2010年-世界发展银行全球_Climate_Change_Impacts_on_Agricultural_Water_Stress_and_Impact_Mitigation_Potential_8页_575kb

Summary of "Climate Change Impacts on Agricultural Water Stress and Impact Mitigation Potential"

Core Content

This document provides a detailed analysis of the impacts of climate change on agricultural water stress and the potential for mitigation through improved water management strategies in both rainfed and irrigated agriculture. It is a background note for the World Development Report 2010, authored by Dieter Gerten and Stefanie Rost from the Potsdam Institute for Climate Impact Research (PIK).

Main Points

1. Current Water Use in Agriculture

• Global Water Consumption: Agriculture currently consumes approximately 17,000 km³ per year of freshwater, with about 50% from cropland and 50% from pasture.
• Green and Blue Water:
  • Green water (rainfall) accounts for the majority of consumption, especially on cropland (about 7,200 km³ yr⁻¹).
  • Blue water (irrigation) makes up a smaller portion, around 1,258 km³ yr⁻¹ on cropland and 106 km³ yr⁻¹ on pasture.
  • The largest blue water consumption occurs in regions like India, Pakistan, and the lower Nile.
• Irrigation Impact: Present irrigation practices have increased global crop production by about 17% compared to rainfed conditions.

2. Water Limitation in Crop Production

• Water Limitation Measure: Water limitation is assessed using the ratio of water-limited net primary production (NPP) to water-unlimited NPP.
• Geographic Impact: Under current conditions, crop production is strongly water-limited in subtropical and inner-continental regions such as Central Asia, southern Europe, southern and eastern Africa, the Sahel, parts of the U.S., South America, and Australia.
• Irrigation Effect: In regions with irrigation, such as the southwestern U.S. and the Indian subcontinent, water limitation is significantly reduced, leading to higher productivity.

3. Mitigation Potential of Water Management Strategies

• Water Harvesting (RH): This strategy involves storing surface runoff for later use. It has a potential to increase crop production by 11% (RH25) and 30.6% (RH85) under current conditions.
• Vapor Shift (VS): This involves reducing soil evaporation and increasing plant transpiration. It can increase production by 6% (VS25) and 24.6% (VS85) under current conditions.
• Combined Strategies (VSRH): The combination of RH25 and VS25 can increase global NPP by 19%, which is nearly equivalent to the increase achieved through current irrigation practices.
• Future Climate Scenarios:
  • Under future climate conditions (2041–2070), the potential for increasing production through these strategies decreases.
  • VSRH25 yields a 12.7% increase, while VSRH85 yields a 37.3% increase, both less than current irrigation benefits.
• CO₂ Effects: Increased atmospheric CO₂ can improve plant water use efficiency, reducing water limitation and thus the potential gains from soil and water management.

4. Future Water Demand and Freshwater Gaps

• Projected Increase in Water Demand: By 2050, the global demand for water in crop production is expected to increase by ~4,500 km³ yr⁻¹, due to population growth.
• Freshwater Gaps: Even with improved water management, ~2,400 km³ yr⁻¹ of freshwater gaps are projected to remain, indicating the need for additional water resources and intensified agriculture.

5. Regional Impacts and Trade-offs

• High Population Growth Regions: Areas like South Asia will experience a disproportionate increase in water demand.
• Population Decline Regions: Regions such as the Former Soviet Union may see a decrease in water requirements.
• Land Use Trade-offs: Future expansion of cropland and intensification of existing cropland may conflict with the preservation of natural ecosystems and bioenergy plantations.

Key Information

• Model Used: The analysis is based on the LPJmL dynamic global vegetation and water balance model, using an enhanced CRU TS2.1 climate dataset and three climate models under the A2 emissions scenario.
• Management Strategies:
  • VS25 and RH25 are considered moderate and realistic scenarios.
  • VSRH85 represents a theoretical maximum potential for increasing crop production.
• Water Productivity: The goal of the study is to explore the potential for "more crop per drop" through improved water use efficiency in rainfed agriculture.
• Data Sources: The findings are supported by field experiments and data from various studies, including Rost et al. (2009) and Oweis and Hachum (2006).

Conclusion

• Despite the potential for increasing crop production through water harvesting and vapor shift strategies, the global freshwater resources may not be sufficient to meet future food demand, especially with population growth.
• Irrigation remains critical, but it is not a sustainable solution for all regions due to reliance on non-renewable groundwater.
• Efficient water use in rainfed agriculture and sustainable land use planning are essential for future food security.