The unusual 2018-2019 rainy seasons in western Uganda through the lens of precipitation isotopes, satellite data, and farmer observations

Abstract:

The onset and cessation of tropical East Africa’s rainy seasons are notoriously variable, presenting an ongoing challenge to food security in the region. In 2018 and 2019, one unusually late and prolonged rainy season and one unusually late but shortened rainy season threatened farming practices in many areas of western Uganda. Rain gauge, satellite, and household survey data suggest that the challenge of too much and/or ill-timed rain may be an increasing trend over the past several decades, and may also be accompanied by a shift in the distribution of wet vs. dry season rainfall (Diem et al., 2019a,b; Salerno et al., 2019). Here, we examine individual and aggregated storm events during the 2018-2019 rainy and dry seasons using daily precipitation δ¹⁸O, δ¹⁷O, δ²H, d-excess and Δ¹⁷O data collected from five study sites in western Uganda. The five sites fall along a northeast/southwest gradient, spanning the transition zone between unimodal and bimodal rainfall regimes. Each site receives precipitation sourced from continental and oceanic sources, but in different proportions. Results suggest that the δ¹⁸O, d-excess, and Δ¹⁷O of rain events occurring during the dry seasons and dry-to-wet transition periods are distinct from rain events occurring during the peak wet seasons. Preliminary data indicate that recycled continental moisture may play a substantial role in these events at some, but not all, locations. We interpret the isotopic and hydrometeorological data with the aid of household survey data and daily weather observations reported by farmers in communities near two of our precipitation collection sites, focusing especially on daily observations of winds, clouds, and storm characteristics during the anomalous wet-to-dry and dry-to-wet transitions of 2018-2019.