Importance of timing: Vulnerability of semi-arid rangeland systems to increased variability in temporal distribution of rainfall events as predicted by future climate change

Natural resources of semi-arid rangelands in the (sub-)tropics are major pillars of food security and livelihoods to millions of people. Besides increasing pressure due to human population growth, high spatial and temporal variability in precipitation leading to highly erratic production of primary biomass has high potential to render the exploitation of ephemeral concentrations of these resources unsustainable. While future climate models predict a general decrease in precipitation for many semi-arid rangelands, the increase in variability of rainfall and its dynamics are important aspects in assessments of climate change impact that only recently gained scientific attention. Building on a spatially-explicit, agent-based model of free-ranging livestock herds in semi-arid rangelands (RaMDry) that accounts for the metabolic energy budget of foraging animals, we assessed the vulnerability of rangelands and livestock production systems as a result of the effects of ongoing changes in precipitation and its variation, as well as its temporal distribution. Based on a coupled-reservoir model conceptually relying on a two-stage cascaded linear reservoir, we developed, implemented and validated a refined forage production module in RaMDry to predict primary productivity of semi-arid grassland and its impacts on livestock production from daily rainfall. Our results show a highly significant effect of increased temporal diffusion of precipitation events on the ecosystem and its productivity. The anticipated diminution in total precipitation and increased inter-annual variation indicated no severe aggravation of the general situation for livestock keepers in terms of system productivity. However, alterations in timing of individual precipitation events resulted in substantial temporal reduction of forage resources in terms of their quantity and quality and major negative impact on livestock body weights. Our findings shed light on the profound consequences of inter-seasonal dry spells as one important aspect of climatic changes on livestock production systems in (sub-)tropical, semi-arid rangelands.