Severe flooding has slashed global rice yields in recent decades, threatening food security for billions of people who depend on the grain. The losses amounted to approximately 4.3%, or 18 million tons of rice per year, between 1980 and 2015, according to research from Stanford University published November 14 in Science Advances .
Damage has accelerated since 2000 due to more frequent extreme floods across major rice-growing regions, a trend likely to be exacerbated by climate change, the researchers found.
Scientists and farmers have long understood that rice yields suffer during droughts. The new study adds to evidence of that damage, finding that droughts reduced rice yields by an average of 8.1% per year during the 35-year study period. But it also highlights a less studied threat. Although rice crops benefit from shallow flooding during early growth stages, too much water for too long can be devastating.
"While the scientific community has focused on damage to rice yield due to droughts, the impacts of floods have not received enough attention," said Steven Gorelick , the study's senior co-author and a professor of Earth system science in the Stanford Doerr School of Sustainability . "Our research documents not only areas where rice yields have suffered due to past flooding, but also where we can anticipate and prepare for this threat in the future."
Defining 'rice-killing floods'
The study clearly defines for the first time what makes a flood deadly for rice crops, said lead study author Zhi Li , who worked on the research as a postdoctoral fellow in Gorelick's lab at Stanford and recently joined the faculty of the University of Colorado Boulder.
A full week underwater during the plant's growth cycle is the key threshold. "When crops are fully submerged for at least seven days, most rice plants die," Li said. "By defining 'rice-killing floods,' we were able to quantify for the first time how these specific floods are consistently destroying one of the most important staple foods for more than half of the global population."
To assess the damage from past droughts and floods, the scientists used information about rice growth stages, annual global rice yields, a global dataset of droughts and floods since 1950, a model of flood dynamics, and a simulation of soil moisture levels in the world's major rice-growing basins over time.
The new analysis suggests that in the next few decades, the most extreme week of rainfall across the world's major rice-growing river basins could bring 13% more rain compared to the average for those regions during the 1980-2015 baseline period.
Flood-resistant varieties can help
Greater adoption of flood-resistant rice varieties could help to avert future losses, especially in regions where the crop is at highest risk. These include the Sabarmati Basin in India, which experiences the longest rice-killing floods, as well as North Korea, Indonesia, China, the Philippines, and Nepal, where the impact from rice-killing floods has grown the most in recent decades. The largest losses overall have been in North Korea, East China, and India's West Bengal.
The research also uncovered exceptions such as India's Pennar Basin, where floods appear to enhance rice yields. According to the authors, this may be explained by such regions' hot, dry climates, which allow stagnant floodwater to evaporate quickly.
For Gorelick and Li, the latest findings underscore the importance of understanding how rice yields respond to floods, droughts, heat waves, and cold stress individually and in sequence. Previous research has shown that sequences of weather whipping from drought to flood and back again result in nearly twice the rice yield loss compared to individual flood or drought events alone. According to the authors, "How these combined effects can be mitigated remains a major challenge."
Additional co-authors not mentioned above include Lorenzo Rosa, who is affiliated with the Department of Earth System Science in the Stanford Doerr School of Sustainability and the Department of Global Ecology at the Carnegie Institution for Science. The research was supported by a Dean's Postdoctoral Fellowship awarded to Li by the Stanford Doerr School of Sustainability.
