Evapotranspiration from a forest clearcut was measured over two growing seasons as part of a larger study of the microclimate of forest clearcuts and microclimate modification by site preparation. Pinegrass is the dominant species on clearcuts in the dry southern interior and is the major competitor with coniferous seedlings. This paper examines the water use of a pinegrass dominated clearcut and the response of surface conductance to environmental variables. Evapotranspiration was derived from eddy correlation measurements of sensible heat flux and measurements of net radiation and soil heat flux. 419 hours of daytime energy balance data from the summers of 1987 and 1988 were analyzed. A rearranged form of the Penman-Monteith equation was used to calculate hourly mean surface conductances for the clearcut. Leaf area measurements were used to calculate stomatal conductance from surface conductances. Stomatal conductance was modelled using boundary-line and non-linear optimization techniques. The most successful model (R² = 0.71) was obtained using non-linear optimization with stomatal conductance as a non-linear function of saturation deficit at the leaf surface (D₀) and solar irradiance. D₀ was calculated from measured evapotranspiration and surface conductance. Response of stomata to saturation deficit would be expected to be better correlated to D₀ than D measured at a reference height above the canopy. Stomatal conductance was also modelled as a function of D (measured at 1.3 m) and solar irradiance. The resulting model (R² = 0.50) was poor compared to that based on D₀. Saturation deficit and temperature were found to be highly correlated both at 1.3 m above the canopy and at the leaf surface. Use of air temperature in the conductance model caused R² to decrease. No relationship between stomatal conductance and volumetric soil water content was found. Hourly evapotranspiration rates calculated using modelled surface conductances agreed well with measured rates.(R² = 0.89). Evapotranspiration was also modelled using the Priestley-Taylor approach. The mean hourly a for all daylight data was found to be 0.81. This simple model was found to give comparable results to the stomatal conductance based model (R² = 0.85). / Land and Food Systems, Faculty of / Graduate

A literature survey indicated that little was known about the effects of commercial clearcutting on stream and watershed solution chemistry. To investigate these effects, five small watersheds were studied in the University of B.C. Research Forest. Three of the watersheds were equiped with weirs, stream height recorders, and soil-air-water thermographs. Soil pits were dug in the three calibrated watersheds and equiped with surface runoff collectors and hanging water column tension lysimeters. Samples of - precipitation above the forest, throughfall (through forest and slash), surface runoff, forest floor leachate, mineral soil leachate near the bottom of the rooting zone, groundwater, and streamwater - were collected at regular intervals and analyzed for pH, electrical conductivity, alkalinity as bicarbonate, K, Na, Mg, Ca, Fe, Mn, Al, Cl, P, N, S, and Si for periods of up to three years prior to clear-cutting and two years after clearcutting. Streamwater was also analyzed for dissolved oxygen and suspended sediment. Sampling was carried out for periods of up to three years prior to clearcutting and up to two years following clear-cutting. The streams were characterized by high discharges from late autumn until early summer and low discharges from May until October, with almost no contribution from snowmelt runoff. Response to precipitation was fairly rapid and it was hypothesized that stormflow arose mainly from flow of water through macrochannels in the soil. Visual observations and chemical data were consistent with this hypothesis. Evapotranspiration from the gauged watersheds was estimated to be about 85 cm per year by subtracting streamflow outputs from precipitation inputs and 65 cm per year using theoretical methods. The discrepancy between these two values was attributed to an unmeasured leakage of water, particularly from the untreated control watershed which rendered too low the streamflow outputs. There was an increase of 30.8 cm in runoff from one watershed, and 27.6 cm from another during the first six months of the dormant season immediately following clearcutting. During this period runoff from the control watershed was 141.5 cm. Stream temperatures underwent annual cycles with winter minima close to 0°C and summer maxima close to 17°C. Diurnal temperature fluctuations were slight and usually less than a few degrees. Clearcutting caused an increase in both maximum and minimum stream temperatures during the first dormant season following clearcutting. The few measurements which were made of suspended sediment, together with visual observations, indicated that concentrations were usually negligible in the streams. Dissolved oxygen concentrations in streams were usually close to 100% saturation and underwent annual cycles with maximum values in winter and minimum values in late summer and early autumn. Clearcutting had little effect on dissolved oxygen values during the cooler wetter months but caused very pronounced decreases during summer and early autumn. This was attributed to the biological and chemical oxygen demands of decaying slash in the streams. Stream chemistry exhibited little diurnal variation but considerable variation with discharge. Sodium, calcium, magnesium, dissolved silica, and bicarbonate concentrations, and electrical conductivity and pH decreased with increasing discharge, whereas potassium and nitrate concentrations exhibited some increases and some decreases. Chloride and sulphate concentrations were generally not significantly related to discharge. In the undisturbed ecosystems, chemical concentrations, pH, and electrical conductivity throughout the systems were generally highest in late summer and early autumn and lowest in winter and early spring. This was attributed to seasonal cycles of geological and biological activity with accumulation of weathering