Assoicate Professor
Department of Agricultural and Biosystems Engineering
Calculating a bubbler irrigation schedule requires the following steps:
. Calculate plant water use in liters per day (LPD) used by the plant.
. Calculate the volume (S) of available water (liters) in soil.
. Calculate days between irrigation (S/LPD)
. Calculate irrigation run time (S/Q), where Q = bubbler or emitter flow rate (LPM)
. Calculate the soil water storage, S, based on soil texture, root zone, allowable soil water depletion for specific plants, and the diameter of the basin.
S = Volume of water storage in soil available to the plant, L.
Z = root zone depth, m (1.0 m for trees and 0.5 m for shrubs),
AWHC = available water holding capacity, % (see table 2),
MAD = management allowable depletion (1.0 for desert plants, 0.5 for other plants),
Db = diameter of the basin, m.
The available water holding capacity (the percent of soil volume that holds water that is available for the plant) for different textured soils is shown in table 1. Assume that depth of the root zone, Z, is 0.5 m for shrubs and 1.0 m for trees unless better information is available.
Soil Texture |
AWHC |
Sand |
8 |
Sandy loam |
12 |
Loam |
17 |
Clay Loam |
18 |
Silty Clay |
20 |
Clay |
23 |
The management allowable depletion is the fraction of total available water in the soil that is available for a specific plant species. Very drought tolerant, moderately drought tolerant, and nondrought tolerant plants might have MAD's of 0.8, 0.45, and 0.2, respectively.
The number of days between irrigations (Days) is the Liters of storage divided by the Liters used per day.
Days = S / LPD (2)
The irrigation cycle run time (Minutes) is the soil water storage divided by the application rate (LPM)
Minutes = S / LPM (Equation 3)
Canopy diameter = 3 m.
The basin diameter, D b, is 1.2 m.
The soil is clay loam. Bubbler flow rate, Q, is 8 LPM.
Late summer reference evapotranspiration, ET 0 , is 5 mm/day
Species crop coefficient, Ks, is 0.5. Microclimate and density crop coefficients are 1.0
Microclimate and density crop coefficients are 1.0
LPD = ET 0 * K L * D 2 LPD = 5 * 0.5 * 32 = 22.5 LPD
Assume that the MAD for a maple tree is 0.5.
AWHC for a clay loam soil is 18 %
Root zone depth, Z, for a tree is 2.0 m.
Days = S / LPD = 207 L / 22 LPD = 9.5 days
Minutes = S / LPM = 207 L / 8 LPM = 26 minutes
A larger basin would also encourage root growth.
Redo the example with 3 m diameter basin.
Same as above = 22 LPD Calculate soil water storage (S)
Days = S / LPD = 1,296 L / 22 LPD = 59 days
Minutes = S / LPM = 1,296 L / 8 LPM = 162 minutes
A bubbler may not fill a 3 m basin, so redo the example with a 2 m basin.
Same as above = 22 LPD
Days = S / LPD = 576 L / 22 LPD = 26 days
Minutes = S / LPM = 576 L / 8 LPM = 72 minutes
The 2 m diameter basin is probably the best choice because there is a long period between irrigations and because the basin is not too large to be irrigated with one bubbler.
