AVAILABILITY OF WATER THE WATER CYCLE Water can come from several different sources; groundwater, surface water, rain harvesting, or water storage. Some growers may have easy access to quite a few of these resources, while others have trouble getting sufficient water from one of them. The availability of water depends on the location of the field. In addition, the crop that is being grown and the irrigation method used has an effect on the amount of water needed. It is important for all growers to manage their water source to ensure a successful growing season. GROUNDWATER Groundwater may be the most reliable water source on earth and its correct management is vital to ensure its supply for the future. Unfortunately, groundwater measurements are fairly complicated, because its flow is hidden. So if the availability of groundwater is not watched carefully then problems such as supply limitations, pump wear, clogging, and over-pumping may occur. The type of irrigation system being used should be taken into account when selecting a pump due to different operating pressures that may be required from, say, a sprinkler system compared to a drip irrigation system. An incorrect selection can lead to inadequate water and/or pressure. The water quality may also cause increased wear on the impellers and other pump components. It is possible that the availability of water in a well is constantly changing. If that is the case, a VFD should be used; they are designed to react to unsteady water flow and safeguard the pump from dry-running. Similarly, if the water table drops during the irrigation season a VFD may be used to ensure the flow and pressure required for the irrigation system. The pump and motor selection must be made for the maximum drawdown level to ensure the proper water and pressure requirements. Over-pumping is when more water is being pumped from a well than the well can efficiently supply, meaning the groundwater cannot be replenished. The pump will dry-run and be damaged, if the pump keeps running. If the pump pulls in sand, this can also damage the pump. Using an in-well sand separator prior to the pump unit will remove unwanted sand. However, the best way for the farmer to try to prevent damage to the water supply and pump is to analyze how much water the well can provide. Water availability on Earth: • 3% is fresh or 1% available for use • 70% is held in the ice caps • 30% is available as groundwater or from lakes and streams When selecting the correct pump for pumping groundwater, the pH of the water, the temperature of the water, the minerals in the water and the salinity are important. The length of the irrigation period and the total run time of the pump must be considered to provide enough time for service and repairs, also if additional irrigation run times are required due to extremely hot temperatures. If electrical use rates are based on when the pump is operating, an important consideration is to select the pump size so that water delivery can be made in off peak operating times. In some cases a farmer may know that if he pumps water straight from the well, he will not have enough water. In that case a reservoir can be built to store the water and pump from this surface supply when required. Water is steadily pumped into a reservoir for longer time periods than it is pumped out while it is pumped out of the reservoir at a faster rate than it is pumped in. The amount of water in the reservoir therefore balances out and can provide a sufficient supply of water for the farmer. This may be done for frost protection systems where a large quantity of water may be required. However, surface storage of water may lead to the need for treatment, due to algae or other organic materials that find their way into the reservoir. Once water is placed into a reservoir it must be treated as surface water.
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