This is an example of a water budget for San Luis Obispo. ETo (evapo-transpiration) is in blue and rainfall is in maroon. Note, for most of the year, the demand for water by plants (ETo) exceeds precepitation for this location. Water must be applied to meet the crop demand (ETo) during the growing season. The amount of water held in the soil and its rate of use, (ETo) will determine the time intervals between water applications to a given soil.