MAD also affects only water amount at one time, but NIPH affects everything.
To summarize the Advanced parameters when used with Flex Daily schedules (abbreviations are mine - not necessarily official):
- Available Water (AW) is determined by the Soil type. The higher it is, the more moisture your soil can hold. There are soil tests you can do to determine the soil type, or look up your actual soil online. Rachio’s values seem reasonable.
- Root Depth (RD) is determined by the Crop Type (grass, shrubs, etc.). It represents the desired depth rather than the actual depth. Rachio’s values seem reasonable, although for warm season grass, my Centipede grass has lower root depth and higher Crop Coefficient.
- Allowed Depletion (AD) is a percent of the total water the soil holds to the root depth of the crop that is allowed to be used up before the crop begins to wilt. This is normally taken as 50%, but is not a hard number.
Those 3 values together (and no other values) determine the INCHES of water that Flex Daily will apply to the zone at one time/day. They are simply multiplied together to get the result:
Inches Water Applied at once = AW x RD x AD
ONLY the above 3 values determine the amount of water applied at once. For newly installed sod, you can save water by setting the Root Depth to a lower level, but always higher than actual root depth, and increase it over time to the desired root depth. And you can play a bit with the Allowed Depletion. But in general, these values can and should stay the same.
- Nozzle Inches per Hour (NIPH) is a measure of the amount of water in inches covering the zone that will be applied in an hour. It is a critical value to the calculation of any irrigation system, and unfortunately, not easy to determine exactly. This information can be gotten from manufacturer’s charts, and by using Catch Cups. IMHO, the easiest and best way is to measure the gallons of water used per unit time for the zone and the area of the zone in square feet to calculate it by the formula:
NIPH = Gallons / Minutes / Area x 96.25
- Efficiency is a measure of how evenly water is applied to a zone. When using a Catch Cup test, it is defined as the average Inches of the lowest 25% of the readings divided by the average Inches for ALL the readings.
NIPH is then used to calculate how many minutes of irrigation is required to put X amount of water down, usually the Inches of water at once. It is modified by the Efficiency by dividing the time by (0.4 + 0.6 x Efficiency). The idea with considering Efficiency is to over-water most of the yard in order to not under-water the lowest flow areas. I feel Rachio’s values are fine. Better values can only be obtained through use of Catch Cups, and my experience indicates that this can be so problematic with actual placement as to be almost worthless.
The only Advanced factor left is:
Crop Coefficient (Kc) is a measure of the percent of water required over time compared to an open pan of water (the FRET value). If a pan of water sitting out in the sun evaporates at the rate of 0.2" per day, and your Crop Coefficient is 0.8, it means your crop would use 0.8 x 0.2" = 0.16" of water per day. The higher the value, the more water it uses, and vice versa. FRET (Forecast Reference crop EvapoTranspiration) is calculated by the NOAA and the NWS and is published in various forms, and takes into account temperature, humidity, wind, sun, etc. Rachio calculates the total ET (Evapotranspiration) for each day from all this information, as well as the Sun setting you input for the zone.
The ET is the value that determines how much water the zone needs over time, and the only factors that determine it are the Crop Coefficient and Sun which we can set, and the weather, which we cannot.
It has been stated many times that the NIPH changes the amount of water received, and it does, but it is best if it’s calculated accurately and left alone, rather than used to change the operation. But reducing NIPH will increase the time which then will increase the ACTUAL water applied during each water application. It doesn’t change the water needed, only what is applied at one time. But while most of the other factors are based on facts, adjusting NIPH just to change watering amount is in effect lying to your system; I think that changing actual factors like Kc or AD make more sense, and using a determined-accurate value for NIPH.