[1] Nitrate (NO 3 À ) movement was studied using a combination of isotopic, chemical, and hydrometric data within the 135 ha Archer Creek watershed in the Adirondack Mountains of New York from January 1995 to December 1996. This research was conducted to identify sources of stream water NO 3 À and the mechanisms that deliver NO 3 À to the stream to test two hypotheses: (1) Soil water NO 3 À concentrations are highest after dry periods and subsequently lower with each storm. (2) Stream water NO 3 À concentrations are controlled by groundwater during growing season low flows and by soil water during the dormant season and during storms. Antecedent moisture conditions and season had little effect on mean soil water NO 3 À concentrations before storms (range of 1.1-5.1 mmol L À1 throughout the study). High soil water NO 3 À concentrations (up to 136 mmol L À1 ) were found only at the watershed ridge top during the 1996 snowmelt and early summer. Results from isotopic hydrograph separations and chemical end-member mixing analysis showed that soil water and till groundwater dominated stream base flow and storm flow during six monitored storms. Near-stream wetland groundwater and event water contributed little to streamflow during most conditions. Near-stream groundwater contributions to streamflow were significant only during very low base flow (<0.05 mm h À1 ) during the summer and fall. Highest stream water NO 3 À concentrations coincided with peaks in the till groundwater contribution according to isotopic hydrograph separations using d 18 O and chloride as conservative tracers. A conceptualization of streamflow generation and watershed NO 3 À release is described in which hillslope hollows are the principal zones of soil water and till groundwater mixing in the watershed and till groundwater is the main source of stream water NO 3 À during both base flow and storms.

No abstract

Changes in snowmelt-related streamflow timing have implications for water availability and use as well as ecologically relevant shifts in streamflow. Historical trends in snowmelt-related streamflow timing (winter-spring center volume date, WSCVD) were computed for minimally disturbed river basins in the conterminous United States. WSCVD was computed by summing daily streamflow for a seasonal window then calculating the day that half of the seasonal volume had flowed past the gage. We used basins where at least 30 percent of annual precipitation was received as snow, and streamflow data were restricted to regionally based winter-spring periods to focus the analyses on snowmelt-related streamflow. Trends over time in WSCVD at gages in the eastern U.S. were relatively homogenous in magnitude and direction and statistically significant; median WSCVD was earlier by 8.2 days (1.1 days/decade) and 8.6 days (1.6 days/decade) for 1940-2014 and 1960-2014 periods respectively. Fewer trends in the West were significant though most trends indicated earlier WSCVD over time. Trends at low-to-mid elevation (< 1,600 m) basins in the West, predominantly located in the Northwest, had median earlier WSCVD by 6.8 days 0.9 days/decade) and 3.4 days (1960-2014, 0.6 days/decade). Streamflow timing at high-elevation (≥ 1,600 m) basins in the West had median earlier WSCVD by 4.0 days (1940-2014, 0.5 days/decade) and 5.2 days (1960-2014, 0.9 days/decade). Trends toward earlier WSCVD in the Northwest were not statistically significant, differing from previous studies that observed many large and (or) significant trends in this region. Much of this difference is likely due to the sensitivity of trend tests to the time period being tested, as well as differences in the streamflow timing metrics used among the studies.Mean February-May air temperature was significantly correlated with WSCVD at 100 percent of the study gages (field significant, p < 0.0001), demonstrating the sensitivity of WSCVD to air

Abstract:In light of recent reductions in sulphur (S) and nitrogen (N) emissions mandated by Title IV of the Clean Air Act Amendments of 1990, temporal trends and trend coherence in precipitation (1984-2001 and 1992-2001) and surface water chemistry (1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001) were determined in two of the most acid-sensitive regions of North America, i.e. concentrations at all sites, and decreasing trends in NO 3 , C B , and H C concentrations and increasing trends in dissolved organic carbon at most sites. In contrast, acid-neutralizing capacity (ANC) increased significantly at only about half the Adirondack lakes and in one of the Catskill streams. Flow correction prior to trend analysis did not change any trend directions and had little effect on SO 4 2 trends, but it caused several significant non-flow-corrected trends in NO 3 and ANC to become non-significant, suggesting that trend results for flow-sensitive constituents are affected by flow-related climate variation. SO 4 2 concentrations showed high temporal coherence in precipitation, surface waters, and in precipitation-surface water comparisons, reflecting a strong link between S emissions, precipitation SO 4 2 concentrations, and the processes that affect S cycling within these regions. NO 3 and H C concentrations and ANC generally showed weak coherence, especially in surface waters and in precipitation-surface water comparisons, indicating that variation in local-scale processes driven by factors such as climate are affecting trends in acid-base chemistry in these two regions.