With a glass half full

One of the many ways of conserving water, saving money and producing more with less water is by being mindful of when and how much water to apply to the crops, plants and grass. This process is generally called irrigation scheduling.

Traditionally, irrigation scheduling has been based on the producer’s observations of plant water needs or calendar date irrespective of variation in weather conditions, growth stages and soil conditions. In areas where the potential evapotranspiration is greater than the rainfall amounts, generally in semi-arid to arid areas, the general notion is that producers are already behind so they shall continuously irrigate until the end of the growing season. Researchers have noted that farmers are reluctant to deviate from traditionally accepted scheduling methods regardless of relative merits of alternative scheduling methods until they are shown that improvements in scheduling results in greater net returns.

USDA scientists defined irrigation scheduling as the process of making decisions on the irrigation amount and timing subject to the irrigation water supply constraints and in concert with labor and cultural crop practices with the goal to maximize profits per unit of inputs. Irrigation engineers in the southern High Plains coined a new definition for irrigation scheduling as being the process of delaying any unnecessary irrigation with the hope that the irrigation season will end before the next irrigation is needed.

These definitions highlight the challenges associated with maximizing profitability when irrigating with limited water supplies. This is particularly true in areas where well capacities from the aquifer or surface water and ponds are dwindling.

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.

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 sensed images. Some types of sensors can detect water stress better than others. The earlier these sensors are able to detect water stress, the more useful they are in helping schedule irrigation.

Considering these three types of feedback, which one is the best? Using all of them is best, but using one is good enough, while having two of them is better. The reason is that not all of them are perfect and that having multiple independent types of feedback adds more confidence in making irrigation scheduling decisions.

As with other technologies, each of these sensors or feedback can fail temporarily or cause significant disruption. I have seen soil moisture sensors fail in the middle of the season due to either circuit or power failure. ET-based water balance may also deviate from reality if crop growth is not optimum (e.g., delay due to wind, hail or pest). The same is true with plant-based sensors; the representative plants being monitored may have some growth issues due to pest and disease. Regardless of the feedback used, it’s still important to check the field occasionally. This also underscores the advantage of using multiple types of feedback in case one fails or the reading is questionable.

For example, during a hot sunny day, one may see the plants starting to curl and the thermal cameras showing stress on the leaves. Checking on the soil moisture sensor, it shows that the moisture content is still 80% or about an inch of water in the soil profile. Checking on the ET water balance, there’s still a ¾-inch of water in the profile. The producer can safely assume that while the plants may look stressed that day, soil moisture is still enough to hold off irrigation and wait for the rain forecasted in the next two days. As one Kansas farmer sums it up, “Knowing that information gave me confidence to shut off my irrigation system and sleep peacefully at night.”

As well capacities in several major aquifers dwindle and surface water sources become critical, irrigators will need to adopt irrigation scheduling to maintain productivity. Making sure that rainwater is fully utilized is the first step in effective irrigation scheduling.

Jonathan Aguilar, PhD, PE, is an associate professor, extension specialist and irrigation engineer for the Kansas State University Southwest Research Extension Center.