Yield Measurement of a Well
Yield has been defined as he amount of water flowing into the well per unit time. It is usually expressed in liters per minute, l.p.m or some times in cubic metres per day.
1.5.1 Gravity well: When gravity well is pumped, the water level in it is lowered. This is accompanied by a lowering of water table in the vicinity and the hydraulic gradient assumes a slope towards the well at the apex. The base of the cone is called the circle of influence. The various factors affecting the yield of a well are.
(i) D, the static head or the vertical distance from the water table to the bottom of the well
(ii) D, the depth of water in the well when it is being pumped.
(iii) D-d, the difference of level in water before and at the time of pumping, called the draw down.
(iv) R2 the radius of the circle of influence
(v) r2 the radius of the well
(vi) Fineness and porosity of the soil
(vii) Inclination of the aquifer
Quantity in l.p.m
Q = ppk x 1,000 (D2-d2) l.p.m
24 x 60 x 2.303 log10 R/r
D, d, R and r being in metres
Or Q = K D2-d2 l.p.m
Log 10 R/r
Which is known as the Dupit Formula,
Where K = ppk x 1,000
24 x 60 x 2.303
Is the transmissibility coefficient?
1.5.2 Artesian Well: The formula for yield of the artesian well is derived in a similar fashion, modified by the fact that the water in this case slows through the confined aquifer having a thickness.
Thus, the area of the substratum becomes A=2 and final expression of yield
Q = K. D-d l.p.m
Log 10 R/r
With K = ppk 2t x 1,000
24 x 60 x 2.303
And with various parameter expressed in their respective units.
Specific yield of a well is its yield per metre of draw down. It is also called the specific capacity of the well. It is used in comparing yields from different wells. A well having high specific yield would be found to give larger yield for the same depth of draw down and hence more acceptable.
1.5.3. Well Tests. These are actual tests carried at site determine the yield of well. These include (1) Pumping (2) Recuperation Test.
Pumping Test involves pumping water from well at a rate so as to maintain a constant draw down. Pump is done at a heavy rate at first till the condition of maximum working head is reached. The rate of pumping is then adjusted that water level in the well remains constant for a number of houses. This means that the rate of pumping equals the rate at which we percolates into the well i.e., yield of the well. The test is usually carried out in the driest part of the year as otherwise it become difficult to maintain the condition of a constant draw down.
Recuperation Test involves depressing by pump water level in the well to any safe level below the normal pumping is then stopped and the water in the well allowed to require its original level. The gradual rise in the water level is observer brief intervals of time. Knowing the cross sectional area of the and the time taken by water to reach different levels, the yield of well at different draw downs can be calculated.
K = 2.33 A/T log h1/h2
The recuperative test is simpler than the pumping test and much used in practice. This has however one limitation and it is since the water level in the well does not remain constant and not the maximum safe working head cannot be maintained through the period of observation, so that this test will not give the maximum safe yield of the well.These wells, which are generally, located 15m. Or more from the riverbank are constructed in open cut so as to reach the water-bearing stratum, which are usually 7.5 to 9m below the ground level. The trench is carried below the minimum level of the water table and gallery or pipe with numerous small openings is constructed. The openings are then back-filled with graded gravel to prevent the entrance of very fine material into the gallery. The gallery is similar covered to prevent the entry of any surface polluted water and provided with manholes at intervals to permit entrance for inspections. The infiltration gallery is generally laid on a grade so that the water may be collected first in a suction well and later pumped into the supply system. Ti is a simple means of obtaining naturally filtered water. Quantity of water an be large depending upon the availability of pocket of coarse sand in the substratum.