Abstract
Groundwater level (GWL) and depth to water (DTW) are related metrics aimed at characterizing groundwater-table positions in peatlands, and two of the most common variables collected by researchers working in these ecosystems. While well-established field techniques exist for measuring GWL and DTW, they are generally difficult to scale. In this study, we present a novel workflow for mapping groundwater using orthophotography and photogrammetric point clouds acquired from unmanned aerial vehicles. Our approach takes advantage of the fact that pockets of surface water are normally abundant in peatlands, which we assume to be reflective of GWL in these porous, gently sloping environments. By first classifying surface water and then extracting a sample of water elevations, we can generate continuous models of GWL through interpolation. Estimates of DTW can then be obtained through additional efforts to characterize terrain. We demonstrate our methodology across a complex, 61-ha treed bog in northern Alberta, Canada. An independent accuracy assessment using 31 temporally coincident water-well measurements revealed accuracies (root mean square error) in the 20-cm range, though errors were concentrated in small upland pockets in the study area, and areas of dense tree covers. Model estimates in the open peatland areas were considerably better.
Highlights
Peatlands are terrestrial wetland environments where the net production of organic matter exceeds its decomposition rate, leading to the accumulation of partially decomposed vegetation known as peat [1]
The position of groundwater table plays a key role in ecosystem functioning, controlling the hydrological flow direction/discharge, rates of peat accumulation, and the chemical processes governing nutrient cycling and greenhouse gas (GHG) flux [4,5,6]
While groundwater tables are commonly characterized as both ground-water level (GWL)—defined as the height of groundwater above mean sea level—and depth to water (DTW)—the depth to groundwater relative to local terrain level—the two metrics are clearly related
Summary
Peatlands are terrestrial wetland environments where the net production of organic matter exceeds its decomposition rate, leading to the accumulation of partially decomposed vegetation known as peat [1]. The position of groundwater table plays a key role in ecosystem functioning, controlling the hydrological flow direction/discharge, rates of peat accumulation, and the chemical processes governing nutrient cycling and greenhouse gas (GHG) flux [4,5,6]. We will use groundwater as a general term, but return to the more specific metrics as we proceed. Both variables, GWL and DTW, are key factors in understanding hydrological processes and estimating GHG (CO2, N2O and CH4) emissions in peatlands, and are among the most commonly sought measurements for researchers working in these ecosystems [7]
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