The use of calculated actual and potential evapotranspiration for estimating potential plant growth
AuthorsRodney J. Arkley
Authors AffiliationsRodney J. Arkley was Lecturer in Soils and Plant Nutrition and Specialist in the Experiment Station, Berkeley; Rudolph Ulrich was Soil Scientist, Soil Conservation Service, Berkeley.
Hilgardia 32(10):443-469. DOI:10.3733/hilg.v32n10p443. May 1962.
Potential evaporation, ETp, can readily be calculated from monthly mean temperatures by means of tables and nomograms now available in the literature. Actual evapotranspiration, ETa, is calculated from ETp by taking precipitation and the water-holding capacity of the soil into account. Either value can be calculated for the whole year (ETp or ETa) if frost-tolerant plants are under consideration; or for the frost-free period (ETp 32° or ETa 32°) if frost-sensitive plants are under consideration.
All four values—ETp, ETa, ETp 32°, and ETa 32°—were calculated for 211 California stations and for 27 Nevada and Oregon stations near California. Plotting these values for 25 stations along a traverse through central California and western Nevada reveals great differences from coast to inland stations, and from low to high altitudes. Comparison of the values with natural vegetation and with crops in the different regions indicates that the values are useful as indexes of expected growth of cultivated crops, range, and forest. ETp values are useful if moisture is not limiting, as in humid climates or where irrigation water is available; ETa values are useful for predicting the suitability of a climate for dry-farmed crops, estimating the potential for increased growth obtainable by irrigation, and for more precise studies of the effects of climate on natural vegetation. The indexes, used together, can be helpful in crop selection and other soil-management decisions.
Isograms of the four ET values, based on all California stations and plotted on maps of the state, furnish much information about those climatic limitations on plant growth that involve moisture and temperature relations.
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