Bursts of rain may deal a double whammy to soil moisture, nitrogen

Two new studies, published in Nature and Nature Geoscience, have found that as climate change concentrates rainfall into fewer, more intense events, the land below may face a double whammy. In their May 13 paper in Nature, Dartmouth College, U.S., researchers concluded that more concentrated rain can reduce the amount of water stored in soils and aquifers across all global climates. This is because heavy bursts of rain can exceed the soil’s infiltration capacity, leading to an “infiltration excess” where water pools on the surface.

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Because these intense events are typically separated by longer dry intervals, the pooled water evaporates instead of recharging deep soil layers. Their work suggests that the drying effect of precipitation concentration is roughly equal in magnitude to the wetting effect of increasing total annual rainfall, effectively neutralising potential gains in the availability of water in many regions.

In their May 25 paper in Nature Geoscience, researchers from across China reported a specific rainfall threshold of around 700 mm/year that seemed to determine how soil retains nitrogen. Below this threshold, the soil generally retains nitrogen because a limited amount of water restricts the movement of nutrients and encourages competition between plants and microbes. But above 700 mm, more moisture encourages leaching, leading to significant nitrogen loss.

These studies together suggest concentrated rainfall could blur the distinction between wetter and drier rainfall regimes. If a region receives 600 mm of rain in concentrated bursts rather than in steady intervals, the downpours could wash more nitrogen out of the soils than the annual rainfall total alone would suggest. An ecosystem could thus lose more nutrients while also suffering water shortage between storms.

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According to the Nature study, 27% of the world’s population will face abnormally dry conditions due to concentrated rainfall alone even if total rainfall doesn’t decrease. A growing body of research suggests the southwest monsoon is itself becoming more episodic, with more seasonal rainfall falling in intense downpours separated by longer dry intervals.

This, together with the Nature Geoscience study’s findings, suggests annual total rainfall alone may no longer capture how ecosystems respond to changing rainfall patterns. Intense rainfall events can wash away nitrogen, a nutrient crucial for plant growth, before it can be used, while the subsequent dry period can quicken water loss by evaporation. Traditional climate models that focus primarily on mean annual totals may therefore underestimate the risk of land degradation.