Limnology

Practical No. : 1

1. Morphometry of lakes, ponds and streams
Morphometry is the measurement of external form or shape of a selected water body. It is that branch of limnology which deals with the measurement of significant morphological features of any basin and its included water mass is known as morphometry. Morphometry defines a physical dimension and involves the quantification and measurement of any basin. These dimensions influence the water quality and productivity levels.
Morphological features, age and geology of the lake basin along with the level of human interference have a direct and significant bearing on the structural and functional attributes of the aquatic habitats. Therefore, before undertaking any limnological investigation it is essential to prepare the maps and generate information on the morphometry and general characteristics of the area.
A. Morphometry of lakes
a. Location : Trace the correct name of the locality and its latitude, longitude and altitude from authentic sources such as government records and publications, maps and topo-sheets of the Survey of India etc.
b. History : It is important to know the history of the area, land use of surroundings, formations or excavation of the basin, past glaciations or tectonic activities if any and other relevant information available from published or unpublished work, district gazetteer and by interviewing the local inhabitants.
c. Area : Surface area is an extremely important dimension for it is at the surface that the solar energy enters into the lake and marks the beginning of the lacustrine energy exchanges. As a result many limnological data related to productivity and heat budget etc. are generally given as unit area of the lake surface, making it possible to compare meaningfully the limnological characteristic of different sized water bodies.
Requirements: Stakes, measuring tape, graph paper, planimeter
Method
i. Mark a base line AB of suitable length on one side of the water body and plot it on a graph choosing a suitable scale.
ii. Depending on the shape of the water stretch, take two or more such base lines at right angles to each other.
iii. Put stakes along the shore line at suitable distances.
iv. Measure the vertical distances between the baseline and stakes on the shore line at regular or suitable intervals and note them against the point number 1, 2 and 3 and so on.
v. On the graph sheet plot the vertical distances in accordance with the scale.
vi. Connect all points to form a shore line diagram.
vii. Compute the area directly from the graph or with the help of a planimeter.





The method is convenient only for small water bodies but for larger water bodies more sophisticated survey techniques should be used.

Bathymetric map (Contour map). It can be prepared by recording the depth at several points at equidistance in the lake; the number of points will depend on the size of the size of the water body. The shoreline map is a pre-requisite for this exercise and the observer should know the cross-wire bearing in the lake at a given point. This method is convenient for shallow lakes only.
a. Maximum Length (L): It is distance between two most distant points on the surface of the lake without land interruptions.
b. Depth (Z): Depth is the minimum vertical distance between the surface and the underlying bottom of the lake at any point, along with the area it gives an idea of the volume of water in the lakes.
Requirement : Heavy metallic plate (circular) with a central hook (Secchi disc can be used), good quality nylon rope and measuring tape.
- Tie one end of the graduated rope to the central hook of the metallic plate
- Lower the plate in water till it reaches the bottom
- Measure the length of the rope in water with the help of the measuring tape and the marking on the rope
Result : Express the depth in meters (m).
c. Maximum Depth (Zm): It is the measured at the deepest point of the lake.
d. Volume (V): Volume of the basin is the integral of the areas at successive close depth starting from the lake surface to the deepest point. It can be computed by the summation of the volume (m³) of truncated cones of the entire column, (V=∑v). The volume of each stratum is calculated as under:

Where, h = Vertical depth of the stratum

A1 = Area of upper surface of the stratum

A2 = Area of lower surface of the stratum

(The area of the upper and lower surfaces of the stratum can be calculated from the contour map).
e. Mean depth (Z): Mean depth is calculated by dividing the volume of the lake by its surface area (Z/A).
f. Geology: This includes the geomorphological, pedological (origin and development of soil), edaphological (soil characters) and topographical information, which can be collected from reliable sources like local and regional offices of survey of India, Geological survey of India, District Gazetteer and relevant published Literature. In case, no such information is available the help of a Geologist should be taken.
g. Catchment area: It is the surrounding area of the lake which influences it either directly or indirectly through run off or otherwise. Information regarding the catchment area includes its geology, drainage pattern, agricultural and other land uses; industrial and sociological aspects; the extent of forest and other natural resources, and water use pattern etc. Information on the above can usually be collected from appropriate government authorities or the concerned University departments.
B. Morphometry of ponds
A farm complex comprises of different types of ponds namely nursery ponds, rearing ponds, production ponds and breeding ponds in aquaculture system. The number and size of these ponds depend upon the water resources, variety, size of fish to be cultured and type of management.
A viable fish culture practice primarily depends on the selection of a suitable pond size, which in turn depends upon water retention capacity of the soil and availability of adequate water supply during the culture period. Knowledge of the construction of different types of fish ponds is a pre-requisite for a profitable fish culture. Areas of silt and clay which retain water to a greater extent are preferred for the construction of fish ponds. Fish pond soil should have a pH of 7-9. It is always better to determine the nature of soil up to 1.5m depth.
Structures of fish ponds
A typical earthen pond will have the following structures, namely bunds or dikes, harvesting pit, inlet, outlet and core trench.
Bunds
Bunds are the protecting structures of fish ponds and are very essential. They may be of three types
a) Main bund or peripheral bund – essential for larger fish farms enclosing a number of fish ponds of varied sizes.
b) Bunds holding water on one side, and
c) Bunds dividing two adjacent ponds.
Slope
The life span and strength of any bund depend not only on the quality of soil but also on its slope and crown. The slope may be defined as “the ratio of horizontal increase in the base of the bund from the point of perpendicular to the top edge of the bund to the edge of the base of the bund on the same side per unit length”. For ordinary ponds of less than 0.5 ha, the wet side slope may be 1:1.5 and the dry side slope 1:1.
Berm
If the production pond is more than 0.5ha, a platform – like space between bund and watery area known as berm or bench line should be made available. The width of the berm may vary between 0.5 and 1.0m, depending on the height of the bund and size of the pond. The berm apart from serving as a walkable space also protects the bund from direct contact with water.
Inlet and outlet
These are required especially for larger ponds, in order to ensure a smooth water supply and drainage. Further, they are helpful in preventing the entry of wild fish from outside and escape of the fish from inside the pond. The size and shape of these structures largely depend on the area and water spread of the ponds.
Crown
The crown width also depends upon the height of the bund. However, a minimum of 1m crown width is invariably needed for any bund.
Other measures to be considered during the construction of a fish farm
1. A sedimentation pond or a filtration system is made as a wall with layers of gravel, sand and mud in order to filter the water, if turbid, before its entry into fish ponds.
2. Nursery ponds may be constructed using brick and cement above ground level in order to reduce mortality of fish fry.
3. If areas of water scarcity and seepage are to be utilized for fish culture, cement ponds have to be constructed there.
4. Turfing or stone pitching may be adopted to avoid the sliding of earthen bunds.
It is also advisable to keep fencing around the fish farm to keep the cattle off ponds.
C. Morphometry of Stream
Only narrow and shallow streams can be surveyed by this method. Measure a 50 m or more stretch of the stream to be mapped. Mark at each interval of one meter and also mark a line called base line, thereafter on the graph paper representing the points on a sustainable scale. Measure the distance from each point to the near boundary of the stream by joining the points and the map of stream can be constructed. (Fig 1 & 2)

