and wear inside the pipes (Clemmens, 1987). Uniformity should be tested as part of the system
maintenance. Problems within the system such as clogging and pressure variations can be diagnosed using
the results from the uniformity test.
Irrigation uniformity has been quantified in several different equations. Low-quarter distribution uniformity (DUIq)
is defined as the ratio of the average of the lowest quarter of samples to the average of all samples:
DU d = (Kruse, 1978) [1]
where diq = the average of the lowest-quarter depths (or volume)
d=g = the average depth (or volume)
Another measure of uniformity that is defined with the coefficient of variation is statistical uniformity.
U, = (1- CV) (ASAE, 1997) [2]
where CV = - [3]
CV = the coefficient of variation
s = standard deviation
x= sample average
Current guidelines for uniformity determination are based on volumetric flow from the emitters. The water
discharged over a constant time is caught from several emitters throughout the sample area. These data
are analyzed according to the above equations to determine the uniformity of the irrigation system or subunit
(ASAE, 1997).
It is becoming increasingly popular for drip irrigation to be installed below the soil surface or under plastic mulch
in vegetable production. As such, it is inconvenient and destructive to evaluate these systems in the field using
the existing procedures described above. Additionally, these results only represent the distribution from the
emitter without accounting for the water's movement through the soil.
A method of determining the soil volumetric moisture content could provide a more practical method for
conducting uniformity tests. Time domain reflectometry, which indicates this soil property, could provide a solution.
Time domain reflectometry (TDR) was initially used to find breaks in coaxial transmission lines, but has been used
in the last two decades to determine the soil moisture content of soils. Electromagnetic waves are propagated along
a wave guide. By timing their return, the dielectric constant of the soil is determined. It has been shown that
the dielectric constant is strongly correlated to soil moisture content, with little effects from texture,
material, temperature or salinity. Thus, water content in the surrounding media can be determined (Topp et.