8.5.3 Determination of the capacity of wells
The efficiency of a well cannot reach 100% since there exist friction losses.As a result，we have to estimate the pumping capacity of each well.
The quantity of groundwater that flows into a deep well（Q）can be expressed as follows：
where
ie＝the entry hydraulic gradient of groundwater flowing into a well；
hw＝the groundwater level at which groundwater flows into the well（see Figure 8.17）
According to Sichart and Kyrieleis（1930），the entry hydraulic gradient cannot be larger than
where
k is the coefficient of permeability.
If one substitutes ie，max for ie in Eq.8.33，the pumping capacity of a deep well（Qw）will be
Figure 8.17 Working merhaniss of pumping
Because the value of hw is difficult to estimate，assume it to be about the groundwater level in the deep well at the preliminary estimation.That is，hw＝h0.Then examine it using the results of the pumping test.When pumping is carried out，the capacity of each deep well in the multiple wells may be smaller than the above calculated Qw for there exists intervention among wells.The influence among the wells in the multiple wells can be estimated by pumping test or numerical simulation of groundwater.
The well point method is also affected by friction loss and group effect，which relate to the type of soil or the coefficient of permeability.Table 8.3 lists some suggested values of the capacities of the well point method（JSA，1988）.
Table 8.3 Relationship between coefficient of permeability and Qw for well points（JSA，1988）
