Frequency
PivotsAdapted to High Frequency
◦Once a day or less
Reasons for long intervals
High frequency increases evaporation losses
For certain soils longer intervals allow more surface drying and cracking – increases infiltration
The maximum intervalIs when one of two conditions are
met◦Plant begins to suffer stress between
irrigations◦Applications become too large to
infiltrate in one pass and runoff occurs
Reasons for not using long intervalsIncreases potential for runoff
Beneficial to use the root zone as a potential “rainfall reservoir”
“Safety buffer” in case of breakdown
Rule of thumbs2.5 days for sandier soils3.5 to 4.5 days for medium
textured soils
Always use fraction of a day.4 to .6 fraction of a day
Minimizes daytime/nighttime impacts
Improves overall uniformity of infiltration and ETc
Example of multiple passes
Partial circlesIf a part-circle machine is operated
“dry” on the reverse leg, then the fraction of operating time, t, should be adjusted as:
Speedwet = the speed of the end of the lateral during application of water, ft/minuteSpeeddry = the speed of the end of the lateral during the dry return, ft/minute
dry
wet
Speed
Speed.
t
1
90
Deficit Irrigation(1) an overall strategy in effectively
stretching a limited water supply, or,
(2) for reducing the costs of irrigation where an adequate water supply exists.
So soil moisture deficit is allowed to occur and the crop undergoes some degree of stress during the season.
The primary objective is to increase the Water Use Efficiency (WUE) by scheduling the timing of irrigations during growth stages having the greatest impact on yield.
The WUE is the yield obtained per unit of water consumed (more crop per drop)
Hopefully , the reduction in irrigation costs from the deficit irrigation are greater than the reductions in revenue associated with any reduction in yield.
Applied Water vs Yield
ET vs Yield
Applied water / ET
Yie
lds
Transpiration
LossesApplied water
Strategies ◦ Soil Surface Modification◦ Residue Management
Soil Surface Modification
Field Residue
Plant Population/Row Configuration Modification
Crop Rotation/Management Modification
Water Supply Management◦restricted Supply- Reduced Irrigation
System Capacity
Peak crop water use management with root zone soil moisture maintenance.
Insufficient Supply- Reduced Water Allocation