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