This is such a cool graph, I had to write a post for it. This is the first empirical evidence that ollas reduce water use in Pacific Northwest gardens.
In 2009, I watered our 650 sqft of gardens by hand and by sprinkler. By subtracting our winter water usage (2992 gallons), I estimate upwards of 3700 gallons of city water went to the garden during June and July of 2009.
By 2010 I had built 8 ollas. I planted green bush beans, potatoes, and cucumbers around these ollas. Our water use dropped by 40%.
In 2011 I had 12 ollas planted with tomatoes, tomatillos, and corn. Our water use dropped another 33%.
In 2012 our June+July city water use for the garden dropped to zero. This was the first year we used cisterns and water use for these two months was entirely from stored rainwater. We had 14 ollas in use in 2012, and we used something like 1500 gallons from the cisterns, so we probably didn’t use less water compared with 2011. But I would expect inter-year variability in water needs to overcome the possible savings of just two ollas in our large garden.
Nonetheless, comparing 2010 and 2011 with 2009 it is easy to acknowledge that ollas played a significant role in our reduced city water use during the peak water use months.