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 * 1.5ft root zone Mad = 30% RAW= 2 inches/ft*1ft*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

Temperature sensor and radiation shield

The Water Balance Spread Sheet Inputs Climate data, paste into spread sheet Soil water holding capacity inch/ft Irrigation amount applied at each irrigation. Output – Date to irrigate on

Conclusion The water balance-check book method can be used to schedule irrigations for lettuce. The water use of lettuce can be calculated from climate data, and this is all that is needed to apply the correct amount of water at the correct time The check book method can also determine the decrease in yield and amount of water stress when time between irrigation is to long.