The method used by LAIS for estimating ET_o is the method described by the Food and Agricultural Organization (FAO) of the United Nations in their document “FAO Irrigation and Drainage Paper 56”. A web version of this document can be found at http://www.fao.org/docrep/X0490E/x0490e00.htm#Contents. Comments appearing in “parentheses” in these notes are extracted from this document which is referred to in the following text as FAO56.

The most common use of ET_o data is to make estimates of irrigation need. This may be done by comparing rainfall and ETo values over the time period of interest to the user. Assuming the soil water holding capacity down to the maximum rooting depth of the crop is known, or estimated, a daily soil moisture balance value can be calculated and used as an index of irrigation need. However, actual cropping conditions may vary in two basic ways from a hypothetical grass reference crop. First, “Due to variations in the crop characteristics throughout its growing season, K_c for a given crop changes from sowing till harvest.” In this case “the crop evapotranspiration under standard conditions, denoted as ET_c,” may be calculated as or ET_c = K_c ET_o using the value of a relevant crop coefficient, K_c. Typical values of K_c for Louisiana crops provided by FAO56 are given at http://www.fao.org/docrep/X0490E/x0490e0h.jpg. The second difference from ET_o is due to “non-standard conditions”. “When cultivating crops in fields, the real crop evapotranspiration may deviate from ETc due to non-optimal conditions such as the presence of pests and diseases, soil salinity, low soil fertility, water shortage or waterlogging. This may result in scanty plant growth, low plant density and may reduce the evapotranspiration rate below ET_c. The crop evapotranspiration under non-standard conditions is calculated by using a water stress coefficient Ks and/or by adjusting K_c for all kinds of other stresses and environmental constraints on crop evapotranspiration.” Information on estimation of Ks may be found in Chapter 8 of FAO56. The process for estimating K_s for a particular location and set of conditions contains multiple steps.

ET_o values could alternatively be used in the Arkansas Irrigation Scheduler system described by Ferguson, James F., Dwayne Edwards, Joel Cahoon, Earl Vories and Phil Tacker. 1996. University of Arkansas Microcomputer Based Irrigation Scheduler User’s Manual and Reference Guide. Version 8.2. Ark. Agric. Exp. Stat., Ark. Soybean Promotion Board and Ark. Coop. Ext. Serv., cooperating. A program for the Arkansas Irrigation Scheduler may be downloaded from http://www.aragriculture.org/computer/default.asp