Carbon_WaterTrade-Off

Learning Objectives

  • Understand the effects of forests on water supply.

  • Explore the idea that increasing forest cover doesn't always equate to better water availability globally.

Effects of Forests on Water Supply

Positive Effects

  • Alter Soil Properties: Improve physical properties of forest soil, increasing infiltration rate.

  • Soil Erosion Prevention: Forests help in reducing soil erosion.

  • Water Storage: Increase in soil water storage capacity.

  • Flood Mitigation: Reduce peak flood events.

  • Low Flow Sustenance: Sustain low flow in water systems.

  • Improve Water Quality: Forests assist in maintaining and improving water quality.

  • Moisture Supply: Supply moisture to the atmosphere, contributing to precipitation.

  • Temperature Regulation: Help regulate air temperature and moisture levels.

Negative Effects

  • Higher Water Usage: Forests, especially evergreen trees, utilize more water than other vegetation, which can lead to reduced stream flow.

  • Drought Exacerbation: Forest cover can worsen drought conditions by increasing water demand.

Case Study: Kathmandu Post Report

  • Report on air quality, news topics in Kathmandu, highlighting environmental issues such as the impact of tree planting and tourism.

  • Plants such as pine trees have been criticized for their effect on soil and local livelihoods.

Water Shortage in Nepal

  • Photos indicate water shortages during the dry season in Nepal, showcasing the tangible effects on local communities.

The Forest-Water-Climate Connection

  • CO2 and Water Loss: Discusses the trade-off between carbon gain and water loss in forest ecosystems.

  • Gross Primary Production (GPP): Relates GPP to the amount of carbon fixed through photosynthesis, emphasizing the balance between productivity and water use.

Water Balance at Watershed Level

  • Water Balance Equation: P = ET + S + R

    • P: Precipitation

    • ET: Evapotranspiration

    • S: Soil water storage

    • R: Runoff

Coupled Carbon and Water (CCW) Modeling

  • GPP and ET Modeling: Equation involving climate factors like Incident Photosynthetically Active Radiation (IPAR), Temperature (T), and Vapor Pressure Deficit (D).

  • Forest Factors: Includes data from remote sensing about Forest Photosynthetically Active Radiation (FPAR) and efficiency parameters.

Global Evapotranspiration Data

  • High range of global evapotranspiration (ET) observed in various areas.

  • Total global ET estimated at 57,000 km3 based on MODIS data.

Coupling CCW with WaSSI Model

  • WaSSI Elements: Incorporates factors affecting water supply and stress including NDVI, VPD, soil attributes, and land cover.

Study Area: Upper Hanjiang River Basin, China

  • The WaSSI model is utilized for modeling water supply at the watershed scale in this significant water source area.

  • The basin has seen considerable vegetation growth, which might influence water supply dynamics.

Significant Greening Observations

  • Analyzes changes in land cover from 2001 to 2018, with trends illustrating increases in cropland, forest, shrubland, and grassland.

Modeling Results

  • Reporting of mean annual water yield (WY) and its trends over time, noting the combined effects of greening and climate factors.

Conclusions

  • Impact on Streamflow: Vegetation greening reduced streamflow by significant amounts, challenging future water supply plans.

  • Drought Severity: Increases in greening could lead to worse droughts as climate change progresses.

  • Management Strategies Needed: Emphasizes the need for active watershed management to mitigate the water supply issues associated with increased vegetation.

Acknowledgements

  • Contributors acknowledged for their research efforts and funding sources highlighted, including NASA and NSF.

  • References to relevant scientific papers and their findings on vegetation trends and hydrological impacts.