There are various approaches for scheduling irrigation. One approach is to supplement rainfall with enough irrigation so that the cumulative rainfall and irrigation, over a specific period of time (e.g., one day, one week, one season), matches the estimated crop water requirement.
The information on this page will allow you to schedule your irrigation in this manner. Here is how it works:
1. First, and this is the hardest part, create an evapotranspiration crop coefficient (Kc) curve for your crop. The following link to the Food and Agriculture Organization (FAO) Document No. 56 provides guidance on the construction of a Kc curve: CLICK HERE. Your Kc curve should look like FAO 56 Figure no. 34 when you are finished. (FYI: Crop coeffiecient curves can also be created using the PRET computer program)
2. Now it gets easier. Go to the appropriate reference evapotranspion (ETo) map(s) for Puerto Rico and determine the ETo for your location CLICK HERE. Note, if you are irrigating ever day, then you only need to obtain the ETo for yesterday’s date. If, however, you are irrigating once per week, for example, then you will need to get the ETo values from the maps for the previous week. In this latter example, you will need to sum the daily values of ETo to obtain a value of the weekly ETo.
3. From your Kc curve, determine a representative value of Kc for the time period of your analysis (e.g., daily or weekly).
4. Estimate the crop water requirement (crop evapotranspiration) ETc = Kc x ETo.
5. Estimatate the required depth of irrigation: Irrigation = ET – Rainfall. If the estimated Irrigation is negative, then you do not need to irrigate.
It is recommended that you measure the rainfall on your farm with a rain gauge, however, if you do not have access to the measured rainfall, you can obtain an estimate of the rainfall (derived from NEXRAD radar) by clicking here.
The irrigation scheduling approach described above is based on various simplifying assumptions (e.g., surface runoff and deep percolation are ignored). Nevertheless, it will vastly improve your water management if you are not currently using an irrigation scheduling method. To evaluate your irrigation management, you may want to construct a graph similar to the example provided below. The graph shows the cumalative depth of irrigation and ETc plotted with time. If these two curves stay close together then you are doing a pretty good job of managing your irrigation.
The methods used to obtain ETo and rainfall can be obtained at the following website: GOES-Puerto Rico Water and Energy Balance (GOES-WEB) Algorithm
For another method of irrigation scheduling that takes into account soil moisture, see A simple Irrigation Scheduling Spreadsheet Program
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