W9 - Measurement and monitoring Flashcards
Why monitor river systems?
Evaluate the impacts of policy
Develop knowledge and understanding (…process)
Testing models
Measure, anticipate, and prevent the deterioration of environmental integrity
What is a sample?
Just one measurement of an environmental variable at one point in time and one point in space
Difference between monitoring and a survey
Monitoring is taking measurements over time all at the same place
A survey refers to measurements spread over space taken at the same time
What is the larger group that a sample is collected from called?
Population
Which variables would you use areal measurements?
Vegetation
Land use
Stream network density
Gully erosion
Which variables would you use point measurements?
Discharge
Water quality
Biological factors
Water table measurements
Soil characteristics
Stream velocity
Acidity (pH)
Types of primary data collection (spatial sampling techniques)
Random
Systematic
Stratified
Non-random
What is random sampling + pros/cons?
Every part of the study area has an equal and independent chance of receiving a sampling point
Gives uneven areal coverage which is a disadvantage if spatial variations are of interest
What is systematic sampling + pros/cons?
Initial point located randomly; all others determined by a fixed interval
Gives even areal coverage
If there is a spatial pattern in the data, the regularly spaced sample points could correspond with some parts of the pattern, giving a biased impression
What is stratified sampling + pros/cons?
Area is divided into sub-areas within which sample points are chosen randomly.
If a regular grid is superimposed, points may be located randomly within each grid square to ensure good areal coverage while preserving the advantages of randomness.
Even coverage of different categories i.e. soil type
Can also tailor sampling points to capture variability
More points in area of higher variability
What is non-random sampling + pros/cons?
Aka purposive or subjective sampling
Sample subjectively chosen by the investigator
* i.e. convenient sampling locations
Likely to be unusual or biased
What is the best size and shape of sampling units?
Regular - grid-like quadrats
Irregular - catchment-shaped quadrats
Spatial sampling of river networks: systematic or Stratified
Systematic would give an even spread over the system
Stratified might be better when trying to capture variability that depends on a specific variable
How frequently should you take measurements
You need to match the frequency with the variability – pilot study?
Collect high frequency data and degrade > data handling issues
Why are there problems with how frequently you need to take measurements?
Many environmental phenomena display regular temporal variation (cycles)
Measurements at fixed time intervals may record the same part of whatever cycles are present, missing the data’s variation
How long should you sample for?
The length of the established cycle when it is possible
What does scale mean?
Cartographic map scale - ratio of map distance to ground distance
Geographic scale - the size or spacial extent of the study, e.g. the watershed vs plot scale
Operational scale - a.k.a. the process scale or characteristic scale. Scale at which a process operates in the environment. Associated with the processes spatial extent and temporal duration (lifetime and scale)
Measurement scale - The space/time between samples (resolution) and the integration of time of a sample
Smallest discernible spacial unit (e.g. pixel) and event.
Fine vs. coarse resolution (space) low vs high resolution (time).
Extensive vs intensive studies
Extensive = large area in little detail
Intensive = small area in a lot of detail
Physical river catchment variables
o Land use
o Topography
o Precipitation
o Water quantity
o Sediment concentration/flux
o Soil properties
o Fluvial geomorphology
Chemical river catchment variables
o pH
o conductivity
o DO
o BOD
o Nutrients
o Metals
o Pesticides
Biological river catchment variables
o Macrophytes
o Invertebrates
o Diatoms
Why should we measure precipitation?
o Need total daily or monthly measurements for water resources
o Need continuous monitoring to assess rainfall intensity for flood warning
How do we measure precipitation?
o Rain gauges and snow gauges
o Suffer from air turbulence, evaporation losses, and wetting losses.
o Point measurements that must be interpolated
o Radar can provide an aerial measure
How do we measure velocity?
Floats
Current Metering
o Need to work out where the average velocity of the stream is – 0.4 Depth (or 0.2 and 0.8)
o propeller or electromagnetic – don’t want these in the stream for long periods of time
How do we measure stage?
Pressure transducer and data logger
* Continuous measurement of stage from a stilling well – a tube with lots of holes cut into it
* Stage related to known discharge data via a rating curve
How do we measure water table?
Dip wells
How do we measure sediment
- Grab sampling and then subsequent lab analysis
- Can use claw samplers
How do we measure suspended sediment
- Collect water samples
- Spatial variation across the channel
- Depth integrated samples account for vertical variation – take water all the way from the top to the bottom then back to the top
- Automated samplers allow remote sampling and storm triggering
How do we measure sediment concentration
- Filtration of water samples of known volume
- Expressed as dry weight of sediment per volume of water (mg l-1 )
What is Turbidity
Cloudiness of water
Secondary data websites
NRFA
Environment Agency
Sampling in space:
Distribution of sampling locations
* Many environmental phenomena are continuous over space
o Precipitation, air temperature etc. changes constantly
o Need to split a continuum into discreet measurement units
* Measurements taken at particular points, along lines (traverses or transects), or in small areas (quadrats)
When do you sample/take measurements for river system dynamics?
Discharges can be highly variable from year to year
You want to measure before the storm, the rising limb, falling limb, and after
Preferably you would measure under different antecedent conditions
What do you have to consider when planning a project?
Include what you’re going to measure
Where you will measure it
When you will measure it
How you will measure it
Why you want to measure it
What else can you use to measure discharge?
Natural Rated Sections
* Cheap and simple to install
* Ideally requires bedrock section
* Problems with overbank flow and gauging of high discharges
What are suspended sediment rating curves?
- Usually collected randomly so don’t have continuous measurements
- Relationship between SSC and Q
- Simple to construct - log-log scale
- Applicable to long runs of pre-existing data – danger of extrapolation – if something in the catchment changes, you would have to alter the curve
- Not a particularly strong line – systems can be prone to sediment exhaustion
- Can be improved by dividing:
o seasonally
o by rising/falling limb
How to measure turbidity and why is it useful?
Turbidity probes
Can help understand sediment conc. and grain size distribution
What chemical things should be measures?
Routine
o pH
o Conductivity
o Dissolved oxygen
o Biochemical Oxygen Demand
Other methods
o Buffering capacity (alkalinity)
o Hardness
o Dissolved organics (carbon)
o Metals and salts (ICP-OES)
o Anions incl. nutrients (IC or spectrophotometer)
spot vs continuous sampling
- Batch aka grab sampling – collected at a specific time and place
- Continuous sampling – uses data loggers over longer time periods
In situ vs lab methods
In situ:
Portable probes
o pH
o Conductivity
o Dissolved oxygen
e.g. Sonde sensor
measures:
o pH
o DO
o Turbidity
o ORP
o Specific ions (e.g. NO3)
* Estimation of Solute Flux
* Can be left out for a long period of time
-ves
– less accurate than the lab ones
- Calibration
Lab methods
-ves = sample degradation
Biological Indicators of Water Quality
- Water quality concerns
o Longer term perspective - Representivity of spot water samples
o Water chemistry is dynamic
o Longer life cycles of biological indicators - Useful biological groups
o Macrophytes (plants)
o Macro-invertebrates
o Algae (microscopic – e.g. diatoms)
What is kick net sampling
Stand in the river and kick the bed sediment so anything that is disturbed will flow downstream into the next
