Effective Rainfall
Effective rainfall refers to the percentage of rainfall which becomes available to plants and crops. It considers losses due to runoff, evaporation and deep percolation. Its always best to use actual observations or measurements to estimate the amount of effective rainfall for a particular site.

In absence of user supplied estimates for effective rainfall,the calculation automatically calculates effective rainfall as follows: assumes the first 1/10 inch of rain is lost to evaporation, gives full credit for rainfall between 1/10 - 1 inch, takes 2/3rd of any rainfall between 1 - 2 inches, and assumes that any additional amounts above 2 inches are lost to runoff and deep percolation.

You may want to adjust effective rainfall values for local conditions.