The term evapotranspiration refers to the total loss of water from the soil as a result of evaporation from the soil and transpiration from a crop growing in the soil. Thornthwaite has established empirical relationships between temperature and evapotranspiration.¹ Using these relationships it is possible to estimate the potential and actual evapotranspiration from a field well covered by a growing crop.

Since plant production of dry matter is approximately proportional to the transpiration of moisture through the plant, a comparison of potential and actual evapotranspiration provides a first approximation of the plant production that can be anticipated under conditions of irrigation and dry farmed conditions. The computed value of potential evapotranspiration (PET) provides information about the amount of moisture a full crop might utilize under existing conditions of temperature, provided the supply of moisture is not limited. This might be the case under irrigation. PET values for the entire year are presented in tabular and in chart form; they show around 25 inches of moisture utilization as a potential along the coast, increasing to 32 inches in warmer areas. This annual figure is of significance in the case of range or timber crops that grow under favorable conditions the year around.

If the same figure is limited to the growing season, as defined by the latest and earliest 32° temperature readings in the spring and fall, respectively, these figures become 25 inches along the coast and 28 inches in warmer areas. Change from the previous values is slight because the growing season covers much of the calendar year. The new figures are applicable to frost sensitive crops that can grow only between frosts.

In computing the amount of moisture a crop might use under dry farmed conditions (Ea) it is necessary to establish the amount of plant-available moisture that can be stored in the root zone of the soil. This will vary from one soil to another, but for purposes of comparison from place to place a figure of 4 inches is used in these computations. Highest readings of 4Ea for the year as whole are found along the coast, where 18 inches of moisture can be utilized by plants growing without the addition of irrigation. This figure drops to around 13 inches in some of the drier areas of the County.

A further reduction in potential plant use of moisture is noted when the period is reduced to the 32° growing season (4Ea32). For this value a total of 17 inches is indicated on the coast and only 8 inches in some interior points. A comparison of these figures suggests the possible production variation that may result from temperature, precipitation, and irrigation differences.

On the basis of these computations it is possible to estimate the date when stored moisture supplies in the soil will be exhausted in a typical year. If we assume that the crop uses soil moisture at the maximum possible rate as long as moisture remains available, the earliest drying of range can be expected around May 15th in some of the low level points in the interior, while coastal points may find soil moisture adequate until early in July.

1. Thornthwaite, G. W., and J. R. Mather, The Water Balance. Publications in Climatology, Vol. VIII, No. 1. Drexel Institute of Technology, Laboratory of Climatology. Centerton, New Jersey, 1955.