Influence of tea, forest and mixed farming land uses on stream flow and sediment flux in Sondu Miriu river basin

Abstract

The changing patterns of land cover and land use in the tropical river basins over time are critical. However, there is limited data and information on the extent to which land use types in tropical regions affect hydrological processes particularly in terms of stream flow and sediment transport. The magnitude to which stream flows and sediment flux differ in tea plantations, forests and mixed farming land covers has not been determined adequately in tropical river basins. The main goal of this study was therefore to determine the influences of different land covers tea plantations, forests and mixed farming on magnitudes of stream flow and sediment flux variability. The study was undertaken in three sub basins namely Timbilil sub basin dominated by tea plantations, Kiptiget sub basin dominated by forests and Kipsonoi sub basin dominated by mixed farming at the upstream of Sondu Miriu River Basin located in the Western Kenya region in period from 1960-2021. Field-based investigations was done using a depth integrated sampler to collect samples for Total Suspended Sediments Concentrations (TSSC), Acoustic Doppler Current meter and Seba current meter were used to measure flow velocities and river gauges were used to measure water levels in the sub basins. Turbidimeter was used to determine turbidity levels in the rivers draining the sub basins. Laboratory analysis and hydrological modelling were used to determine response of hydrological components in dominant land cover types. Spatial data used were obtained from the USGS and FAO databases, while temporal data used were obtained from Kenya Meteorological Services (KMS), Water Resources Authority (WRA) and Ministry of Water, Sanitation and Irrigation. Regression and correlation techniques were used to determine relationship between stream flows and sediment yields. Scenario analysis was carried out to test the effectiveness of various catchment management structures in the sub basins with high sediment generation. The findings of this study showed that the forest land cover in the sub basins dominated by tea plantation and mixed farming land covers declined by 8.4% (4 km2) and 0.3% (26 km2) respectively in the period from 1975 to 2021. Tea plantations land cover showed an increasing trend in all the three sub basins under study from 1975 to 2021. In the sub basin dominated by tea plantation an increase of 7% (24 km2) was observed while in the sub basins dominated by forest and mixed farming land cover, tea plantation increased by 15.2% (23.1 km2) and 6.4 % (101 km2) respectively. The mixed farming land cover portrayed a decline in the sub basins dominated by mixed farming and forest land covers by 6.1 % (96 km2) and 15.4% (23.4 km2) respectively. At the basin scale it was revealed that forest and mixed farming land cover reduced by approximately 16.9% (84 km2) and 3.6% (93 km2) from 1975 to 2021. While the area under tea plantations in the river basin increased by 44% (177 km2) in the same period. It was observed that the sub basin dominated by mixed farming generates high surface runoffs with average of about 30 m3/s compared to average surface runoffs of approximately 4 m3/s generated by sub basins dominated by forest and tea plantation. The sub basin dominated by mixed farming land cover exhibit high turbidity and TSSC levels of about 620 NTU and 630 mg/l in wet seasons. The relationship between TSSC and turbidity in sub basin dominated by mixed farming was positive with coefficient of determination R2 of 0.97. While low levels of turbidity and TSSC of less than 30 NTU and 20 mg/l were observed in the sub basins dominated by forest and tea plantations land cover in wet periods. The sediment loads in the sub basin dominated by mixed farming land cover in the pre planting period were about 900 tonnes/day. The sub basin dominated by the forest and tea plantations land covers exhibited relatively lower sediment generation ranging between 2 and 7 tonnes/day. The sediment loads at the downstream of the river basin ranged from 150 to 600 tonnes/day during the pre-planting and post harvesting periods. The SWAT model simulated the stream flows and sediment yields in the three sub-basins with dominant land covers quite effectively with R2 of 0.8 and Nash–Sutcliff Efficiency (NSE) of 0.78. The SWAT model showed that the mean annual sediment yield in the larger Sondu Miriu River Basin was about 140 tonnes/ha. The identified hotspots’ areas of the sediment generation in the river basin were Kuresoi at the upstream, Ndanai at the middle part and near Sondu market at the downstream. The mean annual evaporation and transpiration was high in the sub basins dominated by tea plantations and forest land covers ranged between 800 mm/a and 1000 mm/a compared to the sub basin dominated by mixed farming land cover with less than 800 mm/a. The relationship between sediment yields and stream flows in the sub basin dominated by forest and tea plantations land covers was weak with R2 of less than 0.24. Whereas a strong relationship between sediment yields and stream flows with R2 of 0.84 was observed in the sub basin dominated by mixed farming land cover. Forest and tea plantations land covers were found suitable for sustainable stream flows and sediment reduction. Also, the terraces, strip cropping and vegetative filter strips were found ideal structures for conserving soils in the sub basin dominated by mixed farming especially the hotspot areas. This study therefore recommends integration of tea plantations and forest land covers to be practiced in the sub basins of the Sondu Miriu River Basin. However, mixed farming should not be replaced completely because it will affect food security in the river basin. Alternatively appropriate soil water conservation measures terracing and strip cropping are recommended to be adopted in mixed farming land uses. This study creates awareness to basin communities, water users and water managers in both County and National Governments on the appropriate land cover and land use that will ensure sustainable availability of water with high water clarity.