Abstract
Wetlands are the largest environmental sources of methane, and
interannual changes in wetland methane fluxes explain most of the
variability in the global flux. Despite their importance, global wetland
maps, a key component of methane models, are inaccurate for at least
three reasons: (1) Their temporal variability is poorly suited for
static maps; (2) Optical remote sensing cannot penetrate foliage, making
water hard to identify; and (3) satellites cannot resolve their
fine-scale features. Furthermore, small, unmapped water bodies may emit
methane disproportionately to their size due their shallow depths
inhibiting bacterial oxidation from the water column and their large
perimeter: volume ratios, which introduce the potential for organic
matter input and plant-mediated fluxes from shorelines. However, in
boreal regions, there is conflicting evidence on the effects of water
body size on methane and carbon dioxide fluxes. Here, we measure methane
emissions in lakes and wetlands in an Arctic-Boreal delta and compare to
open water and vegetated area with the goal of improving methane
emission estimates in this region. We expect small, shallow, and
vegetated wetlands to produce more methane than those bordering deeper
lakes. To test this hypothesis, we map wetlands in the Peace-Athabasca
Delta, a 5,000 km2 inland delta in northern Alberta, Canada containing
abundant open and vegetated wetlands. We use airborne remote sensing
from three sources: (1) High-resolution (<5 cm pixel) unmanned
aerial vehicle (UAV) imagery, (2) Coincident L-band synthetic aperture
radar (SAR) from NASA’s UAVSAR airborne imaging system, and (3) 2017
AirSWOT Ka-band interferometric SAR with color-infrared imagery. With a
wavelength of 23.8 cm, UAVSAR L-band returns are ideal for mapping
vegetated wetlands due to double-bounce backscatter between vegetation
and the water surface. Combining two field campaigns of flux chamber gas
sampling from over twenty lakes, walked shoreline surveys, and over 70
thousand UAV photos, we present a collection of wetland maps and a
methodology for efficiently mapping them from UAV. We then upscale
methane and carbon dioxide emissions to the scale of the delta and
compare to existing estimates. These results will help improve
greenhouse gas emission estimates for boreal zone wetlands.