NOTE: Make sure you read the first three posts (in order!) before tackling the rest, or it could be confusing: Post 1 is Designing the future, Post 2 is Setting up the problem, and Post 3 is Estimating basic requirements.


Thursday, December 07, 2006

Weather effects on Peak Oil Homestead design

In the previous post we picked a specific location for our Peak Oil Homestead Example Problem, which enables us to do quite a bit. With these new assumptions we can better see our true requirements. With 34”/year of precipitation, we can modify our water requirements to take advantage of nature’s irrigation. With knowledge of our latitude, we can run calculations on solar incidence.

In the interest of furthering the design, let’s calculate what we need for growing food. Requirement 2.2.2 in the ORD is for 14,000 liters-per-day (lpd) of water. For four people on a vegetarian diet, we can liberally estimate that 1 acre of food would meet their nutritional needs (NOTE: This will be refined to a more exact number as the design progresses – remember Systems Engineering is an iterative process!).

The precipitation volume rate is therefore:
Vp = (34”/yr)*1 acre = 2.83 acre-ft/yr = 3,490,000 liters/yr = 9560 lpd

In this iteration on the design, we see that we can reduce our daily water requirement of 14,000 lpd by 9560 to 4440 lpd! However, remember that the 34”/yr is an average figure, and we should apply some engineering forethought and bump it up a bit.

Before we do so, however, we should consider the predicted effects of global warming on our particular region. Models call for increased precipitation in the Midwest through at least 2050, which can give us more confidence in our decision to rely more on precipitation for crop irrigation.

(The models also predict only moderate local warming for Iowa from global warming through 2050 for the summer seasons, and more significant winter season warming. The winter warming could mean a reduced need for home heating and is a factor to consider when we get to designing those systems.)

Given this analysis, it seems a refined requirement of 5000-6000 lpd for food production is reasonable. However, we have not yet taken into account possible effects of mulching, greenhouses, or other water-saving techniques! That will be taken account during trade studies in the next iteration.


At 6:36 PM, Anonymous jeff said...

Technically, 34"/yr precipitation for growing food would be lessened to the amount of precipitation that occurs during the growing season. Granted most precipitation usually occurs during the growing season, but the difference can be remarkable.

I take climate models with a grain of salt as many of them have not yet incorporated the last 4 to 5 years of observable climate change. We've seen dramatic climate changes in the last few years. Even with the models that are incorporating the latest observable data, the data represents changes that are unprecedented (in there rapidity), and therefore cannot be tested.

It was interesting to learn that the central Midwest region has an observable temperature and precipitation "hole". Obviously working with this information is far better then not working with any forecasts, but I'll tend to lean on a far more conservative side given the unprecedented future of our climate.

Out of curiosity, how did you come up with the original garden water requirement of 14,000 lpd per acre?

At 10:14 AM, Blogger PeakEngineer said...

Jeff, the orignal requirement was developed in this post

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