Undergraduate Thesis - Feasibility of Rainwater Harvesting in Paarden Island

The “Global Water Crisis”

According to the World Water Commission for Water (WWCW) in the 21st Century (2000, p. 11) freshwater is a “precious resource”. Only about 2.5% of the world’s water is actually freshwater and does not contain any salt. Of this freshwater, two thirds of it is in the form of ice. The remaining water forms part of the hydrological cycle from which 20% is inaccessible to humans. Of the remaining 80% of useable water, three quarters of it comes in the form of floods, which are very difficult to collect. This leaves very little freshwater for humans, to be used as a water source. The remaining water that is actually utilised by humans represents 0.0008% of the total water on the planet (WWCW in the 21st Century, 2000). The WWCW in the 21st Century (2000, p. 11) estimates that 70% of the water used by humans is in agriculture, leaving 30% for municipal water supplies which will be used by households and industry.

 

Research by the United Nations Environment Programme (UNEP) (2002, p. 2) has concluded that increased population growth, industrialisation, urbanisation, agriculture intensification and water-intensive lifestyles has resulted in a “global water crisis”. It is estimated that more than a billion people do not have access to a clean freshwater supply (WWCW in the 21st Century, 2000, p. 13). With an increase in population growth expected, the demand on these water supplies will further increase. The WWCW in the 21st Century (2000, p. 15) forecasts a growth of at least 17% in global water consumption by 2025 if not more.

 

The current supply of freshwater is limited by cost of production and is becoming more and more polluted, which has resulted in water security becoming a major national and regional concern (UNEP, 2002, p. 2). The supply of new sources of freshwater has become more important than ever. Desalination has been seen as the answer to supplying new sources of water for coastal cities but it is unlikely to be used for agriculture due to its high cost (WWCW in the 21st Century, 2000, p. 17).   

 

The use of rainwater harvesting methods and groundwater storage are seen as having potential to supply water to humans and are considered more environmentally friendly but need to be better known and more widely used (WWCW in the 21st Century, 2000, p. 16).

 

Freshwater supplies in South Africa

According to the South African Yearbook 2012/2013 (2012, p. 650) there is still a need to expand the country’s current national infrastructure, although water conservation and demand management has been seen as a very important next step. Rainwater harvesting is able to supplement the provision of freshwater in areas that do not yet have access to piped municipal water or in areas that have limited access to other fresh water supplies.

The South African Government Communication and Information System (2012) states that compared to global standards South Africa has very scarce and limited water resources. The total flow of all the rivers in South Africa is estimated to be about half the flow of the Zambezi River which is the closest large river to South Africa. The country’s climate varies from desert in the west to sub-humid along the eastern coastal areas. The average rainfall of South Africa is about 450mm which is below the global average of 860mm (RSA GCIS, 2012, p. 657). The evaporation rate for South Africa is also relatively high. This low average tends to make complicated rainwater harvesting systems unfeasible in most parts of South Africa and as a result only basic rainwater harvesting systems are usually implemented such as water barrels connected to downpipes which can only be used for food production or irrigating gardens (RSA GCIS, 2012).  

 

The South African Government Communication and Information System (2012) says that  variable rainfall distribution and characteristics have resulted in uneven distribution of water resources whereby 60% of river flow comes from 20% of the land area. The variable rainfall is compounded by the seasonality of the rain as well as the high within-season variability of rainfall (RSA GCIS, 2012, p. 657). The designs of rainwater harvesting systems are highly dependent on the variability of the rainfall in the area. The variability of the rainfall will determine whether the rainwater harvesting system is able to provide freshwater all year round or only during certain periods of the year. The intensity of the rain will determine the possible size of the rainwater tanks that would be required. In areas where high intensity rainfall occurs with long dry periods, large rainwater tanks would be required to compensate for the dry periods. If the intensity is low and occurs regularly then smaller tanks can be used to help reduce the costs.   

 

South Africa is currently able to collect and use about 10 200 million m3 or around 20% of the surface total mean annual runoff. Of this amount about 8% is estimated to be lost due to evaporation and a further 6% due to land use activities identified by the RSA GCIS (2012, p. 658).  

 

Current and future planned supply sources of water in Cape Town

Currently the City of Cape Town has a freshwater capacity of 556 million m3 and a demand of 511 million m3 of which 32% is used for agricultural irrigation and 68% is for urban use (South African Department of Water Affairs, 2011). The City of Cape Town currently has a resource mix of 98% surface water and 2% groundwater (City of Cape Town Water and Sanitation Department, 2010).

 

The City of Cape Town is currently considering doing feasibility studies for the implementation of a large scale desalination plants and large scale re-use of water. A study will also be carried out on large scale groundwater usage of the Table Mountain Group aquifer as a sustainable freshwater supply (South African Department of Water Affairs, 2011). 

 

The City of Cape Town is planning to have a future mix of freshwater supply of (City of Cape Town Water and Sanitation Department, 2010):

• Surface water:        70%

• Groundwater:        7%

• Water re-use:        13% 

• Desalination:        10%

 

If in 2010 the City of Cape Town had a freshwater supply of 556 million m3 and demand of 511 million m3, with a growth rate of 2% per annum it is estimated that by 2014 the demand for freshwater would be higher than the available supply.