Micro-Irrigation Systems for Vineyards
Selecting a Well Pump
A major component of any micro-irrigation system is the pump. An irrigation pump is selected to match two important irrigation system parameters, total irrigation system flow rate and total dynamic head required.
Estimating Total Irrigation Flow Rate
The pump capacity needed, and the discharge pressure should be determined prior to purchase. The capacity is determined by the vine’s water requirements, the efficiency of the micro-irrigation system, and the largest acreage in the vineyard to be irrigated by the pump at a given time. The discharge pressure is determined by the desired operating pressure through the laterals, the pressure loss due to friction, and the various changes in elevation within the system. One of the important aspects to consider with pumping water is to consider the pump efficiency.
Total Dynamic Head
Total dynamic head (TDH) is essentially the equivalent height that a fluid is to be pumped, and one of the most important factors in the pump selection process. The pressure required for operating a given emitter or sprinkler represents only a portion of the total dynamic system head. Additional pressure must be produced by the pump to lift water from the well or other water source, to overcome friction losses in the pipe and other components of the system, and to provide velocity for the water to flow through the pipes.
Static Head
Static head is the vertical distance from the water level at the source to the highest point where the water must be delivered. It is the sum of static lift and static discharge.
Well Drawdown
As a well is pumped the water level in the well declines, which is commonly called the well drawdown. The amount of the drawdown is a function of the pumping rate, the aquifer properties, well size, method of construction (well screen, etc.), and the time the pump is operated.
Operating Head
Some irrigation systems require pressure to operate. The range of this pressure varies among systems. High pressure systems, such as sprinkler systems, may require large operating pressures (up to 100 psi).
Friction Loss
When water flows through a pipe there is a loss of head due to friction. This loss can be calculated using hydraulic formulas or can be evaluated using friction loss tables, nomographs, or curves provided by pipe manufacturers. The pump must add energy to the water to overcome the friction losses.
Velocity Head
Velocity head is the amount of energy required to provide kinetic energy to the water. For systems with a high total head this component is very small compared with other components of the total system head.
Suction Head
A pump operating above a water surface is working with a suction head. The suction head includes not only the vertical suction lift, but also the friction losses through the pipe, elbows, foot valves and other fittings on the suction side of the pump. An allowable limit to the suction head on a pump and the net positive suction head (NPSH) of a pump, sets that limit. The theoretical maximum height that water can be lifted using suction is about 33 feet.
Reading a Pump Curve
Pumps perform differently over a range of conditions primarily due to specific features of the impellers inside the pump housing. Pump manufacturers acknowledge this and publish pump performance curves. Pump curves typically provide three important performance relationships in one graphic display: (1) the relationship between pump capacity (gpm) and total dynamic head (TDH); (2) the relationship between pump capacity (gpm) and bowl efficiency (%); and (3) the relationship between pump capacity and brake (shaft) horsepower. Curves for different impeller diameters may also be presented in the same graphic.
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