Simple Charts for Modifying Crop Coefficients to Local Conditions

摘要

The default crop coefficient values used in FAO-56 were developed for sub-humid climatic conditions with an average daily wind speed of41A mph (2 m/s) and an average daily minimum humidity of 45%. However, coefficient values can be modified with mathematical procedures to compensate for differing climatic conditions. Not modifying default values to local climatic conditions could cause over- or under-irrigation to occur. This paper presents a simplified method to modify crop coefficient values usinglook-up tables for approximately 180 locales in the USA and its possessions and Algiers City, Algeria. The tables are based on long-term climatic data available from the NOAA. The coefficient value that deviates the most from FAO-56 default values is theK-cJni value, which sometimes needs to be increased 200%. This deviation occurs when high numbers of rainfall/irrigation events occur. Since rainfall on two or more consecutive days is considered one rainfall "event", a mathematical estimate was developed to convert total number of days with >0.01 inch or more rainfall per month to the number of non-contiguous rainfall events.