# Scheduling irrigations for lettuce using climate data Ted

• Slides: 22

Scheduling irrigations for lettuce using climate data Ted Sammis

The Water Budget Method • • The lelttuce root zone is a reservoir Water is added by irrigation or rainfall. Water is removed by evapotranspiration. Added water can be lost by deep percolation if the amount exceeds the field capacity of the soil.

Management Allowed Depletion • Management Allowed Depletion Is the percent of water that can be removed before soil moisture stress limits growth. • (For lettuce MAD is % 30. )

Evapotranspiration • Evapotranspiration is determined from climate data to calculate potential Evapotranspiration (water used by grass) and a scaling coefficient or crop coefficient for lettuce • The crop coefficient depends development of the crop or the land area covered by the crop. The crop coefficient changes throughout the growing season increasing to a maximum at maximum leaf cover.

Calculation of Evapotranspiration • • Potential Et= 0. 3 inches / day Et= K*PET K= 0. 8 Et= 0. 8* 0. 3 =. 24 inch/day

Calculated when to irrigate • Readily available water (RAW) = available water (AW) * management allowed depletion (MAD) • Example: • Aw = 2 inches/ft * 1. 5 ft root zone • Mad = 30% • RAW= 2 inches/ft*1 ft*0. 3 = 0. 9 inches

Irrigation Interval • Irrigation interval = RAW / Et / day • Irrigation interval = 0. 9 inches / 0. 24 inches /day = 4 days • Irrigation interval changes with changes in the Et or soil type. • Et changes with cover of lettuce and weather.

Source of Potential Evapotranspiration • New Mexico Climate Center Homepage • http: //weather. nmsu. edu