Trabajos prácticos iniciales

Blelham Tarn contains an in situ automated monitoring buoy located at the lake’s deepest point (14.5 m) which records meteorological and within-lake ecological and physical parameters at a four minute frequency (Figure 3.3). Meteorological variables recorded include: air temperature, wind speed, relative humidity and solar radiation. Water

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temperature is measured at a depth of 0.5 m and then at metre intervals 1-10 m with the final temperature sensor being located at a depth of 12 m. Data collected from the automated monitoring buoy have been used in two chapters in the form of daily averaged data to drive the PROTECH model (Chapter 5) and to calculate heat fluxes, mixed depth and measures of lake stability when assessing the impact of storm events (Chapter 6). The data were quality checked at the hourly scale for each variable and gaps were identified. Causes of the data gaps included sensor errors or data relay errors, in general these errors were identified and resolved quickly over a time period of days rather than weeks. A large data gap of 27 consecutive days occurred in January 2015 due to the replacement of the monitoring buoy infrastructure. Gaps in the meteorological data exceeding three hours were infilled using relationships between the data recorded at Blelham Tarn and data recorded by the nearby buoy located on Esthwaite Water (Lat:54.3593, Long:-2.9858). Gaps in the data less than three hours were infilled using a linear interpolation.

The outflow from Blelham Tarn is not continuously gauged, but discharge data were required in order to calculate nutrient loading (Chapter 5; 2012-2014) and changes in residence time (Chapter 6; 2008-2018). Prior to this study a relationship between daily outflow discharge from Blelham Tarn and the River Leven was established in 1974, when daily measurements were available for both sites. Data from the nearby River Leven (Lat:54.273913, Long:-2.954740; Figure 3.6) were therefore obtained from the Environment Agency and a power law relationship was applied to the River Leven data to convert it into Blelham Tarn outflow (Blelham Beck) data (R2=0.92; p<0.05),

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where QBB is the outflow discharge of Blelham Tarn and QRL is the discharge from the River Leven. This relationship was used to calculate discharge in Chapters 5 and 6, but in order to check this relationship new measurements of discharge were also taken.

Figure 3.5 The Windermere catchment area showing the position of the River Leven relative to Blelham Tarn.

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Blelham Tarn has four main inflows, three of which were accessible (Figure. 3.1). Water level loggers were installed into the outflow (Blelham Beck) and into one inflow (Wray Beck), with water level, depth and water temperature being recorded every 15 minutes. An additional logger located on the bank of Wray Beck recorded air temperature and atmospheric pressure which was later used to adjust the water level pressure recorded at Wray Beck and Blelham Beck. Data from the loggers were downloaded and quality checked every month. Stream surveys were carried out at the inflows and outflow to determine the bed bathymetry of each stream with the depth of the stream at bank full level being recorded every 10 cm. Using a Sontek handheld Acoustic Doppler Velocimeter (ADV), flow speed measurements were taken at three or four equally spaced locations across each inflow and the outflow depending on the wetted width, recording the water depth and the location relative to the bathymetry survey. Using the flow speed and cross sectional area of the flow measurement the stream discharge was calculated using the ‘Mean-Section Method’(Shaw, 2005),

Equation 3.2 Qseg = 0.5(v1+v2) x 0.5(d1+d2) x b,

where v is the flow velocity, d is depth and b is the number of segments. The mean discharge (Q) for the stream was calculated by adding up each Qseg and dividing by the number of Qseg’s. Separate rating curves were then plotted between discharge and water level for the inflow and outflow. The rating curve from the inflow was inadequate and the stream was dredged part way through the measurement period with may have altered the water level and discharge relationship, therefore only the rating curve from the outflow was used (Figure 3.6).

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Figure 3.6 Rating curve derived from the relationship between the outflow discharge and water level.

This rating curve equation was used to convert the remaining water level values from the outflow into daily discharge using a polynomial relationship equation (R2=0.8162), Equation 3.3 y=2.9011x2 _{-1.7618x + 0.2899}

where, y is discharge and x is water level.

The discharge values calculated using equations 3.1 and 3.2 were compared (Figure 3.7). Discharge values derived using both methods compared well, with discharge peaks occurring at similar times and magnitudes (Figure 3.7). The higher baseline flow calculated using the River Leven dataset is likely due to the higher volume of water in the Leven compared to the outflow of Blelham Tarn (Figure 3.7). This comparison confirms that deriving discharge using the River Leven relationship is appropriate.

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Figure 3.5 Discharge of Blelham Beck calculated using the Blelham and Leven discharge in 1974 (Black line) and discharge calculated from the rating curve derived in this study (grey line).