Water Distribution Strategy

An energy assessment was previously done for a fruit farm in the Western Cape. It was found that 78% of all the electricity consumed was for pumping irrigation water from the water sources to the orchards. This case study is about the follow-on project to optimize the cost of water distribution on the farm.

All the water used for irrigation is from rainfall. The water is collected in 8 buffer dams/reservoirs which is also topped up from 10 boreholes. A network of pumping systems distributes the water from these sources to the orchards, as shown in the diagram below.

 

The cost of water distribution was obtained from the cost of electricity, the energy intensity (kWh/m3) and the flow rate per pumping system. This cost per cubic meter from primary sources to reservoir is given in the graph below. It shows a variation between 50c/m3 and R2.00/m3, with the higher cost water sources clearly indicated.

The same analysis was done for the secondary distribution of water (from reservoir to orchard). In this case the results were normalised to take into effect the differences in pumping heights and pumping distances. The results are shown in the graph below. The variation in cost is between 20c/m3 and 80c/m3.

The final step was to link the primary and secondary chains to obtain the total cost for water distribution to the orchards. This analysis also included the volume of water pumped from each primary and secondary source. The results in the graph below show the total cost per cubic meter for the orchards serviced from the secondary sources. It is clear that there is a huge variation in water distribution cost between the different applications.

These results were then used to develop a water distribution strategy to minimize the total energy cost for water distribution on the farm. The strategy included aspects such as:

  • Using the “correct” amount of water per orchard. Too much water is a waste of energy cost and water, and too little water may reduce the production output of the orchard.
  • Give preference to the lower cost pumping systems for water distribution.
  • Improve the efficiency of higher cost pumping systems and distribution lines.
  • Utilize the lower cost storage dams/reservoirs more than others.
  • Combine electricity supply points to lower the unit cost of electricity.
  • Use solar power solutions to replace the more expensive electricity supply points.

Improved Water Management

REDUCING CARBON EMISSIONS ON FARMS WITH IMPROVED WATER MANAGEMENT

Previous CCC industry trend reports have indicated that electricity consumption for the pumping of water is the largest source of farm-level carbon emissions. Irrigation of crops is a necessity in a water-scarce country and predominantly coal based electricity is used as an energy source for pumps. Improved water management has the purpose of saving water, cost and reducing carbon emissions. The following opportunities for savings have been obtained during irrigation pumping system assessments on fruit farms.

The starting point is to determine the “correct amount of water” for each tree or hectare. Too little water may reduce the production output and fruit quality, while too much water is a waste of water and energy cost. A study of water flow rates on one farm has shown that some orchards receive only 50% of the intended water supply while other orchards have received 50% more than target. A system to monitor the soil moisture contents is a necessity for good water management on farms.

A lot of water is wasted by evaporation during the irrigation process, mainly due to wind and heat on the ground surface. Improvement in irrigation technology to reduce evaporation losses can save up to 40% of water and related pumping energy.

The irrigation pipe diameters, piping network and geographical height determines the head and flow rates that the pumps are operating on. Higher water pressures require more energy and the irrigation design should attempt to maximize flow and minimize the pressure required. Many cases were identified where the pump head could be reduced significantly with improved piping and irrigation system, while still providing the required water flow rate.

The operation (pressure and flow) of the pump is determined by the irrigation network design. The selection of the pump and the condition of the pump determine the ability to convert electrical energy into the pressure and flow. The efficiency of a sample of pumps were tested and the results show that the pumps use on average around 30% more energy than needed for the required head and flow.

Pumping of water from boreholes has also shown significant losses due to inefficient borehole pumping systems. A sample of 11 boreholes on one farm consumes an average 0.35kWh per cubic meter. The average consumption of 28 other boreholes at similar borehole depths was 0.62kWh per cubic meter. This shows that the potential energy (and carbon emission) savings in improved borehole pumping systems can be as high as 50%.

Larger farms have water distribution networks where water is pumped from rivers, boreholes and dams to other dams or orchards. Sometimes water is pumped 3 times before it reaches the orchard, while each pumping operation adds to the total energy consumed. The sizes and locations of the dams and the relevant position between the dams and the orchards all have an impact on the total energy requirements for the water distribution network. The water distribution network should be optimized by selecting the lower energy boreholes and pumps to do most of the work where possible. This is a more complex and strategic approach towards energy optimization and can also be used to determine the position and sizing of future dams in the network.

The results obtained from the case studies discussed above are not visible to most farm managers and a technical analysis of the pumping systems are required to identify saving opportunities. It is however clear that an average cumulative saving of 30% to 50% on carbon emissions is possible with improved water management techniques on farms.