Afforestation as a climate change mitigation option has been the subject of intense debate and study over the last few decades, particularly in the tropics, where agricultural activity is expanding.

However, the impact of such landcover changes on the surface energy budget, temperature, and precipitation remains unclear, as feedback between various components is difficult to resolve and interpret.

Contributing to this scientific debate, regional climate models of varying complexity can be used to test how regional climate reacts to afforestation. In this study, the focus is on the gauged Nzoia basin (12,700 km2), located in a heavily farmed region of tropical Africa.

A coupled atmospheric-hydrological model (WRF-Hydro) is used to dynamically downscale a reanalysis product to get a better picture of the land-atmosphere system in the Nzoia region.

To fix the issue of the Nzoia River flooding over its banks, we added an overbank flow routing option to WRF-Hydro. This makes the daily discharge better shown based on Nash-Sutcliffe efficiency and Kling-Gupta efficiency (from -2.69 to 0.30% and -0.36% to 0.63%, respectively).

Changing grassland and cropland into savannas, woody savannas, and evergreen broadleaf forests in three computer simulations lets us figure out how three different strategies for planting trees might affect the climate in different parts of the world.

In all three cases, the afforestation-induced decrease in soil evaporation is greater than the afforestation-induced increase in plant transpiration, thus increasing sensible heat flux and triggering a localized negative feedback process leading to more precipitation and more runoff.

This effect is more pronounced with the woody savannas experiment, with 7% less evapotranspiration, but 13% more precipitation, 8% more surface runoff, and 12% more underground runoff predicted in the Nzoia basin. This study demonstrates a potentially large impact of afforestation on regional water resources, which should be investigated in more detail for policy-making purposes.