchment dominated by smallholder farming. In contrast, a fast sediment response and depletion in sediment supply in the natural forest suggests a dominance of temporarily stored and nearby sediment sources. This study shows that the vegetation cover of a forest ecosystem is very effective in conserving soil, whereas catchments with more bare soil and poor soil conservation practices generated six times more suspended sediment yield. Catchment connectivity through unpaved tracks is thought to be the main explanation for the difference in sediment yield.

Monitoring of Suspended Sediments in a Tropical Forested Landscape With Citizen Science

“…Peak SSC concentrations were reached in early to mid-rainy season (in the months of April-July for long rains and September to October for short rains), in some instances prior to peak flow, and decreased in the transition from rainy to dry season (in the months of December-March). The higher concentrations of suspended sediments during the rainy season and the lower concentrations in the dry season in this study are consistent with other rivers studied in the Mau Forest Complex (Dutton et al, 2018;Stenfert Kroese et al, 2020a). Also notable are the considerable high SSC even in times when flows were consistently lower in March and May 2019 at site CMT and KIP.…”

“…Forests and riparian buffer zones can act as a filter, controlling and decreasing the sediment load by surface runoff (Mello et al, 2018). Similar effects were reported in previous studies that found forested watersheds to have lower suspended sediment concentrations as compared to agricultural landscapes (Tu, 2013;Zhang et al, 2017;Mello et al, 2018;Stenfert Kroese et al, 2020a). Contrasting the effect of land use, we have no clear explanation for the positive correlation with catchment size.…”

“…Other studies have reported similar results showing that the SSC did not necessarily change with discharge values which is an indication that other factors besides rainfall such as natural and anthropogenic disturbances control the SSC in small catchments (Ouellet-Proulx et al, 2016;Rodríguez-Blanco et al, 2018). Stenfert Kroese et al (2020a), in a study in the same catchment, reported that the impulse response between the peaks of discharge and sediment concentration was in general longer compared to the rainfall peak.…”

Water security in sub‐Saharan Africa: Understanding the status of sustainable development goal 6

“…Invasive alien plant species have equally been identified to contribute to freshwater pollution (Chamier et al, 2012). Land use/cover change resulting to increase sediment loads from upstream catchments is also a major source of water pollution (Dutton et al, 2018; Stenfert Kroese et al, 2020).…”

No Difference in Instream Decomposition Among Upland Agricultural and Forested Streams in Kenya

“…Lack of land-use effects on decomposition rates in this study is intriguing and contrary to similar studies on the effects of agriculture on organic matter processing in tropical (Jinggut et al, 2012;Masese et al, 2014b;Silva-junior et al, 2014;Fugère et al, 2020) and temperate (Woodward et al, 2012;Ferreira et al, 2015) streams. Although our agricultural sites had significantly elevated levels of total suspended solids and electrical conductivity, which are indicators of anthropogenic activities on water quality (Minaya et al, 2013;Ontumbi et al, 2015;Masese et al, 2017;Kroese et al, 2020), these were deemed within tolerable limits of most sensitive taxa, including shredders, in these sites. Riparian zones in agricultural streams were well maintained, and this increased the quality of instream habitats, canopy cover and standing stocks of organic matter (Table 1) that are depended upon by shredders.…”