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Appendix D: How New Watering Plan Numbers were Determined
We looked at ETo data for 12/99-11/00. Highest ETo in the summer months of this period was in July (7.81 in/month)
7.81 in/month 
4 weeks/month = 2 inches/week
Highest ETo in the winter months of the period from 12/99-11/00 was October (2.68 in/month) or March (4.15 in/month), depending on when the grounds crew changes the watering schedule from winter to summer. This date is very flexible, although it usually occurs around March/April, so we want to account somewhat for this uncertainty. So, we used the ETo for October to get a ballpark number for the inches that should be applied.
2.68 in/month 4 weeks/month = 0.67 inches/week
We then rounded this number up to 0.75 inches/week to account for the uncertainty in the changeover of watering schedules. This should be plenty of water for the lawns, especially because March has fairly high precipitation (for the period assessed, so this may not always be the case).
Generally, warm season turf like we have on the HMC campus needs 60% of reference evapotranspiration. However, the efficiency of the sprinkler system is not known, and we would like to allow for enough water such that the lawns will receive enough in the case there is a summer with higher evapotranspiration than last. Therefore, we will not suggest using 60% of the numbers obtained above, but 100%.
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