Crop ET is normally related to ETo by a set of coefficients or factors that depend on the type of crop and growth stages and are also applicable to areas of similar climate. Numerically, the crop coefficient (Kc) represents the ratio of crop ET to ETo:

Kc = Crop ET / ETo

Crop coefficients (Kc) are used with ETo to estimate specific crop evapotranspiration rates. The crop coefficient is a dimensionless number that is multiplied by the ETo value to arrive at a crop ET (ETc) estimate. The resulting ETc can be used to help an irrigation manager plan when an irrigation should occur and how much water is to be replaced back into the rootzone.

If you know the ETo from a CIMIS station for the day and the crop coefficient for the crop at a particular growth stage, then by multiplying the ETo by the crop co-efficient (Kc) produces an estimation of how much water was used by the crop for that day. For example:

Crop ET = ETo x Kc

ETo = 0.4 in, Kc = 0.4

Crop ET = 0.4 x 0.4 = 0.16 inches used by the crop

Agronomic crop coefficients (Kc) were developed by research where ETo and crop ET were measured simultaneously. Kc values usually range from 0.1 to 1.2. Values of Kc for annual crops are well under 1.0 in the early growth stages but, at least in dry climates, tend to increase up to 1.10 to 1.20 for many crops when they are fully shading the ground. This means that the ET rate of most crops exceeds the ETo rate by 10 to 20 % during periods of full cover. The range of crop co-efficients varies from very small (~0.15 for early season row crops) to very large (~1.3 for walnuts in mid season).

Although crop coefficients vary from day to day, depending on many factors, they are mainly a function of crop growth and development. The rate of crop growth and development will change from year to year, but the crop coefficient corresponding to a particular growth and development stage is fixed from year to year.

If you cannot obtain a crop coefficient from Wateright's database, there is a rough method to estimate of the Kc.