IRRIGATION SCHEDULING


obtained from local or regional weather station networks or on-site weather stations. While many weather station networks are designed for agricultural use and follow recommended standards for sensors, quality assurance/quality control and siting, others are not necessarily well sited or instrumented for ETc purposes. Evapotranspiration networks, such as such as California Irrigation Management Information System (CIMIS, at cimis.water. ca.gov), Florida Automated Weather Network (FAWN, at fawn.ifas.ufl.edu/tools/ et/), and Oklahoma Mesonet (at mesonet. org), provide near real-time weather data needed to calculate ETref, as well as other agriculturally relevant data. Many sources of weather data may include stations sited for other primary purposes, so they may not be good representations of agricultural field environments. It is also notable that many agricultural areas are not covered by ET weather networks.


Weather data quality depends on instrumentation quality, calibration and maintenance, as well as siting. This applies to public or private networks, as well as on-site weather stations. Some weather parameters (such as solar radiation) are relatively stable with distance, but others (including rainfall) may vary widely over


Irrigation scheduling apps come in many different forms, abilities and ease of use. It’s important that growers find apps that best meet their needs and provide the necessary data.


an applicable weather station or import their own data.


Reference ET crops and crop coefficients


Beyond considerations of weather data availability and quality, application of ET data to irrigation involves calculations of reference ET and crop ET. These steps offer opportunities for errors in application. For instance, there are different reference crops (tall crop or alfalfa reference crop;


It is important to remember that while apps can be useful in integrating


information, their outputs need to be verified (ground-truthed) against the crop in the field.


even relatively short distances. In many instances, other factors (topography, elevation and other local conditions) may indicate that the nearest weather station in a network may not necessarily be the most representative weather station for a given application. Fortunately, many ET networks provide additional information on their websites about their stations, locations and QA/QC, and many irrigation scheduling apps allow the user to select


26 Irrigation TODAY | Summer 2022


short crop or grass reference crop), with different ETref equations and different ETref values. This means that the crop coefficients to convert ETref to ETc are specific to the reference ET equation used. Crop coefficients are crop-specific and growth stage specific, and available crop coefficient curves are limited with regard to other crop factors. Hence, ETc is an estimate of crop water demand, generally assuming ideal crop conditions.


A common mistake is that model users and developers may use ETref in lieu of ETc, which can result in significant overestimate or underestimate of crop water demand, depending on the crop growth stage and reference ET equation used.


Ease of use and data input


Irrigation scheduling apps range from simple and generic to complex and configurable for specific operations. There are tradeoffs, of course. Generic apps may be less precise in their modeling of the crop, but they can be relatively easy to use and widely applicable for a variety of crops and conditions. More configurable apps may provide more specific recommendations, and they require additional inputs. User friendliness depends on data entry (and availability of required data), as well as interpretation and presentation of outputs. Some tools guide data entry by providing default input values or embedding limits to prevent the user from entering unreasonable values (which could result in unreasonable output). Because input data may not be readily available or they may be presented in confusing formats or units, a common mistake is that users may depend too much on default values, limiting the accuracy and benefits of the apps.


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