Drought self-propagation refers to the process by which a drought sustains itself over time or spreads across space, often through feedback mechanisms that reinforce dry conditions. Soil moisture (SM) anomalies—deviations from normal soil moisture levels, particularly when soil becomes unusually dry (negative anomalies)—play a crucial role in driving this process. Below, I’ll explain how SM anomalies affect drought self-propagation, drawing insights from key research papers, and provide links for further exploration.
1. Feedback Loops: How SM Anomalies Sustain Drought
When soil moisture is low, it triggers a land-atmosphere feedback loop that can perpetuate drought conditions. Here’s how it works:
- Reduced Evapotranspiration: Dry soil releases less water vapor into the atmosphere because there’s less moisture available for plants and evaporation. This process, called evapotranspiration, cools the surface and adds moisture to the air.
- Warmer, Drier Air: With less evaporative cooling, surface temperatures rise, and the air becomes drier.
- Less Rainfall: Warmer, drier air reduces the chances of cloud formation and precipitation, keeping the soil dry and reinforcing the drought.
This creates a self-sustaining cycle: dry soil leads to less rain, which keeps the soil dry. This feedback is especially strong in regions where soil moisture strongly influences local weather, like semi-arid areas.
- Research Insight: Hanel et al. (2018) studied European droughts and found that soil moisture deficits amplify drought through this feedback. In summer, when evapotranspiration is typically high, low soil moisture leads to higher temperatures and lower humidity, reducing precipitation further.
- Source: Hanel, M., et al. (2018). “Soil Moisture Drought in Europe: A Compound Event of Precipitation and Potential Evapotranspiration on Multiple Time Scales.” Journal of Geophysical Research: Atmospheres. Link
2. Persistence: SM Anomalies Make Droughts Last Longer
Negative SM anomalies can prolong droughts by making recovery harder, even when rain returns. Dry soil has a lower capacity to hold water, so rainfall often runs off instead of soaking in to replenish moisture levels. This “memory” effect means drought conditions can linger for weeks or months.
- Why It Matters: Even normal rainfall might not break the drought if the soil remains too dry to recover effectively.
- Research Insight: Seneviratne et al. (2010) highlight that soil moisture acts as a memory in the climate system, influencing weather patterns over extended periods. Dry soil alters atmospheric conditions, sustaining drought by reducing precipitation chances.
- Source: Seneviratne, S. I., et al. (2010). “Investigating soil moisture–climate interactions in a changing climate: A review.” Earth-Science Reviews. Link
3. Spatial Spread: How SM Anomalies Propagate Drought
Droughts don’t just stay in one place—they can spread, and SM anomalies help drive this spatial propagation. Dry soil in one area can affect neighboring regions through:
- Atmospheric Teleconnections: Dry conditions can change wind patterns or moisture transport, reducing rainfall downwind.
- Hydrological Connections: Low soil moisture can reduce groundwater or streamflow, impacting downstream areas.
For example, a dry patch of land can “export” drought conditions by altering local weather or water availability elsewhere.
- Research Insight: Van Dijk et al. (2013) modeled drought propagation in Australia’s Murray-Darling Basin. They found that soil moisture deficits in one area led to reduced river flows and soil moisture downstream, spreading the drought across the region.
- Source: Van Dijk, A. I. J. M., et al. (2013). “The Millennium Drought in southeast Australia (2001–2009): Natural and human causes and implications for water resources, ecosystems, economy, and society.” Water Resources Research. Link
4. Amplification by Climate Change
Rising temperatures due to climate change make negative SM anomalies more frequent and severe. Warmer air increases evaporation, drying soil even when rainfall isn’t below average. This enhances drought self-propagation by tipping the balance toward persistent dry conditions.
- Research Insight: Dai (2013) notes that warming-driven increases in evapotranspiration are decoupling soil moisture from precipitation. This means droughts can self-perpetuate more easily as soil dries out faster under higher temperatures.
- Source: Dai, A. (2013). “Increasing drought under global warming in observations and models.” Nature Climate Change. Link