Factors influencing chronic semi-arid headwater stream impairments: a southern California case study Running title: Significance of chronic headwater stream impairment

Sources of stream impairments are well known; however, less attention has centered on characterizing the extent to which human-environmental factors influence headwater stream quality within semi-arid watersheds. This study quantified the extent to which seasonal weather patterns and landscape attributes contribute to the physicochemical characteristics of two perennial headwater tributaries and their confluence within the semi-arid mountainous region of the Santa Ana River Basin, California. In situ sampling of stream temperature (℃), stream flow rate (m/s), nitrate (NO₃^-), ammonium (NH₄⁺), turbidity (NTU), dissolved oxygen (DO), conductivity, pH and lab assessments for. E. coli, total coliform (TC) and total dissolved solids (TDS) occurred during dry and wet season conditions. Across sampling locations, multiple parameters (i.e. NO₃^-, NH₄⁺, TDS, TC) consistently exceeded regulatory standards simultaneously during both the dry and wet seasons, however, the level of concentrations varied between a tributary catchment landscape with high percentage of impervious surfaces (i.e. roads, buildings) and wastewater infrastructure (i.e septic, sewer) versus one characterized by agricultural activities (i.e. crop, livestock) and barren land. Findings illustrate the need for hydrologically comprehensive strategies (i.e. stream headwaters to river mouth) that are community to agency-driven and that support the expansion of monitoring and shared knowledge to mitigate impairments within headwater streams and downstream. Potential avenues for community collaborations that support sustainable water management strategies are highlighted.