Santa Ana Wind Conditions Affect Avocado Tree Water Use

  • Oct 15, 2019

We recently experienced our first significant Santa Ana wind event of the season. Fortunately, this event was relatively mild — both in terms of wind speed and temperatures — for most of our growing areas. However, it’s important to understand how these events, even mild ones, affect your trees’ water use.

Transpiration is the movement of water from the soil, into the roots, through the vascular system of the tree, and exiting through stomates (pores) in the leaf to the atmosphere as water vapor. Transpiration occurs along a water energy gradient, from high energy to low energy, which is largely driven by atmospheric conditions. When humidity is low, the gradient from inside a moist leaf to the dry atmosphere is very large causing transpiration to increase. Wind also affects the gradient by disrupting the boundary layer, a layer of high humidity air right at a leaf’s surface. When the boundary layer is disrupted, the humidity surrounding the leaf is effectively decreased, thus increasing the gradient and transpiration. Temperature also affects transpiration because warm air is able to hold more moisture. Thus, as temperatures go up, relative humidity goes down, and transpiration goes up.

During a Santa Ana wind event all three of the above-mentioned conditions can, and often do, occur. However, even during a mild event such as the one that occurred last week, plant water use can more than double. The table below shows how evapotranspiration (the combination of transpiration and evaporation from the soil surface) was forecast to change from Friday during the Santa Ana event through Tuesday, October 15 for several locations around southern California. Note that the forecast temperatures during and after the Santa Ana event did not fluctuate severely. The high evapotranspiration was largely driven by the high winds and low humidity air they brought.

As we go through future Santa Ana wind events, it is important to keep their effect on tree water use in mind, and irrigate prior to the onset of the winds, even if temperatures are not forecast to be very warm. Additionally, California public utilities can now conduct Public Safety Power Shutoffs (PSPS) during wind events to help prevent wildfires. These power shutoffs may make it impossible to irrigate your trees during periods of high wind and high tree water need, making proactive irrigation even more important.

There are many forecast options available to predict evapotranspiration. A good one is the National Weather Service’s Forecast Reference Evapotranspiration (FRET) tool. To get a localized forecast for your area, including predicted FRET values, enter your address or zip code in the search box in the upper left corner of the map. 

Forecast evapotranspiration and daily high/low temperature for several locations throughout southern California for
Friday, October 11 (Santa Ana winds) through Tuesday, October 15 (post Santa Ana winds).
Source: National Weather Service.

 

Oct. 11

Oct. 12

Oct. 13

Oct. 14

Oct. 15

 

Forecast evapotranspiration (inches)

Fillmore

0.45

0.32

0.17

0.16

0.15

Oxnard

0.38

0.23

0.14

0.12

0.11

Oceanside

0.28

0.16

0.10

0.10

0.11

Escondido

0.38

0.25

0.13

0.11

0.11

 

Forecast daily high/low temperatures (°F)

Fillmore

82/54

82/50

81/49

77/50

77/51

Oxnard

83/53

80/49

75/50

73/48

73/49

Oceanside

78/54

76/54

73/54

74/55

76/56

Escondido

87/52

83/49

79/50

81/50

84/51

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