The head at the well node rises above the value of

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Unformatted text preview: ue to decrease with time (fig. 29). In the 14th time step of the first transient stress period, the head in the aquifer drops below the value of hlim, so at this time the discharge ceases, and hWELL equilibrates with the head in the aquifer. Water-level recovery begins at this location about 40 days into the second stress period (after pumping ceases in the three upgradient single-node wells). The head at the well node rises above the value of hlim at a total elapsed time of about 306 days, and at that time the well begins to flow again. The flow rate then increases with time as the head in the aquifer increases. The water level in the well, however, remains constrained at a value of hlim = 0.0 ft. Pump Capacity The capacity of a pump installed in a well to deliver water depends on several factors, including the size of the pump and the power of the motor. It also depends on the lift, or vertical distance over which the water must be raised. That is, for a constant-speed pump with given characteristics, the yield (or discharge) will vary depending on the lift requirements and other factors. There are a number of reasons why well yields and pump performance might decrease over time (see, for example, Driscoll, 1986). Some involve damage or deterioration to the pump or well screens. Others simply are related to changing heads over time. Driscoll (1986, p. 583) gives an example for a deep-well turbine pump where “the total head would be as low as 60 ft [18.3 meters (m)] during a season of high water level or minimum withdrawal of water; but during another season, the total head might be 100 ft (30.5 m) because the water level in the aquifer has decreased or interference from adjacent wells has increased. Under these Figure 29. Plot showing relation between computed net discharge (Qnet) from a nonpumping, free-flowing, single-node well located close to the downgradient boundary in a variation of the Reilly problem. Shown for comparison are the computed head in the aquifer at the well location (hn) and the computed water level in the well (hWELL) for a case in which three upgradient single-node pumping wells are active during the first 150-day transient stress period and inactive during a second 300-day transient stress period. Model Features and Processes conditions, the rate of pumping would range from nearly 1,340 gallons per minute (gpm) [7,300 cubic meters per day (m3/ day)] down to about 620 gpm (3,380 m3/day).” Driscoll (1986, p. 585) also shows that there is a “shut-off head” at which no flow will occur, which is consistent with Halford and Hanson’s (2002, p. 11) statement that it is unrealistic for pump discharge to vary smoothly from the specified rate down to zero. Boonstra and Soppe (2007) relate pump efficiency and pump performance to the total dynamic head, which they state “is made up of (1) the water-level depth inside the pumped well …; (2) the above ground lift; and (3) head losses due to friction and turbulence in the discharge pipelines.” Conceptually, after pumping starts, the water level in the well will decline over time, and the lift (and total dynamic head) required to discharge at a fixed point and elevation above the land surface will increase. As the total dynamic head increases, more work is required to lift and discharge a unit volume of water and so the discharge from a standard constant-speed pump will tend to decrease. The methods described in this report are not applicable to a variable-speed pump designed to maintain a constant discharge under conditions of changing lift. Most pump manufacturers provide performance curves for their products that typically include a head-capacity curve relating the total dynamic head to the discharge rate (Boonstra and Soppe, 2007). A hypothetical example set of performance curves having representative shapes is shown in figure 30. Near the design capacity of the pumps, the curves are steeper and there is a relatively small change in discharge for a unit change in total dynamic head. However, as the lift increases, the curves tend to flatten out and there may be a relatively large change in discharge for a unit change in total dynamic head—until a point is reached where the pump can no longer provide water, and the discharge decreases to zero. In developing, calibrating, and using a ground-water flow model, there may be cases where it is deemed valuable to incorporate the reduction in well yield with increases in drawdown. Where historical data on discharge from wells are based on metering or other estimates of the total volume produced over a given time period, incorporating these relations may provide little or no added value for model calibration. However, if the model is used to make predictions of future behavior, evaluation of management scenarios, or for small-scale studies near a pumping center, the use of these head-capacity curves may add more realism and defensibility to predictions of future conditions. The new MNW2 Package allows the user to specify a performance curve (head-capacity curve) for each well. This capability is of...
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