G. Z. Sass

Vegetation class dependent errors in lidar ground elevation and canopy height estimates in a boreal wetland environment

An airborne scanning light detection and ranging (lidar) survey using a discrete pulse return airborne laser terrain mapper (ALTM) was conducted over the Utikuma boreal wetland area of northern Alberta in August 2002. These data were analysed to quantify vegetation class dependent errors in lidar ground surface elevation and vegetation canopy surface height. The sensitivity of lidar-derived land-cover frictional parameters to these height errors was also investigated. Aquatic vegetation was associated with the largest error in lidar ground surface definition (+0.15 m, SD = 0.22, probability of no difference in height P < 0.01), likely a result of saturated ground conditions. The largest absolute errors in lidar canopy surface height were associated with tall vegetation classes; however, the largest relative errors were associated with low shrub (63%, –0.52 m, P < 0.01) and aquatic vegetation (54%, –0.24 m, P < 0.01) classes. The openness and orientation of vegetation foliage (i.e., minimal projection of horizontal area) were thought to enhance laser pulse canopy surface penetration in these two classes. Raster canopy height models (CHMs) underestimated field heights by between 3% (aspens and black spruce) and 64% (aquatic vegetation). Lidar canopy surface height errors led to hydraulic Darcy–Weisbach friction factor underestimates of 10%–49% for short (<2 m) vegetation classes and overestimates of 12%–41% for taller vegetation classes.

Digital Terrain Analysis Approaches for Tracking Hydrological and Biogeochemical Pathways and Processes in Forested Landscapes

Digital terrain analysis (DTA) comprises a set of tools that use digital elevation models (DEMs) to model earth surface processes at a range of scales. DEM and its derivatives are part of a larger set of digital terrain models (DTMs) used in various fields to model the flow of energy and materials across surfaces. The ubiquity of DTMs in the hydrologist’s toolkit has led to the widespread use of terrain attributes such as slope and upslope contributing area to characterize the way water and associated nutrients move across landscapes. Algorithms to compute terrain attributes are now programmed into all commercial Geographic Information System (GIS) software (e.g., ArcGIS, Idrisi) and with a push of the button users can map patterns of potential surface hydrological flows. While the derived layers always look visually stimulating, field hydrologists have often raised the question: are DTMs often merely interesting spatial patterns with not much relevance to predicting actual hydrological behavior? This synthesis critically answers this question by discussing the relevance of DTA for practicing forest hydrologists in the twenty-first century.

Mapping hydrologically sensitive areas on the Boreal Plain: a multitemporal analysis of ERS synthetic aperture radar data

Characterizing the spatial and temporal dynamics of hydrologically sensitive areas (HSAs) is vital to the effective management of the boreal forest. HSAs are defined as saturated or inundated areas that, if disturbed, might result in a significant change in the movement of water, nutrients and biota within landscapes. This study presents a remote sensing technique that uses archived European Remote Sensing Satellite (ERS)‐1 and ERS‐2 synthetic aperture radar (SAR) images to monitor HSAs in the Willow River watershed (1030 km2) on the western Boreal Plain of Canada. ERS images were used to generate a probability of HSA occurrence map for a 10‐year period (1991–2000). This map revealed the complexity of HSAs on the western Boreal Plain, where some areas remained consistently dry or wet whereas others were dynamic, transitioning from dry to wet and vice versa. A probability map of HSA occurrence provides spatial and temporal information previously unavailable for this region that may expand our understanding of the hydrological behaviour of drainage basins and serve as a planning tool for land management decisions.

Bird’s-Eye View of Forest Hydrology: Novel Approaches Using Remote Sensing Techniques

Without question, better scientific understanding of hydrological processes in forested environments will be a product of the synergistic play of theory and data. Remote sensing (RS) from satellite and airborne platforms, along with many other sources of hydrological data such as wireless sensor arrays and ground-based radar networks, is playing and will continue to play a vital role in better understanding the hydrosphere by providing the next generation of datasets to the hydrological community. RS systems are planetary macroscopes that allow the study of ecosystems from a completely new vantage point, facilitating a holistic perspective like viewing the Earth does for astronauts.