Water Bodies
SPECTRAL RESPONSE OF WATER The spectral response of a water body is a function of water characteristics, depth of the water, and organic or inorganic matter suspended in the water.
Reflection within Water & at the Surface Spectral characteristics of turbid river water and clear lake water.
Reflection from the Bottom
Spectral transmittance for 10 m in various water types.
WHAT CAN BE DETECTED, MEASURED, AND MAPPED IN WATER?
• Material floating on the water surface is usually easy to detect and to map its areal extent.
• Certain things cannot be detected.
• This includes gasses (oxygen, carbon dioxide) dissolved in water, inorganic salts (sodium chloride), and acidity (pH).
Algae, for example, appears prominently in near infrared as a bright surface on a dark background.
Floating oil is detectable, though not so prominently, in shorter wavelengths, 350–450 nm, and again in the thermal infrared.
TIMING OF WATER MEASUREMENTS
• Water bodies are some of the most dynamic features on earth, and certain aspects of them may change in a day, or even a few hours.
• The rate of change in water level will vary with the area and volume of the water body and the shape of the basin containing it.
• Sediment concentrations—and algae—are not uniformly distributed in lakes and reservoirs, and significant areal variation in concentration is often present.
TIMING OF WATER MEASUREMENTS
• Currents in a lake can be due to wind and stream inflow, producing a visible pattern in the suspended sediment that changes continuously.
• All these variables make field work on water bodies highly time dependent, and may require making observations and taking samples at the exact time of an imaging overflight.
FIELD MEASUREMENTS
Suspended Sediment & Turbidity
• The terms “suspended sediment” and “turbidity” refer to the same phenomenon but are not interchangeable.
• Suspended sediment is the concentration of particles suspended in the water.
• Turbidity is an optical quality referring to the amount of light transmitted through the water.
Suspended Sediment & Turbidity Sediment in the water may be observed in two ways:
• as suspended sediment measured in weight of sediment per unit volume of water,
• or as turbidity measured in terms of light transmission through the water.
Suspended Sediment & Turbidity
• Suspended sediment may be collected in a sample bottle designed so that the operator can open and fill it, at a selected depth.
• Also, it is possible to measure suspended sediment in situ using a sediment gauge.
Sediment gauge
Suspended Sediment & Turbidity
• Turbidity is measured according to the attenuation of light transmitted through the water (absorbiometric), or it may be measured by the amount of scattering as light passes through water (nephelometric).
The Secchi Disk
• A useful device for evaluating water transparency is the Secchi disk.
• Using this disk has the advantages of simplicity and immediate results.
• The Secchi disk itself provides no information about the reason for variation in water clarity.
• Water color is also a factor determining the depth of visibility of the Secchi disk, and recorded observations should include notes on water color that may relate to types of algae present.
The Secchi Disk
• Also, readings should only be taken during the high sun period when light penetration is greatest.
• Between 10 A.M. and 2 P.M. is the optimal period.
• For best accuracy and precision 6–10 Secchi measurements should be averaged and corrected for solar altitude before attempting to compare data from different lakes.
Sampling Plans
• The sampling plan should consist of several transects across the water body collecting data or samples at regular intervals along the way.
• A GPS receiver is essential on a water body in order to know when one has reached a selected sample site.
• Plan on making enough observations with spacing close enough to have 6–10 points in various sediment concentrations.
Collecting Suspended Sediment Samples
Depth Measurements
• Water depths may be measured by a sounding rod, a weighted cord or cable, or an ultrasonic sounding device.