Grasslands are the dominant, natural biome in semi-arid environments where precipitation is <600 mm/yr. Understanding the hydrologic processes which contribute to drought-tolerance in grasslands is critical given an increase in droughts. Few studies have evaluated condensation’s role as an augmenting source of water in grassland ecosystems. The objective of this research was to quantify the volumes of water captured through night-time condensation by prairie grasses relative to the total water budget, and identify key microclimatic drivers. The approach used was to compare morning and evening weight changes among pots with (1) soil but no grasses (evaporative CONTROLs – 6 reps), (2) pots with grasses without night-time covers so that condensation could occur (COND treatment – 9 reps), and (3) pots containing grasses that were tightly enclosed each night with a plastic cover that precluded dewfall (LID treatment – 9 reps). Experiment 1 monitored condensation while the grasses were embedded within a 30 cm high canopy using two prairie species (Panicum virgatum) and Pennisetum setaceum) and Experiment 2 (ESA SEEDS / Doris Duke CSP Undergraduate Scholar) compared rates with and without a surrounding grass canopy. Supplemental data were collected on soil water contents, temperature, RH, and plant foliar biomass.
Results/Conclusions
Condensation resulted in significant increases in daily water budgets. In Exp1, the CONTROLs lost an average of -72.7 ml/day (+/- 25.5 SD) due to evaporation alone and only gained 0.6 ml/night (+/- 0.9 SD) from nightly condensation across the 17d study. The daily water loss to evapotranspiration was ~twice that in the COND treatment, at -150.2 ml/day (+/-25.5). 52% of the daily loss in the COND pots was due to transpiration alone. Dewfall rate in the CONDS was +14.7 ml/night (+/- 9.6 SD; n=9), but reached a maximum of 43 ml/night. These gains represented an average +23.3% (+/- 15%) replacement of the same day’s transpiration losses. The LID pots lost 141.0 ml (+/- 22.2)/day, not statistically different from the COND treatment but also consistently lost -3.3 (+/-2.3) ml/night. COND pots had higher soil moisture contents. Microclimate was critical - RH was higher within the canopy, reducing the dew point, and resulted in condensation gains, as compared to night-time losses without a canopy. Scaling up, grassland ET accounts for 4.54 +/- 0.75 mm/day or 694.6mm/season but condensation can return 67.3 mm or more per season. We conclude that condensation is a critical but undervalued supplemental source of water to grassland ecosystems.