The Irrigation Association settled on Lower Quarter Distribution Uniformity as the measurement for sprinkler performance many years ago. Easy, but time consuming, to measure and very simple to calculate.
LQDU, or DU, provides an irrigation system manager a valuable tool to measure performance or quality, improve scheduling, & determine when system renovations are due.
Measuring DU is accomplished by placing collection containers within a defined
boundary covered by an irrigation system. Sprinklers are then operated for timed period. How much water was collected in each container is then measured and documented.
From there DU can be calculated with the following formula;
LQDU = LQ Ave / Total Ave
Where:
- LQ Ave = the average collection amount of the 25% of the collection containers with the lowest amount of measured water. (measured in milliliters, ml)
- Total Ave = the average collection amount of all the collection containers. (measured in milliliters, ml)
Example;
16 collection containers were laid out in an area. After the sprinklers ran, the water collected in each container was measured in milliliters.
Cup # ML’s Cup # ML’s
1 – 25 9 – 25
2 – 20 10 – 27
3 – 27 11 – 23
4 – 23 12 – 17
5 – 19 13 – 18
6 – 15 14 – 16
7 – 21 15 – 25
8 – 19 16 – 20
To calculate the LQ Ave, we need to isolate the 25% of the collections with the lowest collection amount. Since there are 16 collection containers there will be 4 containers in our lower quarter sample. (16 cups x .25= 4)
We look for the 4 lowest collections, which would be cup numbers 6, 12, 13, and 14 in the list above. Now we determine the average collection for just those containers. First total the collection amounts;
Cup # ML’s
6 – 15
12 – 17
13 – 18
14 – 16
Total = 66 ml’s
Next, determine the LQ Ave by dividing the total collections in those 4 cups by the number of collection cups in the sample;
LQ Ave = 66 ml’s / 4 cups = 16.5 ml’s
Next we determine the average of all the collection cups by adding up the total collections in all cups and dividing it by the number of all collection cups.
Total Ave = 340 ml’s / 16 cups = 21.25 ml’s
Now we divide the LQ Ave by the Total Ave to get our DU ratio;
LQDU = 16.5 ml’s / 21.25 ml’s = .7764705
OR
LQDU = (round to two places after decimal) = .78.
There are a variety of forms and Excel spreadsheets to help you do the math. One is linked on our web site in our I-Team page.
