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Appendix C: Water Needs of Lawn under Different Plans
Currently
Summer and winter weekly watering totals are known (see Appendix B). Assume the system spends six months on each schedule (from information given by Mike Barber).
Summer: 1,083,000 gal/wk 
26 weeks = 28,158,000 gallons
Winter: 419,000 gal/wk 26 weeks = 10,894,000 gallons
Total for a year: 28,158,000 gallons + 10,894,000 gallons = 39,052,000 gal/yr
New water plan
New water plan suggests 2 in/wk on grass in summer months and 0.75 in/wk in winter months. This equals a total of 597,400 gal/wk to the lawns in the summer and 224,000 gal/wk in the winter. Again, assume system spends six months on each schedule.
Summer: 597,400 gal/wk 26 weeks = 15,532,400 gallons
Winter: 224,000 gal/wk 26 weeks = 5,824,000 gallons
Total for a year: 15,532,400 gallons + 5,824,000 gallons = 21,356,400 gal/yr
New sprinkler system
This total represents the water we would need to put on the lawn in a year if we were to implement a new sprinkler system that could respond to daily evapotranspiration data. Warm season turf (the Kikuyu and Bermuda grasses that occur on the Harvey Mudd campus are both of this type) needd 60% of ETo, the reference evapotranspiration. Precipitation was subtracted from water needs for each month (as rain will take care of some water needs). Then the positive values (water that is needed to supplement rain) were added together to get the yearly supplemental water needs of our plot. This is an estimate of yearly water needs based upon last year's evapotranspiration and precipitation data, and is subject to change from year to year with changing weather. Data obtained from http://wwwdpla.water.ca.gov/cgi-bin/cimis/cimis/hq/main.pl.
| ET (in) | precip (in) | 60% ETo | minus precip | |
| Dec-99 | 2.51 | 0 | 1.506 | 1.506 |
| Jan-00 | 2.07 | 0 | 1.242 | 1.242 |
| Feb-00 | 1.99 | 7.17 | 1.194 | -5.976 |
| Mar-00 | 4.15 | 2.68 | 2.49 | -0.19 |
| Apr-00 | 5.21 | 3.84 | 3.126 | -0.714 |
| May-00 | 6.16 | 1.71 | 3.696 | 1.986 |
| Jun-00 | 7.25 | 0.62 | 4.35 | 3.73 |
| Jul-00 | 7.81 | 0 | 4.686 | 4.686 |
| Aug-00 | 7.07 | 0.01 | 4.242 | 4.232 |
| Sep-00 | 5.23 | 0.35 | 3.138 | 2.788 |
| Oct-00 | 2.68 | 1.88 | 1.608 | -0.272 |
| Nov-00 | 2.52 | 0.13 | 1.512 | 1.382 |
| Add positive values: | 21.6 in/yr |
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Water needs of lawn for 12/99 - 11/00:
21.6 in H2O/yr 
12 in/ft = 1.8 ft H2O/yr
1.8 ft H2O/yr 479,160 ft2 (area of lawns) = 862,488 ft3 H2O/yr
862,488 ft3 H2O/yr 7.48 gal/ft3 = 6,451,400 gal H2O/yr
This number assumes very good efficiency of the new watering system (even watering) and that the water soaks evenly into the soil in all areas, which may not be true because of variability in soil type throughout the landscape. As such, more water than the number above may need to be applied to deal with these problems, but this can not currently be known.
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