Study finds that bioenergy crops could have a significant impact on the hydrologic cycle of a region
A new study led by Praveen Kumar at the University of Illinois at Urbana-Champaign, details the links between the hydrologic cycle and large-scale land conversion for the cultivation of bioenergy crops, both now and as growing conditions change in the future. The findings, published this week in the journal Proceedings of the National Academy of Sciences, highlight the potential for bioenergy crops having a significant impact on hydrologic cycle of a region.
Phong et al. used a mechanistic multi-layer canopy-root-soil model to (i) capture the eco-physiological acclimations of bioenergy crops under climate change, and (ii) predict how hydrologic fluxes are likely to be altered from their current magnitudes. They used observed data and Monte Carlo simulations of weather for recent past and future scenarios to characterize the variability range of the predictions.
Under present weather conditions, miscanthus and switchgrass utilized more water than maize for total seasonal evapotranspiration by approximately 58% and 36%, respectively. Projected higher concentrations of atmospheric CO2 (550 ppm) is likely to decrease water used for evapotranspiration of miscanthus, switchgrass, and maize by 12%, 10%, and 11%, respectively. However, when climate change with projected increases in air temperature and reduced summer rainfall are also considered, there is a net increase in evapotranspiration for all crops, leading to significant reduction in soil-moisture storage and specific surface runoff. These results highlight the critical role of the warming climate in potentially altering the water cycle in the region under extensive conversion of existing maize cropping to support bioenergy demand.—Phong et al.
Miscanthus and switchgrass have a very different above-ground foliage structure from corn—more surface area and much denser growth. This is good for maximizing the amount of biomass that an acre of land can produce, Kumar said, but it also increases water use. Miscanthus and switchgrass intercept light and rain differently from corn and lose more water through transpiration, causing them to pull more water from the soil. The result of large-scale adoption would be a reduction in soil moisture and runoff, but an increase in atmospheric humidity.
All these together account for changes in hydrology, just from land-use change. Then, if you impose further—higher carbon dioxide in the atmosphere, higher temperatures and changes in rainfall patterns—they add more modulation to the water-use pattern.—Praveen Kumar
Using the predictive model, the researchers found that net water use will increase further as a result of rising temperatures and carbon dioxide. Higher levels of carbon dioxide alone make the plants more water-efficient, since their pores are open less time to absorb carbon dioxide. However, rising temperatures counteract this effect, as the plants will transpire more while their pores are open, losing more water than they save. This additional water loss compounds the increase in water usage from land conversion.
In the US Midwest, rainfall should remain sufficient to meet water demand, according to Kumar. However, areas that rely on irrigation could find they have less water to meet higher demands, which could increase the net cost of large-scale land conversion and put pressure on already stressed water resources.
If we’re going to solve energy problems through bioenergy crops, there are collateral issues that need to be considered. Water is a significant issue. It’s already a scarce resource across the globe, and the need for it is only going to increase. The cost of that should be factored in to the decision making.—Praveen Kumar
Graduate student Phong V.V. Le and former postdoctoral researcher Darren Drewry, who is now at the Max Planck Institute in Germany, are co-authors of the paper. The Vietnam Education Foundation also supported the research.
Phong V. V. Le, Praveen Kumar, and Darren T. Drewry (2011) Implications for the hydrologic cycle under climate change due to the expansion of bioenergy crops in the Midwestern United States. PNAS doi: 10.1073/pnas.1107177108