08 - ASSESSMENT OF WATER QUANTITY

• important in planning a water supply system

• gives the planner basis to decide for or against its development

PRODUCTION CAPACITY

The vertical distance from the center line of the discharge to the water surface in the well when there is no pumping

STATIC WATER LEVEL

The vertical distance from the center line of the discharge to the water surface in the well while pumping

PUMPING WATER LEVEL

The difference between the static water level and the pumping water level.

DRAWDOWN

The volume of water per unit time that could be safely pumped from the well, as determined by a pumping test

YIELD OF WELL

The volume of water per unit time discharged by the spring.

YIELD OF SPRING

SOURCE CAPACITY MEASUREMENTS

1. PRODUCTION CAPACITY

2. STATIC WATER LEVEL

3. PUMPING WATER LEVEL

4. DRAWDOWN

5. YIELD OF WELL

6. YIELD OF SPRING

MEASUREMENTS OF DISCHARGE

1. VOLUMENTRIC METHOD

2. V - NOTCH WEIR METHOD

3. FLOW FROM A HORIZONTAL BLOW - OFF PIPE

• Done by measuring volume

• Necessary equipment: 1) wrist watch or timer and 2) bucket or drum of known volume

• Repeated measurement for accuracy then average time is taken

• Bigger container = greater accuracy

VOLUMENTRIC METHOD

• an overflow structure built across an open channel for the purpose of measuring the rate of flow of water

• may be rectangular, trapezoidal or triangular in shape

V - NOTCH WEIR METHOD

• procedure for measuring flow from a horizontal pipe as shown in the figure.

Two conditions required for this method:

• The pipe must be flowing full

• The pipe must be horizontal.

FLOW FROM A HORIZONTAL BLOW - OFF PIPE

GIVE THE STATIC WATER LEVEL

• THE WATER TABLE IS FALLING. THE AQUIFER IS BEING DEPLETED FASTER THAN IT CAN RECHARGE.

DROPPING

GIVE THE STATIC WATER LEVEL

• THE SCREEN OR STRAINER MAY BE CLOGGED AND WATER IS NOT FREELY GOING INTO THE WELL.

UNCHANGED

GIVE THE STATIC WATER LEVEL

• THERE IS A LOSS IN PUMP EFFICIENCY

UNCHANGED

uses electrical signals to determine the distance from the surface to the water level in the wellbore.

ELECTRIC SOUNDER OR ELECTRICAL DEPTH GAUGE

• An electrode suspended by a pair of insulated wires is lowered into the well as shown in the figure. When the electrode touches the water surface, the circuit is closed and current flows. An attached ammeter may be used to indicate the current’s flow.

• Equally effective is to use a light bulb and flashlight batteries instead of an ammeter as shown.

• To improve the accuracy of readings, the electrode and cable should be left hanging in the well for a series of readings.

ELECTRIC SOUNDER OR ELECTRICAL DEPTH GAUGE

• This method involves lowering a tape into the wellbore until it reaches the fluid level, and then withdrawing the tape to measure the length of tape that is wetted by the fluid.

• can be an accurate method of measuring depth of water and can be readily used for depths up to 25 m.

WETTED TAPE METHOD