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.

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  dam - aged, 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.

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.

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 bal- ances 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.

SURFACE WATER

Surface  water  is  the  second  most  common  source  of  fresh- water. It comes in many different forms, from lakes, springs, rivers, ditches, streams, and so on. Water can easily be pumped from any surface water location. Nevertheless, there are some factors that need to be considered in order to design an efficient surface water pumping system.

The first thing that one should realise is that if a public surface water source is being used, several users will be pumping from it,  and  the  water  level  can  be  significantly  lower  during  dry seasons. In order to ensure an adequate amount of water and to prevent dry-running a pump, a water storage facility should be created.

If the water source is prone to flooding, submersible pumps should  be  used;  dry  motor  pumps  will  be  damaged  beyond repair if they become submerged in water. If a pump is in an area where it may be stolen, it should be protected with a fence or structure to prevent theft or at least make it diffucult to steal.

Another  factor  to  consider  when  designing  a  surface  water pumping system is the water quality. Surface water can carry a substantial amount of mud, silt, and other particles during the  rainy  and  snow-melting  season.  It  is  important  to  con - struct a settling canal before the pump suction to prevent these particles from entering the pumping system. The canal needs  to be at least twenty feet long, and its flow velocity should be no more than 0.05 feet per second when water is being pumped. The particles that settle in the canal need to be removed before every irrigation season so that it can function efficiently. Organism growth in the canal may also affect its performance. The canal should therefore be covered so that sunlight will not stimulate organic growth. The water can also be treated or pre-screened at the intake of the pump with self-cleaning screens or a series of screens to prevent trash and other organic materials from entering the pump and interfering with the operation of the pump.

RAINWATER HARVESTING

When groundwater and surface water is not available or does not provide enough water, artificial reservoirs can be created to bring an ample amount of water to a farm. Two types of man- made reservoirs are rainwater harvesting systems and water storage systems. This option is of course limited in many dry climates  where  there  may  not  be  sufficient  rainfall  to  make this practical. It can however be another source to supplement surface or groundwater supply.

Rainwater  can  be  harvested  and  stored  for  future  use.  Sur - faces such as roofs, roads, and paved areas are used to collect the water, and it is then stored in tanks.  In order to design a sufficient  rainwater  harvest  system,  the  demand  of  water, average amount of rainfall, and size of storage tanks have to be considered. This system alone usually does not capture enough water to perform all irrigation applications, but it has the ability to carry out most. Treatment of the water during storage may be required as runoff from roofs and other structures may create water quality issues.

WATER STORAGE

A water storage system can either be built as an above ground, open-air basin, or as an underground water tank.  These two different  types  of  water  storage  systems  perform  the  same function, holding water that will be used during peak demand periods, because the main water source cannot meet the peak demand  for  water.  The  system  that  is  above  ground  is  less  expensive to build or remove. However, water can evaporate, which may lead to a build-up of salt. Since it is above ground, algae and moss are more prone to grow, the basin takes up productive  space,  and  there  is  the  risk  of  drowning  when working in or around the basin. Underground tanks do not pose the problems that open-air basins do. However, they are more expensive to build and remove.

For  both  types  of  water  storage  systems,  parallel  operating boosters should be used for the distribution pump system. They offer several benefits, including smaller motor sizes, starting amp reduction, and water hammer reduction.

A water bank is a further type of water storage. This involves pumping  water  down  a  well  in  one  place  to  replenish  the aquifer, thereby storing it.  The water is then drawn up again when needed.