Tool 13
TOOL 13. WATER QUANTITY MEASUREMENTS FROM WATER RESOURCES AND INFRASTRUCTURE
To obtain good insight in the amount of water available from water resources and infrastructure.
OBJECTIVE
To obtain good insight in the amount of water available from water resources and infrastructure.
METHOD
Rainwater
To determine the rainwater supply, it is necessary to know how much it rains, when it rains, the pattern of dry and rainy seasons and how they vary from year to year. Data on this can be collected from a weather station. The reliability of the data depends on the location of the station in relation to the intervention site, its calibration and the accuracy of measurements.
As well as getting general information on precipitation from a weather station, it is possible to install rain gauges at the implementation site.
Surface water
There are several methods for measuring the quantity of water available from surface water resources. It is worth noting that the method used depends on the size of the watercourse. In a big river it is possible that the application of simple methods of estimating the capacity cannot be applied.
Velocity-area method
Step 1: Measure the width and depth of the water course.
Step 2: Measure the velocity of the surface flow. This can be done by measuring the time it takes for a float to go along an established section of the watercourse (as straight as possible). When the water depth is less than 1 m, the average velocity is considered 80% the surface velocity. Measure the velocity at least 3 times.
Step 3: Calculate the discharge of the water course.
The discharge of the watercourse can be calculated as follows:
Q = 800 * V * A
Q = Discharge in l / s
V = superficial velocity in m / s
A = cross sectional area in m2
Volumetric method
Step 1: Divert the stream of water to flow in its entirety into a container with a certain volume.
Step 2: Measure the time it takes to fill the container. Do this at least 3 times.
Step 3: Calculate the discharge:
Q = V / t
Q = Discharge in l / s
V = Volume of container in l
t = average time in s
Ground water
Information of the quantity of ground water resources can be obtained in the following ways:
Secondary data: Collect hydrogeological data, such as geologic and topographic maps, data from borehole drilling, reports from geological survey, meteorological records, hydrogeological data, etc.
Community geophysical research: This can include well testing and electrical resistivity measurements. Through this you can set the location, depth and quantity of groundwater. Alternatively, when wells are present in the community, the community itself can be a good source of information on groundwater. In that case, consult the residents or do simple measurements on the locations of the wells and the depth of the ground water.
Based on the collected data, a hydro-geologist can determine the rate at which water can be abstracted without danger of depleting the surface water resources.
BASED ON
CINARA, 2007. Lineamientos para el diseño y administracion de sistemas de abastecimiento de agua bajo el enfoque de usos multiples, Cali: Instituto Cinara – Universidad del Valle. Available at:http://www.musgroup.net/home/activities/the_cpwf_mus_project/basins_countries/andean_basin_cpwf_mus_studies/andes_in_english/andes_basin_outputs/guidelines_for_planning_and_management_of_mus_systems/(language)/eng-GB.