and decomposition products occurring during dry, warm summers. These were flushed through the system in autumn, with solutions becoming progressively more dilute throughout the winter until the onset of warmer weather. Nitrate concentrations tended to be higher in winter than in summer which was attributed to greater nitrogen uptake by organisms in summer. The most abundant ions in precipitation and throughfall were hydrogen, sulphate, and chloride, while calcium, bicarbonate, and sulphate were dominant in all the other types of water samples. There was a general increase in chemical concentrations to maximum values in forest floor leachate followed by a decrease to minimum values in groundwater, and a slight increase again in streamwater. The lowest pH values were in throughfall (4.0-4.5) followed by a steady increase through the system to maximum values in stream-water (6.5-7.0). Clearcutting increased the pH of water reaching the forest floor and surface runoff but decreased the pH of mineral soil leachate, groundwater, and streamwater. It generally decreased chemical concentrations in water reaching the forest floor and in surface runoff, and, to a lesser extent, in forest floor and mineral soil leachates, but it increased concentrations in groundwater and, to a lesser extent, in streamwater. A most notable increase throughout the system was in the concentration of potassium which was attributed to the relative ease with which potassium is leached from decaying vegetation. Increases in nitrate concentrations were particularly high in groundwater. Streamwater concentrations of potassium, iron, calcium, dissolved oxygen, and probably manganese, were significantly affected by clearcutting; concentrations of all these chemicals increased, except dissolved oxygen which decreased. Slight increases in magnesium, nitrate, sulphate, and chloride concentrations, and electrical conductivity, and decreases in pH and bicarbonate concentrations were also observed. All changes were most noticeable during the low flow periods of late summer and early autumn. There were no obvious effects on sodium, aluminium, ammonium, dissolved silica, and phosphate concentrations. In terms of chemical budgets, there was a general net loss of calcium, sodium, magnesium, potassium, and sulphur from all the watersheds, in their undisturbed state, while nitrogen was accumulated and phosphorus underwent very little change. The chloride balance changed from year to year with losses one year and gains the next. Chemical outputs increased relative to inputs with increasing precipitation so that net losses were greater in winter than in summer. Chemical budgets and stream chemistry at Haney were compared to the results of other studies, particularly one in the nearby Seymour watershed (Zeman, 1973). At Haney, clearcutting significantly increased potassium losses and decreased nitrogen gains in one watershed and significantly increased potassium, sodium, magnesium, and chloride losses in another watershed. From the nutrient viewpoint, it appears that clearcutting has not impaired the mechanisms for nutrient retention in the ecosystems of the type present in the study area. This may not be the case for all ecosystems in coastal B.C., or for other forestry practices, such as slashburning. The study has pointed out the need for further work to quantify the role of macrochannels in soils with respect to hydrologic and chemical behaviour of watersheds. It has also pointed out the danger of extrapolating to larger ecosystems the results of lysimeter studies. Chemical analysis of groundwater may offer a more accurate means of estimating chemical losses from soils than do lysimeters. / Forestry, Faculty of / Graduate

Debris torrents (channelized debris flows) are a geomorphological process only relatively recently recognised in southwest British Columbia. They are of interest both because of the tremendous amount of geomorphic work they do, and because of the hazards they pose to engineering works and residential developments. Fourteen torrents on the east side of Howe Sound, near Vancouver, since October 1981, have claimed twelve lives. Debris torrents differ from water floods in that they involve large amounts of coarse organic and inorganic debris. Therefore, a major requirement for torrent occurrence in a given channel is a supply of mobilizable debris. This thesis examines debris supply mechanisms and rates of debris supply in four small watersheds along Howe Sound, near the village of Lions Bay. An inventory of major debris sources has been compiled, and selected typical sites are examined in detail. Study methods include airphoto interpretation, ground surveying and reconnaissance, field instrumentation and site monitoring, dendrochronology, and materials sampling and testing. Debris supply is controlled by natural factors such as the nature and distribution of the bedrock and surficial materials, topographic gradient, vegetation, weather, and surface and groundwater hydrology, and by human activities such as logging and road construction. A wide variety of debris supply mechanisms operate in the study area, including rockfall and rockslide, talus shift, debris slide, soil wedge failure, ravelling, and snow avalanche. In addition to delivering debris to channel systems, some of these processes are capable of triggering debris torrents. Debris redistribution in channels occurs through debris torrents which do not reach the fans, fluvial processes (bedload transport), and snow avalanches. Active debris removal from main supply points, and storage elsewhere in the channel system, can decrease the frequency but increase the magnitude of torrent events in the basin. The wide variety of debris supply, debris redistribution, and torrent triggering mechanisms acting in this relatively small area points to a need for careful study of individual basins if the torrent potential in an area is to be understood. Regionally-based climatological or hydrological models of torrent occurrence should be employed for preliminary hazard assessment only. / Arts, Faculty of / Geography, Department of / Graduate