Agriculture Reference
In-Depth Information
where
t i =
time required for infiltration of net irrigation depth
t L =
time required for waterfront to reach the end of run
R t value of 1.0 represents the length of run in which the time required to infiltrate
the irrigation depth is equal to the advance time.
The advance ration is an important factor for managing a furrow irrigation sys-
tem. Generally, water should get to the end of a furrow in less than 1 / 2 of the set time
to achieve good distribution uniformity. Whether that should be as quickly as 1 / 4 of
the set time would depend on the soil texture and conditions.
3.4.3.5 Cutoff Ratio or Advance Ratio
Advance ratio or cutoff ratio is defined as the ratio of time to reach the water-
front at the end of furrow, to the time set for irrigation. Mathematically, it can be
expressed as
t e
t irr
R cut =
where
t e =
time required for water advance to reach the end of furrow
t irr =
time of irrigation
Runoff and the uniformity of water infiltrated along the furrow are related to the
cutoff ratio.
A cutoff ratio of 0.5 is desired. For example, for an 8-h irrigation set time, the
advance time should be about 4 h. The easiest way to change the advance time is by
altering the furrow stream size, i.e., by changing the size of the irrigation set. This
will affect the cutoff ratio and hence the uniformity of water application.
For both level and sloping furrow systems, high uniformities (greater than 85%)
require a reasonably small advance ratio.
3.4.3.6 Irrigation Set Time
It is the total time for irrigation. Irrigation set time is determined by furrow inflow
rate, furrow shape, roughness, and length.
3.4.3.7 Advance Rate
It is the rate at which the waterfront advances through the furrow. Advance rate is
influenced by both soil conditions (size, slope, and roughness of the furrow) and
inflow rate.
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