The effects of climate and forest cover variability on the hydrological regulation of an eastern Andean Cusiana river sub-basin

The hydrologic regulation analysis on a basin scale requires the understanding of the spatio-temporal heterogeneity of the climate regime and the land cover. The Cusiana river sub-basin (4992 km²) is part of the Colombian Orinoco River basin and its important because of the ecosystem services it provides. However, there is limited knowledge about the factors that govern its hydrological functioning. In this study we aimed at describing the relation between the forest cover and the hydroclimatic tendencies with the hydrologic regulation of this basin. For this, we evaluated rainfall and evapotranspiration spatial variability and the temporal tendencies of these variables and streamflow employing the Mann-Kendall test for a dataset covering from 1980 till 2020. Streamflow data was obtained from various gauging sites at different points along the main streambed between 290 and 3135 m a.s.l. We also evaluated the basin's forest cover changes between 2003 and 2019 using official land cover classification maps. With the hydroclimatic and land cover analyzes we created potential scenarios of extreme dry and wet years and of deforestation and restoration. These extreme scenarios were forced in the previously calibrated and validated Seasonal Water Yield module of the InVEST model to evaluate their effects on base and quickflows. Results show that there are no homogeneous tendencies in the rainfall, evapotranspiration, or streamflow within this basin. Furthermore, streamflow tends to decrease in the basin segments that show an increase in forest cover. The model outputs suggest that the magnitude of the baseflow decrease under a deforestation scenario is up to 3.6 times larger than the increase in baseflow under a forest cover restoration scenario, and that the quickflows can increase up to 1.4 times more in a combined wet year and deforestation scenario, than in an average year under current forest cover. From our results we identify that the spatial relation between land cover, climate and hydrologic regulation must be explicitly evaluated for the design of water resources management plans for any Andean River basin.