Todd Stong, an associate of ours, is a retired US engineer, a US military veteran and a volunteer engineer working on a water storage feasibility project in Lake Chapala, Mexico.
Here is a proposal he has presented to the Mexican authorities of the region:
Assumptions
- Population of the metro area: – 5 million (1.5 million in the central are)
- Household water daily usage: 200 liters/day/person (0.2m3)
- Water required during dry season (Mid Oct- Mid June, 8 mon) = 48 m3/person
- Water storage for total population (with no evaporation) = 240 million m3
- Area of a storage tank if 5m deep = 48 million m2 = 4800 hectares
- Dimensions of a square tank: 6928m = 6. 9 km on a side
- Rainfall (mid June-mid Oct, 4 mon): 800 mm
- Rainfall Collectable (less 25% evaporation &10% infiltration): 528 mm, 0.528m
- Rainfall collection area required to collect 240 million m3: 445 million m2 = 44,500 hectares = 21096m on a side of a square, 21 km
- Area of Guadalajara: 72.6 sq mi, 8.52 mi on a side, 14,2 km on the side of a square
- Metro area: 1056 sq mi, 32.5 mi on a side, 54.2 km on a side. The Metro area is 14 times greater than the area of the central portion of the city
Considerations
Population growth in 10 years at 2%/year = 19.5% increase to total of 5.98 million
The paved area in central city assume: 50% (challenge of removing auto oil deposits on the streets from sewers, assuming 30% may reach separate stormwater collection sewers and 70% may reach existing combined storm and household sewage sewers)
Due to the dense building of the central city and the level of pollutants expected in any collected rainwater it may be assumed that a rainwater collection tank of 6.9 km on a side would be impossible to be built inside the city. Thus, it is more likely that a collection of rainwater storage tanks could be planned for existing undeveloped areas in the metro area.
The central city area currently suffers from serious flooding during the rainy season given the lack of separate stormwater sewers.
Roofed area in central city assumed: 40% (no separate system of collection sewers exists. Dust and bird droppings could be removed. Separate roof water collection piping only possible by individual buildings and be at private owner expense.
The most feasible means to address an increase in household water to the city will be to increase the annual withdrawal of water from Lake Chapala. The current annual withdrawal is 20cm (15-25cm). For each 1 million increase in population assume an added 4cm of withdrawal would be required.
To assure an increased amount of water via the Lerma River is delivered to the lake (6-8%) it will be necessary to reduce the current 80% use of the Lerma River from its highly inefficient (Babylonian-like era), unmetered irrigation, of low valued crops. For each 1% less use from the current 80% it will be possible to provide water for an increase of 400,000 population to Guadalajara.
Due to the construction of over 500 dams in the Lerma River basin during the period 1930 to 1990, the flow of the river to the lake has been reduced from 90% to its present maximum of 10%. It is now possible for the 4 states on the river above Jalisco to capture 100% of the flow. If all 43communities (130,000 citizens) about the lake were permitted to use the lake water versus current deep wells, expensive to pump, and many with very excessive levels of arsenic, they in total would require 2 cm of water off the lake each year.
Todd D. Stong, PhD
Licensed Professional Engineer (USA)
Volunteer engineer for 19 years to the communities about the lake
E-Mail: todd.stong@aridgroup.com
