Abstract
Stream permanence classifications (i.e., perennial, intermittent, ephemeral) are a primary consideration to determine stream regulatory status in the United States (U.S.) and are an important indicator of environmental conditions and biodiversity. However, at present, no models or products adequately describe surface water presence for regulatory determinations. We modified the Thornthwaite monthly water balance model (MWBM) with a flow threshold parameter to estimate flow permanence and evaluated the model’s accuracy and precision for more than 1.3 million headwater stream reaches in the U.S. Pacific Northwest (PNW). Stream reaches were assigned to one of eight calibration groups by unsupervised classification based on sensitivity to MWBM parameters. Suitable MWBM parameter sets were identified by comparing modeled stream permanence estimates to surface water presence observations (SWPO). Parameter sets with accuracies > 65% were considered suitable. The MWBM estimated stream permanence with high precision at 40% of reaches, with poor precision at 20% of reaches, and no suitable parameter sets were identified for 40% of reaches. Results highlight the need for increased SWPO collection to improve calibration and assessment of stream permanence models. Additionally, implementation of the MWBM to estimate surface water presence indicates potential for process-based models to predict stream permanence with future development.
Highlights
In the United States, stream permanence classifications are a primary consideration to determine stream regulatory status under the Clean Water Act and are an important indicator of environmental conditions and biodiversity [1–3]
National Hydrography Dataset (NHD) stream permanence classifications (SPC) have been shown to exhibit up to 50% disagreement with in-situ observations [5,6] and the highest disagreement rates occur on headwater streams [7]
Most headwater streams exhibited some degree of sensitivity to the Runoff factor (RF) and flow threshold (FT) parameters
Summary
In the United States, stream permanence classifications (i.e., perennial, intermittent, and ephemeral) are a primary consideration to determine stream regulatory status under the Clean Water Act and are an important indicator of environmental conditions and biodiversity [1–3]. A primary reason for the uncertainty of NHD SPC is that designations originated from observations made by topographic survey crews throughout the 1900s [5,7,8]. Climatic conditions during the survey year may not represent long-term average conditions and survey crew observations may not capture the full range of variability for surface water conditions at a stream reach [7]. Another reason for uncertainty is the SPC designations themselves. SPC designations for part of a watershed could have been made under very different climate conditions than other portions of the same watershed [7]
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