In the summer of 2011, we developed an on-line Crop Water Use Calculator program which uses the Penman-Monteith equation. Preliminary field testing began in Missouri in our summer (northern hemisphere) and now we are testing it in South Africa (southern hemisphere summer). To make the program, we began with published crop coefficient curves for cotton, corn (maize), and soybean. For rice, Earl Vories (USDA-ARS) and Paul Counce from University of Arkansas developed an experimental coefficient curve for center pivot irrigation. Below is a screen grab from the Ukulima Farm weather page.
Note the new box in the center right side of the weather screen. Click on the Crop Water Use Calculator image. This will bring you to a new screen with options to select crop and planting date. As shown below, if you have a smart phone with internet access, you can do this in the field as you select the irrigation rate on a center pivot system.