On the plane to SF today, I spent a little time reviewing the information provided by the Peterson and Haug article. A few data points:
- Rainfall varies based on location, from 500 mm/year (19.6 inches) to 4000 mm/year (157.5 inches).
- 90% of the rainfall occurs between June and September.
- City/states such as Tikal had water holding facilities able to support up to 10,000 people for 18 months.
- The typical drought periodicity was around 200 years, but the time of focus (760-910 A.D.) had significant drought period approximately 50 years apart.
- Again, based on location (and particularly in the northern regions), there was little surface water that could be used to support the population.
- The lack of water could be considered an amplifying effect on other behaviors, and thus population stress was likely to be greatly increased during times of drought.
Given this, and in order to simplify, I had to make some basic assumptions (and perform some initial calculations:
- Assuming a daily water consumption of 1 gallon per day, having enough water for a population of 10,000 requires about 300,000 gallons per months, or 5,400,000 gallons of water for an 18 month period.
- Evapotranspiration would have a significant effect on standing water supplies. Without further details, and assuming some attempt at mitigation, we'll assume that 10% of water supply would evaporate every month. This will be a variable that we can adjust if that assumption is proven invalid. (Some information on this retrieved from http://www.eoearth.org/article/Evapotranspiration).
- A cubic foot of storage will hold 7.48 gallons of water (found here). That means our 5400K gallon water storage would take roughly 721925 cubic feet - so we will start with an assumption of roughly 120000 square feet to a depth of 6 feet. This is important, because the size of the "opening" will also (probably) be the collection area.
- One gallon of rain is collected in 231 cubic inches (1.6 s.f. of area to a depth of an inch). Hence, the rainfall collected in a 120000 s.f. collection area is roughly 75000 gallons per inch of rain.
All of this is sort of conjecture, and needs to be modeled up. So I'm going to run off and do that now...