Quantifying the sensitivity of ephemeral streams to land disturbance activities in arid ecosystems at the watershed scale

Abstract

Large areas of public lands administered by the Bureau of Land Management and located in arid regions of the southwestern United States are being considered for the development of utility-scale solar energy facilities. Land-disturbing activities in these desert, alluvium-filled valleys have the potential to adversely affect the hydrologic and ecologic functions of ephemeral streams. Regulation and management of ephemeral streams typically falls under a spectrum of federal, state, and local programs, but scientifically based guidelines for protecting ephemeral streams with respect to land-development activities are largely nonexistent. This study developed an assessment approach for quantifying the sensitivity to land disturbance of ephemeral stream reaches located in proposed solar energy zones (SEZs). The ephemeral stream assessment approach used publicly-available geospatial data on hydrology, topography, surficial geology, and soil characteristics, as well as high-resolution aerial imagery. These datasets were used to inform a professional judgment-based score index of potential land disturbance impacts on selected critical functions of ephemeral streams, including flow and sediment conveyance, ecological habitat value, and groundwater recharge. The total sensitivity scores (sum of scores for the critical stream functions of flow and sediment conveyance, ecological habitats, and groundwater recharge) were used to identify highly sensitive stream reaches to inform decisions on developable areas in SEZs. Total sensitivity scores typically reflected the scores of the individual stream functions; some exceptions pertain to groundwater recharge and ecological habitats. The primary limitations of this assessment approach were the lack of high-resolution identification of ephemeral stream channels in the existing National Hydrography Dataset, and the lack of mechanistic processes describing potential impacts on ephemeral stream functions at the watershed scale. The primary strength of this assessment approach is that it allows watershed-scale planning for low-impact development in arid ecosystems; the qualitative scoring of potential impacts can also be adjusted to accommodate new geospatial data, and to allow for expert and stakeholder input into decisions regarding the identification and potential avoidance of highly sensitive stream reaches.

Validation of Rapid Assessment Methods to Determine Streamflow Duration Classes in the Pacific Northwest, USA

“…The disagreement in flow classification by the NHD tended to be largely overestimations in the dry (34.2 % overestimation vs. 7.9 % underestimation), semiarid (54.8 vs. 4.8 %), and moist (48.3 vs. 6.9 %) climate classes, whereas it tended to be more evenly split in the wet (30.8 vs. 27.9 %) and very wet (33.3 vs. 27.4 %) classes. While hydrogeographic databases are important tools for watershed and water resource management, our findings emphasize and corroborate (e.g., Fritz et al 2013a;Caruso 2014;O'Connor et al 2014) that limitations in portraying the extent and flow status associated with existing databases must be considered in their application, and in conclusions drawn from their use.…”

“…These findings corroborate those reported in Fritz et al (2013a) where high-resolution NHD and topographic map classifications of streamflow duration agreed with *50 % of field determinations across *300 headwater sites and agreement increased to 73.8 % when considering 'at least intermittent' study reaches. The lack of high-resolution identification of ephemeral stream channels in the existing NHD has been noted by others (e.g., O'Connor et al 2014), including a recent study in a Colorado mountain watershed in which the high-resolution NHD underestimated the number of intermittent and ephemeral streams by 22 % (Caruso 2014). Because the original delineation of most streams in NHD was based upon interpretations from stereo orthophotographs, the ability to discern headwater streams on the landscape likely varied not only among cartographers but also with surrounding vegetation and season.…”

“…Historically, headwater streams were too extensive for cartographers to comprehensively document on most paper maps, yet the extensive distribution of headwater streams contributes to their likely importance to global biodiversity and water and biogeochemical cycling. More broadly speaking, intermittent and ephemeral streams have received less protection than their perennial counterparts, despite the fact that many of these streams drain into perennial water bodies and may represent vital sources of freshwater in semiarid and arid regions (Taylor et al 2011;Steward et al 2012;Acuña et al 2014;O'Connor et al 2014). Accurate classification of flow permanence is fundamental to water monitoring programs and watershed models.…”

Southwestern Intermittent and Ephemeral Stream Connectivity

“…O'Connor et al. () note that development of solar energy zones will significantly affect ephemeral channel systems. They developed a scoring system to conduct ephemeral stream assessments using publicly available geospatial data and high‐resolution aerial imagery.…”

“…Due to the abundant solar resources in the arid and semiarid Southwest, numerous, large-scale solar energy projects are envisioned or already under development. O'Connor et al (2014) note that development of solar energy zones will significantly affect ephemeral channel systems. They developed a scoring system to conduct ephemeral stream assessments using publicly available geospatial data and highresolution aerial imagery.…”