Objective: To estimate the water yield in a catchment (of a lake, pond, stream or river).
Catchment (Drainage area, Drainage Basin or Watershed): The area of land draining water into a water body (fig1). Two neighboring catchments are separated by a ridge (highest land that separates two watersheds). The areal extent of a catchment is obtained by tracing the ridge on a Topographical Map (fig2). Based on the spatial extent, catchments are classified given in table 1.

Table1: Classification of Catchments in India

 Source: http://fes.org.in/source-book/SWC%20Source%20Book_final.pdf

Contours: Contours are the imaginary lines on the earth surface with equal elevation. In a topographic map of 1:50000 scale, contours are at every 20 metre interval. Contours with decreasing altitudes with respect to an higher altitude contour indicates hillock, on the contrary increasing contours along a low altitude contour indicates Valley. 
Steps involved in Delineating a Catchment:

step1. Scan the respective topographic map
step2. Use QGIS, Geo-reference or assign the co-ordinates
step3. Digitise waterbody as polygons and Stream network as line features.


a. Create New features as line or polygon

Go to Layers, New Shape file Select Line or Polygon feature based on the feature to be delineated, add attributes and save.











Assessment of water yield: Water yield or Surface Runoff is the precipitated water that drains to a water body in a catchment. Surface runoff occurs during monsoon. Factors affecting Runoff are the Slope, Drainage, Land use, Soil Characteristics, Rainfall. The total quantity of water that can be expected in a stream in a given period of time such as monthly, annual etc… is referred to as Runoff Yield.

Runoff Yield (Q) as kilo.cubic metre (Million Litres) is estimated empirically (eq.1) as a function of Rainfall (P) in mm and Area under different land uses (A) in Hectares.
Q = (C*P*A)/100                               1
Where  Q = Runoff Yield in Million litres
C = Runoff Coefficient of a particular land use
A = Area under land use in Ha
P = Mean Monthly rainfall in mm (average of 10 – 15 years)

Runoff Coefficient under different land use is as specified in table 2.

Land Use	Runoff Coefficient
Forests	0.1 – 0.2
Plantations	0.2 – 0.6
Agriculture	0.4 – 0.7
Open Spaces, Grasslands	0.5 – 0.8
Built-up	0.7 – 0.9
Rocky Areas	0.8 -  1.0




 

Water Yield Estimation

Location Description
Catchment Extent			Latitude				Longitude
			min				min
			max				max
Catchment Area in Ha
Annual Rainfall in mm
Land Use			Area in Ha A				Runoff Coefficient C
Forests
Plantations
Agriculture
Open Spaces, Grasslands
Built-up
Rocky Areas
Monthly rainfall P in mm	January	February		March		April		May	June
	July	August		September		October		November	December
Seasonal Catchment Yield in Million Litres Q = (C*A*P)/100
Land Use	January	February		March		April		May	June
Forests
Plantations
Agriculture
Open Spaces, Grasslands
Built-up
Rocky Areas
Gross Yield  Q
Land Use	July	August		September		October		November	December
Forests
Plantations
Agriculture
Open Spaces, Grasslands
Built-up
Rocky Areas
Gross Yield Q
Annual Catchment Yield  = (Σ Q)					Million Liters

 

 

 

 

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