IRRIGATION SCHEDULING


Let’s look more closely at the second definition. In areas where it rains, this definition allows rain to be the major driver of scheduling in the hope of conserving irrigation water overall. Producers will always look into the weather forecast to see if there is chance for rain and make the decision on whether or not to irrigate on top of this free rainwater. This is looking at the glass as half full, rather than half empty.


If it happens to be a wet year with ample and distributed rain events, we just saved the potentially full irrigation amount plus the money associated with pumping water and system maintenance. During a dry year, this approach will always maximize whatever rainfall that comes because it prioritizes rain over irrigation water.


Now the next question is: How do you know if you need to irrigate? Before anything else, the first step is to check the chance of rain tomorrow or the following day.


If rain is not on the horizon, we then check the field if it really needs irrigation through some feedback. We group all irrigation scheduling methods or feedbacks into three categories: soil-, weather- and plant- based.


Having multiple independent types


of feedback adds more confidence in making irrigation scheduling decisions.


Soil-based feedback relies on the amount of moisture on the soil available for plant use. It can be as simple as the hand-feel method where a handful of soil is pressed on the hand to see if moisture is present. Probing the soil profile with a metal rod is a variation of this method. The standard or direct method of soil moisture measurement is the gravimetric method. Water content on the soil is measured by weighing a soil sample, both in its wet or fresh condition, then its dry condition after oven-drying the sample. The use of soil moisture sensors also falls into this general category. There are multiple types of soil moisture sensors and some of them are better in certain applications and conditions over the other. However,


while the expectation for these sensors is to measure the amount of moisture, most of the commercially available sensors are typically only good at tracking the moisture gain and removal. Nonetheless, it is still very useful, particularly if considering a newly defined irrigation schedule.


Weather-based feedback relies on the measurement of weather parameters that relates to crop water use also known as evapotranspiration. Weather stations and atmometers provide weather information to estimate evapotranspiration and crop water needs. ET data is used to gauge how much water is currently being held in the crop root zone and generate recommendations on when and how much to water. Models, apps and management tools have been developed to help in tracking the water lost (i.e., ET), added (i.e., rain and irrigation) and retained (i.e., soil moisture content), much like monitoring a bank account with withdrawals, deposits and current balance.


Plant-based feedback could be as simple as seeing the leaves curl or wilt, to the use of sensors such as dendrometer, thermal cameras, micro-tensiometer, osmotic potential gauge or even remotely


22 Irrigation TODAY | Spring 2024


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