No abstract

The hydrological, thermal, and chemical characteristics of two small streams flowing through relatively undisturbed, low-elevation mountain watersheds in southwestern British Columbia were investigated. All observations and chemical analyses of ecosystems were consistent with the hypothesis that stormflow originated mainly from flow of water through soil macrochannels to groundwater and thence to streams. Water budgets indicated unmeasured groundwater losses. The streams exhibited annual chemical cycles for most parameters, with maximum values in late summer and early autumn and minimum values in winter and early spring. Nitrate concentrations displayed no consistent seasonal variation, whereas potassium and sulphate concentrations were relatively uniform throughout the year. Most chemical parameters decreased with increasing discharge, whereas dissolved oxygen concentrations increased.Potassium concentrations exhibited some increases and some decreases, and chloride, nitrate, and sulphate concentrations were generally not significantly related to discharge. Concentration-discharge relationships were used to infer the origin of stormflow water. Differences in the chemistry of the two very similar streams have important ramifications for the design of watershed nutrient studies. Nutrient budgets were very similar to those of other watersheds in humid temperate regions, with net losses of calcium, sodium, magnesium, potassium, chloride, and sulphur. Nitrogen and phosphorus exports in dissolved or particulate organic form were not measured. Based on dissolved inorganic measurements, nitrogen was accumulated, while any gains or losses of phosphorus were extremely small. THE STUDY AREATwo watersheds [Fdler, 1977] were studied at the University of British Columbia Research Forest, located at Haney, approximately 60 km east of Vancouver (Figure 1 ). The area has a marine, warm to temperate, rainy climate with an annual precipitation of 220-270 cm. Owing to the low elevation (140-450 m), snow falls only occasionally, and most precipitation is Pseudotsuga menziesii (Douglas fir) forests originating from a fire in 1868. Small amounts of Alnus rubra (red alder), Acer macrophyllum (big-leaf maple), and Betula papyrifera (western white birch) occur in occasional openings or wet sites. Part of one watershed (watershed C) is covered by forest which grew up following logging in the 1920's. METHODSSharp-crested 120 ø V notch weirs were constructed on the streams draining two small watersheds. The larger watershed was split into upper and lower sections by constructing a rectangular broad-crested weir on the stream about 600 m upstream from the V notch weir. This resulted in the collection of data from three distinct drainage areas: a small entire watershed (stream A) and the lower (stream B) and upper (stream C) sections of the second watershed. Thus 'stream B' and 'stream C' are the lower and upper sections of the same stream, respectively. Instrument shelters at the weirs housed Richards-type water level recorders and soil-air...

Two small forested watersheds near Haney in southwestern British Columbia were partially clearcut, and the slash on one of them was subsequently burned. Streamwater chemistry was monitored in these treated watersheds and an undisturbed control watershed for 2 years prior to treatment and up to 9 years after treatment. The chemical parameters that were studied responded differently to the treatments, but there was a general pattern of increased concentrations and fluxes in Streamwater for the first 2–3 years following treatment followed by a decline to, and sometimes below, pretreatment values. The most pronounced increases were observed for K and NO3. It was not possible to determine the exact causes of these changes in concentrations and fluxes, due to the great variability in the ecosystems present. This variability precluded determination of statistically significant changes in annual terrestrial nutrient fluxes and pools. Stream nutrient exports usually were <10 kg/ha/yr for each of N, P, K, and Mg, <20 kg/ha/yr for Na and Cl, and <30 kg/ha/yr for Ca. These values were considerably less than nutrient exports in harvested logs and in losses to the atmosphere during the slashburn. Clearcutting and burning caused greater nutrient losses than Clearcutting alone, particularly in the case of N, where the Clearcutting and Clearcutting and burning treatments resulted in total losses of 245 kg/ha and 1293 kg/ha, respectively, for the first two years after treatment.

The solution chemistry of forested streams primarily in western North America is explained by considering the major factors that influence this chemistry — geological weathering; atmospheric precipitation and climate; precipitation acidity; terrestrial biological processes; physical/chemical reactions in the soil; and physical, chemical, and biological processes within streams. Due to the complexity of all these processes and their varying importance for different chemicals, stream water chemistry has exhibited considerable geographic and temporal variation and is difficult to model accurately. The impacts of forest harvesting on stream water chemistry were reviewed by considering the effects of harvesting on each of the important factors controlling this chemistry, as well as other factors influencing these impacts ‐ extent of the watershed harvested, presence of buffer strips between streams and harvested areas, nature of post‐harvesting site preparation, revegetation rate following harvesting, pre‐harvesting soil fertility, and soil buffering capacity. These effects have sometimes reinforced one another but have sometimes been counterbalancing or slight so that harvesting impacts on stream water chemistry have been highly variable. Eight major knowledge gaps were identified, two of which — a scarcity of detailed stream chemical budgets and knowledge of longitudinal variation in stream chemistry — relate to undisturbed streams, while the remainder relate to forest harvesting effects.

Climate data from the Malcolm Knapp Research Forest (MKRF) in the Coast Range mountains of southwestern British Columbia were used to examine relationships between climate and hydrology and variations in the El Niño Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO). Air and water temperatures were higher and precipitation was lower during in‐phase or warm PDO/E1 Niño events than in other years. In contrast, in‐phase or cool PDO/La Niña years were generally cooler and wetter than other years. Precipitation and East Creek discharge were positively related to the Southern Oscillation Index (SOI) and negatively related to the PDO index. Conversely, air and water temperatures were negatively related to the SOI and positively related to the PDO index. Differences in precipitation and air temperature were also evident at longer time scales when separated by PDO phase. Because of drier conditions during in‐phase El Niño events, the flow of organic matter from East Creek to downstream portions of the channel network was lower compared to other years. This reduction has implications for downstream communities, as these subsidies provide a major source of energy for stream food webs. Therefore, short term and long term shifts in climate, discharge, and water temperature may have profound impacts on the ecology of Pacific Northwest (PNW) watersheds due to changes in a number of ecosystem processes such as altered flux of organic matter from headwater streams to larger rivers.