Water vapor transport through bioenergy grass residues and its effects on soil water evaporation

Miscanthus is a productive perennial grass that is suitable as a bioenergy crop in "marginal" lands (e.g., eroded soils) with low water holding capacity. However, little is known about the impact of miscanthus residues on vapor transport and soil water budgets. Laboratory experiments were conducted to measure the vapor conductance through miscanthus residues and its effect on soil water evaporation. The ranges for the length, width, and thickness of residue elements were 0.5-9.0, 0.1-0.5, and 0.1-0.5 cm, respectively. Average residue areal, bulk, and skeletal densities were 0.88 kg m-2, 24 kg m-3, and 1006 kg m-3, respectively, giving a porosity of 0.98 m3 m-3. A power function described the decrease in conductance with increasing residue load. The corresponding conductance for a residue load of 0.88 kg m-2 was 1.6 mm s-1. During the first days of a 60-day drying experiment, cumulative evaporation showed logarithmic decay with increasing residue load. Conversely, cumulative evaporation during the last days of the study showed little difference between treatments. Measurements indicated that there is a "critical" residue load (similar to 1.0 kg m-2) beyond which evaporation no longer decreases appreciably when the soil is under the stage 1 evaporation regime. Results suggest that soil water conservation in marginal lands may be accomplished by maintaining moderate amounts of bioenergy grass residue covering the soil. Determining "critical" loads for different residue types is a knowledge gap that merits further research. Residues had skeletal density of 1006 kg m-3 and porosity of 0.98 m3 m-3.Residue conductance to water vapor decreased with increased areal density.Evaporation decreased with increased residue load during the first 15 days of drying.After 60 days of drying, treatments showed little difference in cumulative evaporation.