Climate change impacts on rainwater productivity across agricultural landscapes of Ethiopia

In this study, the spatio-temporal changes in Rainwater Productivity (RP) and its sensitivity to the changes in precipitation and temperature predicted by climate models in various climatic zones across the rainfed agricultural areas of Ethiopia were analyzed. First, the future precipitation, air temperature, and shortwave radiation from multiple GCM projections were downscaled to a 0.05°x0.05° grid resolution, considering three shared socioeconomic pathways (SSP1-2.6, SSP2-4.5, and SSP5-8.5) and three future periods: 2020-2049, 2045-2074, and 2070-2099 using the present climate (1981-2010) as a reference. Next, the reference potential evapotranspiration was computed using the FAO Penman-Monteith and the actual evapotranspiration was simulated using a daily soil water balance model. Then, the relative crop yield (i.e., the ratio of the actual and water-limited potential yield) was determined as a function of the evaporative stress index and crop yield response factor (Ky) for the two growing seasons -- the main (meher) growing season (May-Sep) and the shorter (belg) growing season (Feb-May) for the present and future climates. The computed relative yield was used as a proxy for RP, under the assumption that effective rainfall is the limiting factor for crop yield. Finally, the sensitivity of RP to projected changes in precipitation and temperature was analyzed based on the one-at-a-time (OAT) approach for warmer and drier versus warmer and wetter climate scenarios.

The results show that under the present climate, the median RP (percent of the potential RP) during the Meher and Belg seasons ranges from about 52% and 34% in semi-arid climates to 93% and 45% in humid climates. The projected Meher RP in the future shows either a slight change or a decrease by up to 10% across the majority of the RFA regions under all SSPs and future periods. Conversely, the Belg season RP is likely to increase by up to 15% across the major Belg-producing regions by the end of the century. The observed changes are the combined effects of the nearly consistent but spatially variable increase in precipitation (for example up to 30% under SSP5-8.5 in the 2080s) and rising temperature (up to 5°C under SSP5-8.5 in the 2080s) over the RFA region. The OAT sensitivity analysis reveals that RP under warmer and drier climates is strongly sensitive to precipitation. However, under warmer and wetter conditions the climate sensitivity of RP is determined by the rainfall regime, i.e, in the areas with unimodal rainfall regimes, changes in RP are dominated by the changes in precipitation while in areas with strongly erratic or bimodal rainfall distribution, temperature, or both precipitation and temperature control the changes in RP. Such analyses are useful for assessing the future climate risks to crop yield due to water stress associated with the expected increases in atmospheric evaporative demand, identifying vulnerable areas across the RFA region as well as possibilities for agricultural expansion.