The profile of any section of a stream can be prepared by recording the depth across the stream at small intervals prepare a graph between the distance and depth to obtain the profile.

Stream morphometry is the measurement of physical dimensions of a (fluvial) object. This is done in the similar manner of taking measurements with a tool and is applied them to define a dimension. So, quite often the use of stream morphometry is to get an accurate representation morphology, but more importantly, an accurate characterization of stream morphology.

Velocity (m/sec) : Rate of movement
Water in a stream moves fastest near the surface and slows down near the bottom, where the flow is slower by friction from the roughness of the bed material. To compute the discharge of a stream, we need to compute velocity, which changes with depth. To make the best estimate of a stream's velocity hydrologists use the average velocity of a stream.
Quick stream flow measuring is best done with a meter to measure water current velocity. Stream flow measuring is easily accomplished using a water current meter and a tape measure. The current velocity meter allows to measure stream flow velocity in feet or meters per second and measure water depth in hundredths of a foot up to three feet. The average stream flow velocity times the cross-sectional area of the stream determines the stream flow measurement in cubic feet or meters per second. The area for a channel is known for pipes, or is determined for a stream flow measurement by measuring the distance from shore and water depth at various points across the stream flow to construct a channel profile.
The water current meter offers two unique methods for determining average water velocity:
1) For small stream flows and pipes, the current velocity meter may be moved smoothly and uniformly throughout the stream flow profile until a steady average reading is displayed. This steady reading is the true average velocity for the stream flow.
2) For larger streams, the current velocity meter may be used to measure a vertical profile of water velocity at several points across a stream channel. The stream flow measurement for the profile is the sum of the average velocity of each subsection of stream flow times its cross-sectional area.
Gradient (m/km or %) : drop in elevation over a distance
Stream gradient is the ratio of drop in a stream per unit distance, usually expressed as feet per mile or meters per kilometer. A high gradient indicates a steep slope and rapid flow of water (ie. more ability to erode); whereas a low gradient indicates a nearly level stream bed and sluggishly moving water, that may be able to carry only small amounts of very fine sediments. High gradient streams tend to have steep, narrow V-shaped valleys, and are referred to as young streams. Low gradient streams have wider and less rugged valleys, with a tendency for the stream to meander. These are older streams, in geological time.
A stream that flows upon a uniformly erodable substrate will tend to have a steep gradient near its source, and a low gradient nearing zero as it reaches its base level. Of course a uniform substrate would be rare in nature; hard layers of rock along the way may establish a temporary base level, followed by a high gradient, or even a waterfall, as softer materials are encountered below the hard layer.
On topographic maps, stream gradient can be easily approximated if the scale of the map and the contour intervals are known. Contour lines form a V-shape on the map, pointed upstream. By counting the number of lines that cross a stream bed within a measured distance, and converting this to the actual measurements of the land surface, will determine the actual gradient. For example, if one measures a scale mile along the stream length, and counts 3 contour lines crossed on a map with ten-foot contours, the gradient is approximately 30 feet per mile, a fairly steep gradient.
Stream Gradient Calculations

Cross-sectional area (m²) : Area of the stream perpendicular to main flow
The cross-sectional area of the stream is determined by multiplying channel depth by channel width along a transverse section of the stream. For a hypothetical stream with a rectangular cross-sectional shape (a stream with a flat bottom and vertical sides) the cross-sectional area (A) is simply the width multiplied by the depth:

A= (W x D)

Discharge (m³/sec) : Rate of water movement - influences:
1) substrate conditions
2) disturbance of biota
Stream discharge is the volume of water passing through a particular cross-section in a unit of time, measured in cubic meters per second or cubic feet per second. The discharge of a perennially flowing stream is provided by the influx of groundwater into the channel. This influx provides what is called the ‘base flow’ of the stream. Water is added to the stream by runoff from the surrounding terrain during storm events.
Discharge (Q) can be expressed as

Q = A x V

where,

A= cross-sectional area

V= velocity