Using the Soil and Water Assessment Tool to develop a LiDAR-based index of the erosion regulation ecosystem service

Abstract

Maps of ecosystem services are becoming increasingly useful for reporting on the potential impacts of human activity on the environment. However, interactions in watersheds are complex, and mapping hydrological ecosystem services (HES) requires indicators which accurately measure underlying processes. The main objective of this study was to take advantage of the Soil and Water Assessment Tool (SWAT) and Light detection and ranging (LiDAR) data to map the erosion regulation service for a managed boreal forest watershed. To do so, SWAT and partial least-squares (PLS) regression were used to select explanatory variables for sediment yield. Variables of importance in projection (VIP) with a score > 1 were selected to develop LiDAR-based ecological indicators. Four categories of variables were identified as VIP from the PLS: (i) climate: annual precipitation, (ii) land use: forest, cutovers; (iii) land use patterns: cutover patch cohesion index, and (iv) morphometric: main channel length, channel length and sub-watershed area. The height of the 95th percentile of LiDAR returns (p95) < 5 m provided the most accurate spatial representation of cutovers and the optimal cutover patch cohesion index. Other morphometrics were obtained from a LiDAR-based digital terrain model. Explanatory variables for sediment yield were combined in a sediment erosion control (SEC) index, except for yearly average precipitation because the SEC index is not actually used as a temporal index. As expected, a negative relationship was found between sediment yield and SEC index rankings for the 2006-2015 period (Spearman, rho = -0.6, p < 0.05). Moreover, the overall agreement between SWAT and SEC index classes was 87% for 31 sub-watersheds. The study provides a list of relevant explanatory variables for modelling sediment yield in a boreal forest watershed where timber harvest activities occur. It also demonstrates the use of LiDAR data for deriving an index of the erosion regulation ecosystem service in a proxy-based approach as it had not been demonstrated previously at the watershed level. The validation method applied here fills a gap in ecosystem services mapping that could benefit studies in other watershed contexts.