Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

Simple method

The Simple method is done by calculating what the demand storage requirements would be for 3 months without rainfall (Krishna, 2005). This method often results in an error in calculation since it assumed a fixed 90 day period without rainfall. In some regions where the numbers of dry days are less than 90 will result in over estimations which will result in more expensive systems to due to higher storage costs. In regions where the numbers of dry days are longer than 90 days will result in a water deficit. A deficit is acceptable if a secondary source of water is available but it is not acceptable if rainwater harvesting is the only source of freshwater.

This method also does not consider the actual supply capabilities of the runoff surface and the rainfall quantities. In dry arid regions the monthly demand of a household in 3 months could be greater than the total amount of water that can be supplied in 3 months which would result in the storage tank never being filled and as such an over design.

 

Water balance method

This method is used to determine the feasibility of a proposed rainwater harvesting system by balancing the monthly water supply with the monthly water demand (Krishna, 2005). The supply of rainfall is added to the balance and the demand of water is subtracted from the balance every month. The resulting deficit represents the amount of water that will be required to be supplemented from another source such as the municipal conventional water system. The resulting surplus from the water balance will be the amount of water that will be required to be store in the storage tank for future use.

The water balance of each month is carried over to the following month and the process is continued for about 2 years so that a full cycle of data is available (Porter, et al., 2008). The total available storage requirement is adjusted so that every month is able to provide a water supply to the system i.e. the resulting shift in the available storage should result in 1 month having a minimum storage requirement and 1 month having a maximum storage requirement. The size of the storage tank is then designed to store the water for the month with the maximum requirement. Figure 3-7 shows a visual representation of how to calculate the design volume.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 3‑4: Graphical method to determine the required storage

Source: (Khoury-Nolde, 2008)