Irrigation System Designing - Practices of Irrigation and On-farm Water Management - page 88

3.4.3.2 Infiltration Opportunity Time

Infiltration opportunity time is defined as the amount of time that water has the
opportunity to infiltrate the soil. Or, it is the difference between the times when the
water recedes and when it advanced to a specific location.
The infiltration opportunity time is different at different points along the length
of the furrow. This is because the length of time that the water is present on the
surface of the soil at any location is the difference between the time the water
arrives (advance) and the time the water leaves (recession). The rate at which
water advances down the field is different to the rate at which it recedes. Making
opportunity time more uniform down a furrow is a desirable strategy for improving
distribution uniformity for furrow irrigation.

3.4.3.3 Distribution Uniformity

The measure of how evenly the infiltration occurs is called distribution uniformity
(DU). In general, depth of infiltration at low quarter is considered as reference for
expressing DU (Burt et al., 1997). That is,

DU = (average low quarter infiltration depth) × 100/(average infiltration depth)

During a furrow irrigation event, distribution uniformity is primarily influenced
by the soil infiltration characteristic (soil type, variability, and moisture content),
rate of water flow into the furrow (inflow) and the length of time this water is flowing
(time to cut off). Other factors that influence uniformity include field slope and
variability and field length.
Irrigation efficiencies are directly related to the uniformity of water appli-
cation (distribution uniformity) on the individual fields. Furrow-irrigated field
distribution uniformity is directly related to the advance ratio and the aver-
age depth of water infiltrated per hour. For a uniform soil with good land
grading, the distribution uniformity of water infiltration for furrows is depen-
dent upon the uniformity of opportunity times. Making opportunity times more
uniform down a furrow is a desirable strategy for improving DU for furrow
irrigation.
A DU value of 80% is considered excellent for all irrigation methods.

3.4.3.4 Time Ratio

Time ratio ( R t ) is defined as the ratio of the time required for the infiltration of total
net amount of water required for the root zone to the time when the water front
reaches the end of the run. It plays a key role in determining optimum furrow length
to achieve maximum irrigation efficiency.

t i
t L

R t =