Warming changes forest water yield by catchment type
Across 21 North American headwater catchments, modest warming can raise or cut streamflow depending on elasticity, with conifers often yielding more water.
Source
Changing forest water yields in response to climate warming: results from long-term experimental watershed sites across North America
What they did
Authors compared cool versus warm 5-year periods in 21 forested headwater catchments at 12 sites, using Budyko dryness/evaporative indices and an elasticity metric of how evaporative index tracks dryness as climate warms.
What they found
Low-elasticity catchments (e<1) more often increased water yield with warming (r²=0.34). With ΔT<1.5°C, deviation fell as elasticity declined (r²=0.91); with ΔT>1.5°C the opposite (r²=0.81). Conifer catchments generally increased yield; alpine sites warmed little but dried >10% in precipitation.
The limits
What it doesn't show
Catchments were not a random sample of North American forests. Extreme wet/dry years were removed. The study cannot isolate vegetation change from snowmelt or stomatal responses.
Key terms
- Water yield
- Streamflow leaving a headwater catchment, a water-supply metric.
- Budyko curve
- The expected relationship between dryness index (DI) and evaporative index (EI).
- Elasticity (e)
- How much EI changes when DI changes with warming; low e means yield is more labile.
- Dynamic deviation (d)
- Shift off the Budyko curve; negative d means higher water yield than expected.
- Dryness index (DI)
- Potential evaporation relative to precipitation; DI>1 is water-limited.
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Common questions
Does warming always dry streams?
No. Some conifer and low-elasticity catchments increased yield, especially with modest warming.
What is hydrologic elasticity here?
Whether evaporative index keeps pace with a changing dryness index.
Why might conifers yield more water when warmer?
Possible stomatal control of transpiration or extra meltwater.
What about alpine sites?
Small warming (<1.5°C) but large precipitation declines (>10%).
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