The expansion of rainfed grain production can generate spontaneous hydrological changes that reduce climate sensitivity

Abstract

As global warming intensifies climatic extremes, the need to understand their effects on farming systems, particularly under rainfed conditions, grows. During the last three decades the Argentine Pampas, a major global grain exporter, hosted an unprecedented expansion of cultivation under unirrigated and undrained conditions. Simultaneously, the extreme flatness and lack of water infrastructure favored groundwater level raises where agriculture expanded. However, the effect of climate extremes and elevated water tables buffering droughts but increasing flooding risk on the sensitivity of regional grain production remains unknown. Based on agronomic, weather, water table, and remote sensing data, we analyzed the production response to dry, wet, and flooded periods over the last 35 years, and to the ongoing water table raises during the last 15 years, focusing on sown and effective harvested area (harvested/sown area) and yield. Soybean and maize production increased 5.9 and 3.3-fold, respectively, as a result of area and yield growths. On average, droughts decreased production (−25 % for soybean and −14 % for maize) and wet periods increased it (+14 % for soybean and +17 % for maize) through their effects on yields and effective harvested area. Floods reduced production (−8 % for soybean and −10 % for maize) by decreasing sown and effective harvested area, leaving yields unaffected. As water tables rose, a positive yield effect during drought was detected, with counties with shallow water tables (< 3 m depth) halving yield cuts during dry years. Lacking water infrastructure, this South American grain belt is currently matching the annual production variability levels observed under intense irrigation and drainage in North America. The unexpected water table level raises of the Pampas had an overall positive effect on grain production, with flood disruptions being more than compensated by drought buffering. This balance may change in the future, calling for a deeper understanding of these complex relationships between climate, hydrology and agriculture.