Extreme floods and droughts are considered to be one of the greatest challenges to society in the 21st century. Historically, they have led to the collapse of civilizations, fatalities, and economic losses. In Indiana, recent droughts, such as the drought of 2012, have affected agriculture and water supplies. Recent flooding, such as summer 2021’s disasters in downtown Bloomington, have damaged businesses and homes. Now, the prevalence of these extreme events is increasing around the world; heavy rainstorms are increasing the severity and frequency of floods, while warming temperatures and a thirsty atmosphere are making droughts more severe.
It is clear that landscapes can have a major influence on how severe these disasters are. For instance, soils that quickly become saturated with water during a rainstorm can cause more precipitation to run off into streams, rather than soaking into the ground. Also, soils that lose water quickly to the atmosphere can reduce water supplies during droughts. But, can these influences go in the other direction? Could the presence of large floods alter soil or vegetation conditions (particularly in floodplains) so that the next flood is more extreme? Or, could prolonged periods of drought cause soils to crack or blow around and lose scarce water more quickly?
One thing that worries me is that there could be positive feedback between extreme floods and droughts. In other words, extreme events could become self-reinforcing, such that an extreme flood or drought makes the next flood or drought more severe. As an example, we know that intense dryness can reduce the vegetation cover of a landscape, causing more bare soil that quickly sends rainwater away. This event could create even less vegetation via positive feedback, resulting in even more future drying. In addition, heavy rainstorms could cause hard crusts to form on the soil, meaning the next rainstorm will produce more runoff and flooding. Droughts could also make soils repel water, keeping them drier and extending the drought.
We can prepare our communities to minimize the impacts of extreme floods and droughts. One way to do this is to ensure that forests are protected. Forests allow rainwater to soak into the ground more quickly than grassy or barren lands because the water must follow paths among leaf litter and roots, thereby reducing the size of floods. Another way to diminish these effects is to keep barren soils covered by vegetation such as cover crops, which allows rainwater to soak more rapidly into the soil and reduces flooding, but still protects the soil from positive feedbacks during drought.
Further research is needed to fully understand these feedbacks and the influence they may have on extreme floods and droughts around the globe. In addition, there are a lot of questions about scales, or how far through space or time these feedbacks last. For instance, would localized feedback in a small area have an influence on drought over a larger region? Also, would immediate changes to soil conditions from a flood last until the next flood, even if it occurred months in the future? Would the same feedback occur in forests, farmlands, and urban areas?
Another possibility to explore is whether there could be negative feedback between these events, meaning that the landscape corrects itself so that a large flood could make the next flood less severe. An example of negative feedback caused by humans is building a dam and reservoir to have more control over flooding and water shortages, although these solutions often come with negative environmental consequences.
We should — and must — determine the answers to these questions in order to prepare ourselves for a future with these disastrous events.