After setup, all the grower needs to do is to add the irrigation amounts on the dates that field is irrigated. The model keeps track of the soil water content over time and displays the soil water deficit (how much more water the soil can hold before it is lost to deep percolation) and the current percent of the total soil available water. It displays the soil water content as a gas gauge. The model can be corrected or updated on any date with soil moisture measurements or estimates. The model uses a daily time step and takes into account the effects of a growing root zone as well as the decrease in plant water use due to water stress. A one- week forecast based on projected maximum and minimum daily temperatures from the National Weather Service is also included.

The most important screens for the user will probably be the Daily Budget Table (see fig. 1) and the Soil Water Chart (see fig. 2). The model also outputs graphs of the daily crop water use, the cumulative water use, the crop coefficients and root zone depth over time, the estimated amount of water lost to deep percolation, and the degree of water stress and estimated yield loss to the crop due to water stress. Online help for each screen is available within the Irrigation Scheduler Mobile application. A full

help manual is also available online at /is/ISMManual.pdf.

The inclusion of many additional considerations could increase the accuracy of this model, such as differences in water-holding capacities and water contents at different depths in the soil, infiltration rate limitations, the time dependency of water movement through the soil, differing irrigation efficiency and effective rainfall fraction estimates with the changing seasons, and salinity effects. However, including these requires growers to input information that they likely will not have. Because of this, these were deliberately left out of the model to enable and encourage use of the tool.

Our hope is that growers have and will use irrigation scheduling tools to give them better information to improve their irrigation water management for their own benefit, as well as for the benefit of us all.

R. Troy Peters, PE, PhD, is a professor and extension irrigation specialist at Washington State University located at the Irrigated Agriculture Research and Extension Center in Prosser, Washington, where he has served for the last 13 years.





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