Carbon&WaterTrade-Off

Carbon and Water Trade-Off

Learning Objective

  • Understand the effects of forests on water supply.

  • Question: Is it universally true that more forests equate to better water supplies globally?

Two Contrasting Effects of Forests on Water Supply

Positive Effects

  • Alteration of Soil Physical Properties:

    • Increases infiltration rates.

    • Reduces soil erosion.

    • Enhances soil water storage capacity.

  • Flood Regulation:

    • Mitigates peak flooding.

    • Sustains lower flows, maintaining water availability in dry periods.

  • Water Quality Improvement:

    • Filters pollutants and enhances overall water quality.

  • Climate Regulation:

    • Supplies moisture to the atmosphere, fostering precipitation.

    • Regulates air temperature and humidity levels.

Negative Effects

  • Water Utilization by Forests:

    • Forests, especially conifers, consume more water than other vegetative types.

    • This can lead to reduced stream flows and exacerbate drought conditions.

Current Context in Kathmandu Post

  • Challenges in air quality and environmental management.

  • Criticism of specific forestry initiatives, such as pine tree planting, which may not be beneficial for local livelihoods despite increasing green cover.

Water Shortage During Dry Season in Nepal

  • Illustrates the critical issue of water availability, particularly during dry spells.

The Forest-Water-Climate Connection

  • Trade-off Dynamics:

    • Understanding that losing water can be a compromise for gaining CO2 uptake.

  • Photosynthetically Active Radiation (PAR):

    • Relationship between carbon fixation during photosynthesis and water use.

Water Balance at a Watershed Level

  • Water Balance Equation:

    • P = ET + S + R

    • Where:

      • P = Precipitation

      • ET = Evapotranspiration

      • S = Soil water storage

      • R = Runoff

Coupled Carbon and Water (CCW) Model

  • Used for modeling Gross Primary Production (GPP) and Evapotranspiration (ET).

  • Key Variables Include:

    • Climate factors (IPAR, Temperature, Vapor Pressure Deficit).

    • Forest factors (FPAR, light use efficiency, water use efficiency).

Global Evapotranspiration

  • Measurement of Global ET highlights variances, with total global ET at 57,000 km³.

Study Area: Upper Hanjiang River Basin in China

  • Applied CCW model to assess water supply at watershed scale.

  • Notable greening in vegetation from 2001-2018 affecting water dynamics.

Modeling Results

  • Observations of water yield (WY), evapotranspiration (ET), and precipitation (P) trends emphasizing interrelations.

Conclusions

  • Vegetation Impact:

    • Significant reduction in streamflow linked to greening efforts.

    • Predicted climate changes could heighten drought severity.

  • Management Recommendations:

    • Importance of active watershed management to balance vegetation impact on water supply.

    • Strategies include thinning, species selection, and encouraging natural regeneration of vegetation.

Acknowledgements and Sources

  • Contributions from esteemed researchers and funding organizations (NASA, NSF, USDA).

  • Key publications detailing the studies conducted, methodologies, and findings related to carbon and water interactions in vegetation.