Evaluation of Biofuels as an Alternative Energy Source

Overview

There is increasing interest in the use of biofuels as an alternative source of fuel, both globally and within the US. However, it is critical that any potential negative impacts of biofuel production on the environment are understood before large scale developed of biofuel production. Biofuel production can negatively impact water resources by depleting resources through irrigation and by contaminating resources from fertilizers.

Traditional biofuels include corn in the US and sugarcane in Brazil. Biofeedstocks being proposed for second generation biofuel production include perennial grasses, such as switch grass and miscanthus and annuals such as sugar cane and energy cane, and oil seed crops. Our research in areas of native vegetation in the High Plains can provide valuable insights into potential impacts of perennial grasses on water resources. Unsaturated zone pressure monitoring indicates that perennial grasses and shrubs result in zero recharge in (semi)arid regions. Converting annual cropland to perennial grasses for biofuel production would therefore decrease recharge and also resultant surface water flows that are dependent on groundwater discharge.

Figure 1. Time series of matric potential head monitored in the unsaturated zone and daily precipitation at a rangeland site in the Texas High Plains. (a) Different colored lines represent heat dissipation sensors installed at varying depths. Increased precipitation in 2004 resulted in deep percolation to at least 3 m; however, roots of perennial grasses and scattered shrubs removed the excess water by evapotranspiration over the following 2 years (Scanlon et al., 2005).

Soil profiling also provides a valuable approach to assessing impacts of different types of biofuels on nutrient cycling. Previous studies in the Texas High Plains also showed large scale increases in soil water nitrates as a result of mineralization and nitrification associated with the beginning of cultivation. Elevated nitrate in the unsaturated zone has resulted in groundwater contamination with nitrate in this system.

Proposed Future Work Related to Biofuels

Our future research will evaluate impacts of biofuel production on water and nutrient cycling in semiarid regions by soil profiling systems that are appropriate analogs to proposed biofuels. These soil profiles will provide information on impacts of biofuel production on recharge and nutrient availability/transport and effects on groundwater quantity and quality.

Remote sensing using satellite data and energy balance models such as METRIC or TSEB will be used to develop regional estimates of evapotranspiration (ET) in areas of biofuel production. Quantifying ET in these settings will provide important information on consumptive use for biofeedstock production. A unique aspect of our proposed studies is ground-referencing of satellite based ET estimates using large aperture scintillometers (figure 2). Ground-referencing satellite data is critical to increase the reliability of the satellite based estimates. This program will build on our existing remote sensing ET work.

Figure 2.  Large aperture scintillometer monitoring ET in an irrigated cotton field.

In areas where biofeedstocks are being irrigated with groundwater, the GRACE satellite will provide useful information on groundwater depletion in remote areas where groundwater level monitoring is minimal. This approach has proved useful in the US High Plains (figure 3) and could be used in many developing countries. This approach will build on our existing work on using GRACE data for water resources management.

Figure 3.