A Network Approach Linking Spatial Patterns in Water Chemistry to Post-Fire Landscapes
Abstract
Water supply in the Western US largely originates in forested watersheds, which are increasingly threatened by wildfire. Some water quality constituents, such as sediment, can recover to pre-fire levels relatively quickly (<5 years). However, elevated stream nitrate (NO3-) can persist for decades after fire, which causes both ecological and human health concerns. To investigate the drivers of elevated stream NO3- following wildfire, we utilized a spatially distributed water chemistry dataset. This included 71 stream sodium (Na) and NO3- samples across 9 watersheds that were burned to varying degrees by the Hayman Fire. We specifically contrasted Na and NO3- because in-stream Na concentrations are strongly regulated by weathering patterns and hydrologic flow paths, whereas spatial patterns of in-stream NO3- are more closely linked to spatial distributions of vegetation across the terrestrial landscape. We compared two statistical modeling approaches multiple linear regression and spatial stream network modeling to determine the degree to which environmental predictor variables and spatial autocorrelation can predict observed Na and NO3- concentrations. No topographic, vegetation, or fire variables were strong predictors of stream Na. However, a majority of variance in stream Na was explained by flow-connected spatial autocorrelation. This confirms that spatial patterns in stream Na are dominated by downstream transport rather than spatial distributions of fire and vegetation. In contrast, mean catchment normalized differenced wetness index (NDWI), which represent vegetation wetness, was the strongest predictor of stream NO3-. Furthermore, stream NO3- had weak flow-connected spatial autocorrelation, but instead exhibited fine-scale variability reflective of patchy post-fire vegetation. Interestingly, the location of burned hillslopes with low NDWI within a watershed had unique implications for stream NO3-. Stream reaches that received large lateral inputs of burned area in headwater positions were more susceptible to elevated stream NO3- than inputs from burned hillslopes further downstream. This suggests that burned hillslopes that generate large lateral inflows to small streams should be targeted for post-fire rehabilitation to aid stream NO3- recovery to pre-fire levels.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.H54H..04R