Can Aquaponics Help Conserve Water in Times of Drought?

[TechDIYaspirant]

TL;DR Summary

Could lake Meade's evaporation be slowed by aquaponic design principles?

I've seen a number of recent videos on lake Meade's declining water levels and as I have an amateur interest in aquaponics I've wondered if a large scale project might go some distance towards tackling at least one source of water loss, evaporation. Probably the biggest water draw is downstream usage and the massive increase in population there so this isn't likely to significantly effect that problem, but it's an interesting thought experiment to me nonetheless with a number of potential positive side-effects. It's also been tried at scale before in what is now Mexico city by native cultures and other locations. The priniciple of covered water reservoirs is also attested by the use of black plastic balls in some california reservoirs to decrease surface evaporation.

Essentially you put a bunch of lettuce plants in styrofoam sheets that float on top of the water. Fish waste feeds the nutrient uptake of the plants. I can forsee some objections to the produce but it could be harvest for fodder or just green manure if it's not food grade to add biomass to the region.

https://bluedoormedia.co/2020/07/07...ztec-era-floating-gardens-to-feed-its-people/

https://www.meetrv.com/wp-content/uploads/2019/04/aquaponics.png

https://www.ctvnews.ca/sci-tech/sha...s-open-air-reservoirs-water-quality-1.2515018

 

[Baluncore]

I believe that plants will increase the transpiration of water from the surface when compared to covering the surface with a high MW material such as Cetyl alcohol that might greatly reduce evaporation.

If you want to prevent loss of water and grow plants, you will need a floating greenhouse. To reduce water vapour loss, the greenhouse should not be vented, but then you have a cooling problem.

 

[TechDIYaspirant]

I believe that plants will increase the transpiration of water from the surface when compared to covering the surface with a high MW material such as Cetyl alcohol that might greatly reduce evaporation.

If you want to prevent loss of water and grow plants, you will need a floating greenhouse. To reduce water vapour loss, the greenhouse should not be vented, but then you have a cooling problem.

Would transpiration be greater than evaporation? I suppose the added surface area of the plant would account for that?

 

[Baluncore]

Would transpiration be greater than evaporation?

There are no certainties. You will need to do some research and crunch every number.

Trees bring rain because they increase the transfer of groundwater to the air. That is good news for trees down wind, that recycle water in a massive rainforest, but bad news for a small oasis in a desert, or Lake Meade with open-air hydroponics.

 

[TechDIYaspirant]

There are no certainties. You will need to do some research and crunch every number.

Trees bring rain because they increase the transfer of groundwater to the air. That is good news for trees down wind, that recycle water in a massive rainforest, but bad news for a small oasis in a desert, or Lake Meade with open-air hydroponics.

I see your point.

 

[jrmichler]

Evaporation from open water in a desert climate is significant. Evaporation on those 100 degree F days with low humidity is on the order of 25 mm per day.

One scheme for reducing evaporation is to float solar panels on the water. The Wikipedia article is a good place to start reading: https://en.wikipedia.org/wiki/Floating_solar. Of course, that would interfere with the rights of boaters...

Keep in mind that all rivers and lakes with fish in them have natural biological systems for removing fish (and duck and bird) waste. Full details in the standard reference by Wetzel: https://www.amazon.com/dp/0127447601/?tag=pfamazon01-20. It's a little dense, but anybody with high school chemistry and physics can get a lot out of it.

If you want to learn more about transpiration from vegetation, the standard reference is Brutsaert: https://www.amazon.com/dp/9027712476/?tag=pfamazon01-20.

If you want a VERY ROUGH approximation for the Midwest US, the average evaporation from open water and the average transpiration from vegetation is about 2/3 of the average rainfall of 30 to 35 inches per year. The evaporation from open water in a hotter, dryer climate would be a lot more than that.

Yes, there are things than can be done to reduce evaporation from reservoirs, but there is more to it than just covering the water.

 

[JT Smith]

https://www.nps.gov/lake/learn/water-budget.htm

 

[jrmichler]

If the OP is interested enough to run some experiments with vegetation in water, here is a link to a paper on measuring water budgets using low cost instrumentation: https://www.researchgate.net/publication/335029148_A_low-cost_hydrologic_observatory_for_monitoring_the_water_balance_of_small_lakes. Evaporation is measured directly using an evaporation pan. And the second author does have a copy if somebody sends a PM.

As a side note, earlier today I helped install an Onset Computer MX2001 data logger (https://www.onsetcomp.com/products/data-loggers/mx2001/ to measure lake water levels. A nonprofit is looking for better ways to measure water levels in 27 lakes than having people checking staff gauges. A photo of the installation process taken by my wife:

I lead a rough life.

 

[jim mcnamara]

FWIW there are standardized ways to measure spatiotemporal drought and water loss

SPI Standardized Precipitation Index
SPEI Standardized Precipitation Evapotranspiration Index
Example use of both methods in China:
https://www.nature.com/articles/s41598-020-80527-3