Southwest Plant Selector Water Calculator

Enter the variables below for a more accurate estimate of how much you'll need to water a xeric (low water use) plant:

Water-use designation
Low (little or no supplemental water)
Medium (1x/month, 2x/summer)
High (up to weekly in summer)
Very High (every four days)†
The categories shown map on to those listed in the Southwest Plant Selector App.
† "Very High" water use plants are not included in the app, however, you may have them in your landscape.
What month are you watering in?
Where are you watering?
North of Albuquerque
South of Albuquerque
El Paso region
When did you last irrigate?
A month ago
Two weeks ago
One week ago
Four days ago
How much precipitation since last irrigation?
How large is the canopy diameter?

How long do I water?

How long you water depends on how much the plant needs (computed above) and how fast your method of watering delivers water. Use the form below to estimate how long to run your watering system to deliver the amount of water needed.

How much water do you need to deliver?
Results of the water requirements form above are automatically entered here.
How fast does your water system deliver water?
gallons per hour (gph)
See below for information about computing your water delivery rate.

How do I water? How do I compute how much water I'm using?

When you water a plant, try to cover a soil area equal to at least 70% of the plant's canopy area. Try to reduce spillage outside the watering area.

To compute your water flow rates, see the example below:

Suppose you are watering a Desert Willow with an 8' diameter tree canopy once a month in Albuquerque due to prolonged drought. It's June, and there has been a .2" rainfall since the last time you watered. The Desert Willow is a "low" water use plant, so, using the formula above, this yields a recommended 28 gallons of water.

If you are using a drip system with seven 0.4 gph (gallons per hour) emitters around its base, you are using 2.8 gallons per hour:

seven times .4 gallon-per-hour emitters equals 2.8 gallons per hour

So, we divide the gallons needed by the flow rate to get how long we should water:

10 hours equals 28 gallons divided by 2.8 gallons per hour

Thus, you would run the drip irrigation system for 10 hours once or twice that month depending on observed plant stress.

Alternately, you can measure the flow rate from a hose by the length of time it takes to fill a 5-gallon bucket at the flow that will be used.

If it takes 30 seconds to fill a 5-gallon bucket, the flow rate is 10 gallons per minute:

five gallons per thirty seconds times sixty seconds per minute

You would then move the hose around the tree in an attempt to cover the canopy area in about 3 minutes:

twentyeight gallons divided by ten gallons per minute equals two point eight minutes, or about three minutes

In this case, 6 sets around the tree at 30 seconds per set would provide the needed water supplement.

How is the water requirement computed?

The formula we are using is:

Requirement = (ET × F − P) × D2 × KL × 0.5


  • ET = reference evapotranspiration from table below (inches)
  • F = irrigation frequency (days between irrigation)
  • P = precipitation (inches)
  • D = plant (or zone) canopy diameter (feet)
  • KL = adjustment factor from table below
MonthNorth of AlbuquerqueSouth of AlbuquerqueEl Paso County†
March - 0.200.220
June and July0.300.300.330
October - 0.200.220
Shows approximate daily reference evapotranspiration (ET) in inches.
Results applicable for areas within New Mexico and El Paso County only.
† This calculator adds +10% to the ET values for "South of Albuquerque" to compute values for "El Paso County"; a 10-15% adjustment is typical depending on site conditions.
Water UseAdjustment Factor (KL)Irrigation Frequency (F)
Precipitation Only0†Never†
Low 0.10 Once per month (30)
Medium 0.20 Twice per month (15)
High 0.30-0.40Once per week (7)
Very High‡0.50-0.60Every 4 days (4)
Shows the approximate Adjustment Factor (KL) for plants of the given water use designation, and the recommended irrigation frequency for plants of that designation. Results applicable for areas within New Mexico and El Paso County only.
† Supplemental irrigation usually not required after establishment except during unusual drought.
‡ Very high water use plants are not included in the Southwest Plant Selector mobile app, but you may have them in your landscape.


These are general guidelines. Actual ET, irrigation requirement, and irrigation frequency may require adjustments based on weather abnormalities, local climate and microclimate of the planting, soil type, etc.

Note that in the summer (May thru August), reference ET seems to be pretty much the same throughout the state (based on comparisons of Farmington to Las Cruces and Artesia). It is in Spring and Fall when there are larger differences between the two regions.


If you have questions about the above data, contact:

Dan Smeal
Irrigation Specialist, New Mexico State University
Agricultural Science Center at Farmington

P.O. Box 1018 Farmington, NM 87499-1018
dsmeal at