Ionic Pulse Research Articles

Development of a Hydrochemical Model for Seasonally Snow‐Covered Alpine Watersheds: Application to Emerald Lake Watershed, Sierra Nevada, California

We have developed and tested a model to assess the hydrologic and biogeochemical responses of seasonally snow covered alpine areas to changes in inputs of water, chemicals, and energy. This alpine hydrochemical model (AHM) is capable of incorporating a detailed understanding of watershed processes in order to simulate events critical to biota such as the ionic pulse associated with spring snowmelt, which is only a few days long and may involve only a portion of the catchment. The model computes integrated water and chemical balances for multiple terrestrial, stream, and lake subunits within a watershed, each of which can have a unique and variable snow‐covered area. Two years of data from the Emerald Lake watershed in the southern Sierra Nevada were used for fitting and testing by comparing observations with modeled daily output. To the extent possible, model parameters were set on the basis of independent physical or chemical measurements, leaving only a few fitted parameters. In its current application, model capabilities include (1) tracking of chemical inputs from precipitation, dry deposition, snowmelt, mineral weathering, flows external to the watershed, and user‐defined sources and sinks; (2) tracking surface and subsurface water and chemical movements through vegetation canopy, snowpack, soil litter, multiple soil layers, streamflow, and lakes; (3) calculating chemical speciation, including precipitates, exchange complexes, and acid‐neutralizing capacity; (4) simulating nitrogen reactions; (5) using a snowmelt optimization procedure to aid in matching observed watershed outflows; and (6) modeling riparian areas. Using one year of stream data for parameter estimation and a second for evaluation, the agreement between model and data was judged to be quite good. AHM is a flexible, precise algorithm for simulating watershed hydrochemistry and can readily be adapted to other alpine catchments using the Emerald results as a guide. Application of AHM to forested catchments should also be feasible.

Processes Controlling the Chemistry of Two Snowmelt‐Dominated Streams in the Rocky Mountains

Time‐intensive discharge and chemical data for two alpine streams in the Loch Vale watershed, Colorado, were used to identify sources of runoff, flow paths, and important biogeochemical processes during the 1992 snowmelt runoff season. In spite of the paucity of soil cover the chemical composition of the streams is regulated much as in typical forested watersheds. Soils and other shallow groundwater matrices such as boulder fields appear to be more important in controlling surface‐water chemistry than their abundance would indicate. The chemical composition of the major source waters (usually thought of as end‐members whose chemical composition is relatively constant over time) changes at the same time that their mixing ratio in streams changes, confounding use of end‐member mixing models to describe stream‐water chemistry. Changes in the chemical composition of these source waters are caused by the ionic pulse of solutes from the snowpack and the small size of the shallow groundwater reservoir compared to the volume of snowmelt passing through it. The brief hydrologic residence time in the shallow groundwater indicates that concentrations of most dissolved constituents of stream water were controlled by fast geochemical processes that occurred on timescales of hours to days, rather than slower processes such as weathering of primary minerals. Differences in the timing of snowmelt‐related processes between different areas of the watershed also affect the stream‐water chemical composition. Cirque lakes affect discharge and chemical composition of one of the streams; seasonal control on stream‐water NO3 and SiO2 concentrations by diatom uptake in the lakes was inferred. Elution of acidic waters from the snowpack, along with dilution of base cations originating in shallow groundwater, caused episodes of decreased acid‐neutralizing capacity in the streams, but the streams did not become acidic.

Fluxes and transformations of nitrogen in a high-elevation catchment, Sierra Nevada

