Water Year as a Major Challenge to Small Scale Water Projects

Appropriate scale technologies have risks.

• large scale water projects spread out the risk of low precipitation over large geographic areas
• small scale projects are dependent on local water supply and variation in availability

Potential solutions are:

• shift behavior (change crops or water use patterns)
• storage

Storage is sensitive to variations in water supply and risk of drought periods

example water year in Portland

from www.weather.com

 

probabiliity of periods of drought - determine failure of storage

sept 2012 - greater than 44 days without rain

1951 - 51 rainless days

can be up to 3 months - http://www.oregonlive.com/weather/index.ssf/2012/10/record_three_month_dry_spell_i.html

drought predictions - http://droughtmonitor.unl.edu/

 

Calculation of water needed

• water efficiency of a crop
• dependence on temperature and humidity

Calculation of water collected

(inches of rain x area of roof in sq feet)/(231 cubic inches per gallon) = gallons

cm of rain x area of roof in sq meters/10 = liters of water

http://www.thecenterforrainwaterharvesting.org/2_roof_gutters4.htm

Need to take into account inter-dependent risk factors

• summer drought periods are hotter and drier
• increases the water demand

It would be a mistake to treat length of a drought and water use efficiency as independent risks. (see The Black Swan, Naseem Taleb, 2007)