Rainfall cycle causes the inter-seasonal instability of ecosystem service trade-offs: A case study in the Tana River Basin, Kenya

As a crucial climate condition, the inter-seasonal rainfall cycle directly impacts the livelihoods and productivity of billions residing in monsoon regions. However, there is still limited understanding regarding its effects on trade-offs among ecosystem services (ESs). This knowledge gap may impede the sustainable development of human well-being in these regions. In this study focused on Tana River Basin, a representative monsoon region, we assessed the seasonal supply and trade-offs of three ESs including water yield (WY), soil conservation (SC), and net primary production (NPP), which are closely linked to local human well-being. To comprehend the driving mechanism of inter-seasonal variation, the influential factors were identified using the Geographical Detector Model. Furthermore, by quantifying the seasonal trade-off intensity among three ESs, the managemental optimization priority areas were identified. The results showed that (1) all three ES supply presented an increasing trend from 2000 to 2020. Seasonally, both the supply and trade-offs of ESs were significantly enhanced in the rainy seasons, with spatial heterogeneity observed in the response to inter-seasonal rainfall cycle. (2) Natural condition factors presented stronger influence on the seasonal variation of ES trade-offs, which is crucial to consider the comprehensive hydrological response resulting from vegetation growth as the foundation for these seasonal variations. For the WY-SC tradeoff, precipitation was identified as the primary influential factor, while vegetation coverage impacted on SC-NPP and WY-NPP trade-offs. Higher precipitation and vegetation coverage would intensify the trade-off between corresponding ESs. (3) Regarding the spatial distribution of seasonal instability, several sub-watersheds aggregated in the midstream hill region were identified as the primary optimized zones, more flexible management strategies should be implemented preferentially. The findings contribute to a better understanding of how the inter-seasonal rainfall cycle impact on the ES trade-offs, supporting effective ecosystem management in monsoon regions.