The fluxes and transformations of nitrogen (N) were investigated from 1985 through 1987 at the Emerald Lake watershed (ELW), a 120 ha high-elevation catchment located in the southern Sierra Nevada, California, USA. Up to 90% of annual wet deposition of N was stored in the seasonal snowpack; NO 3 − and NH 4 + were released from storage in the form of an ionic pulse, where the first fraction of meltwater draining from the snowpack had concentrations of NO 3 − and NH 4 + as high as 28 μeq L−1 compared to bulk concentrations of <5 μeq L−1 in the snowpack. The soil reservoir of organic N (81 keq ha−1) was about ten times the N storage in litter and biomass (12 keq ha−1). Assimilation of N by vegetation was balanced by the release of N from soil mineralization, nitrification, and litter decay. Mineralization and nitrification processes produced 1.1 keq ha−1 yr−1 of inorganic N, about 3 1/2 times the loading of N from wet and dry deposition. Less than 1% of the NH 4 + in wet and dry deposition was exported from the basin as NH 4 + . Biological assimilation was primarily responsible for retention of NH 4 + in the basin, releasing one mode of H+ for every mole of NH 4 + retained and neutralizing about 25% of the annual acid neutralizing capacity produced by mineral weathering in the basin. Nitrate concentrations in stream waters reached an annual peak during the first part of snowmelt runoff, with maximum concentrations in stream water of 20 μeq L−1, more than 4 times the volume-weighted mean annual concentrations of NO 3 − in wet deposition. This annual peak in stream water NO 3 − was consistent with the release of NO 3 − from the snowpack in the form of an ionic pulse; however soil processes occurring underneath the winter snowpack were another potential source of this NO 3 − . Concentrations of stream water NO 3 − during the summer growing season were always near or below detection limits (0.5 μeq L−1).

Solute chemistry of snowmelt and runoff in an Alpine Basin, Sierra Nevada

Snowpack runoff contributions to the hydrochemistry of an alpine catchment in the Sierra Nevada were evaluated in 1986 and 1987 by analyzing snowpack, meltwater, and stream water samples for major inorganic ions, conductance, acid neutralizing capacity (ANC), and silicate. An ionic pulse in meltwater with initial concentrations twofold to twelvefold greater than the snowpack average, varying with site and with ion, was measured in lysimeters placed at the base of the snowpack. Maximum concentrations of ions in meltwater were inversely related to the rate of snowmelt; melt‐freeze cycles increased the concentration of solutes in meltwater. Hydrogen concentration in meltwater was buffered by ANC produced in part by the dissolution of particulates. The anionic pulse in meltwater was observed in stream waters during the first 30 days of snowpack runoff, with NO3− concentrations in stream waters at this time about 1.6‐fold greater than the average NO3− concentration for the time period of snowpack runoff, Cl− about 1.5‐fold greater and SO42− about 1.3‐fold greater. Maximum H+ concentration during snowpack runoff (increase of 170% over winter concentrations) occurred near maximum discharge. ANC minima occurred at maximum discharge as a result of dilution, with a decrease from winter concentrations of 70% in 1986 and 60% in 1987. Interactions between snowpack runoff and soils were important to the chemistry of stream water. Eighty to ninety percent of the H+ stored in the snowpack was consumed before it reached the base of the catchment. Soils were a sink for NH4+ from snowpack meltwater, with less than 1% of the NH4+ released from snowpack storage exported from the basin as NH4+. Sulfate concentrations in stream waters were less variable than NO3− or Cl− concentrations; sorption processes in soils were a likely cause for the regulation of SO42− concentrations.

Ionic tracer movement through a wyoming snowpack

A meltwater ionic pulse with initial concentrations of 5–10 or more times the average was observed in lysimeters set at the base of a 2-m snowpack in an unpolluted, alpine watershed. Both background chemical species and added tracers exhibited the initial pulse. About 10 days after the onset of meltwater release, solute concentrations collected in the lysimeters dropped to near the average snowpack level. Silica was elevated more than major ions in some initial lysimeter samples, which is evidence of a soil-water or overland-flow contribution. Lysimeter flow rates were equivalent to about 2 cm of melted snow per day, approximately equal to the daily melt estimated from a temperature-index model. Less than 10 per cent of the ionic-tracer mass applied to snow directly above the lysimeters was present in the meltwater. Snow pits dug in an area adjacent to the lysimeters showed that ice lenses played a significant role in solute movement through the pack, influencing both solute depletion and horizontal vs vertical flow. Snow pits also showed depletion of at least 25–35 per cent of the solute mass during release of the first 10 per cent of meltwater.

The effect of atropine on acetylcholine action at the neuromuscular junction.

The action of atropine at the motor end plate was studied by measuring its effect on acetylcholine-induced fluctuations of membrane potential (‘acetylcholine noise’). Atropine, in contrast to curare, modifies the elementary depolarization produced by the impact of acetylcholine molecules; it reduces the voltage amplitude and shortens the underlying ionic current pulse.