More and more turf managers are using soil moisture probes to manage their soil moisture. This trend is most prominent in the golf industry, but I hope to see it continue to expand to sports turf and beyond. The value of a moisture probe for calibrating an irrigation system and understanding your soils is hard to overstate. Before the moisture probe, we relied on irrigation audits by measuring the distribution uniformity (DU) and then acting on the results to improve the DU. Irrigation DU is quantified by placing flat-bottomed, straight-sided water collection containers at regular intervals (every 6-10 ft.) on an irrigated area (fairway, athletic field, home lawn etc.). The irrigation is run for a set time and the amount of water in the collection vessels is measured. The DU is calculated by taking the average of the lowest 25% of the readings and dividing by the overall average. A simple example is shown below. Ideally, you’ll have many more than eight measurements.
A DU of 80 is considered about as good as it gets, and anything below 60 is generally frowned upon in the textbooks (but common in the real world). The DU is used to adjust run times, as more irrigation needs to be applied to the entire area to make sure the lowest 25% doesn’t get too dry. This obviously means that the other 75% is over-irrigated to compensate for the lowest 25%. While this makes perfect sense on paper, there is usually more than meets the eye.
However, by making soil moisture maps with a GPS equipped moisture probe, we learned that striving for a high DU is too simplistic at best, and can even do more harm than good in certain situations. On a particular sand root zone with a 1% surface slope we noticed that the downslope side was usually wetter than the upslope areas. After mapping it with our (See Figure 1) we found the upslope moisture was to be around 15% while the downslope moisture was around 35%. Next, we tested the DU of the irrigation system and found that it was 80%. So despite nearly perfect irrigation coverage the moisture uniformity in the soil was horrible.
So, we made an adjustment and capped off the irrigation head at the downslope side. We came back about a month later, measured the soil moisture and measured the irrigation DU with only three heads operating (See Figure 2). The DU was now only 12%, but the moisture content of the soil was visibly more uniform. Interestingly, the wettest point on the green remained the downslope area nearest the head that was capped off. This is likely because of rainfall events, and subsurface flow/drainage from the higher points in the soil to the lower spots.
Clearly, irrigation DU is only a tiny piece of the water management puzzle. By ignoring the actual water content patterns in the soil, achieving a high DU is a meaningless activity. In fact, in this example the turf manager should be striving towards a lower DU, as a means to improve the moisture distribution of the green. Soils are not flat, uniform bodies. They are highly varied and have different drainage rates, particle distributions, and slopes, among other things. Remember, you are growing plants in the soil and the goal should be to have even moisture distribution in the soil, not necessarily coming out of the irrigation heads.