The Watering % Adjust is based on ET (evapotranspiration) data that measures the loss of water evaporated from the soil and transpired by plants. ET is calculated from specialized local weather stations that measure such information as solar radiation, air temperature, relative humidity and wind velocity. ET values are primarily dependent on the amount of solar radiation and wind speed. A watering schedule should be designed to periodically replace the amount of water lost via ET.
The Watering % Adjust compares the average ET data for the most recent week with the highest average weekly ET value recorded over the last 10 years. The historical high occurs in July or August and equals 100% on the Watering % Adjust.
The graph above shows a typical ET curve and the step-curve created from Watering % Adjust values calculated over a twelve-month period. The third curve shows the water used when the irrigation schedule is adjusted just twice a year, a common approach for many people.