Trees don't care about evapotranspiration..

True. But we certainly do.. It tells us when and how much to water our farm.

We can determine optimum time and amount of water to irrigate the orchard by tracking how much water is lost from the soil via evapotranspiration (the loss of moisture in the soil thru evaporation + the loss of water thru tree transpiration).

Here's an example. For almond orchards, we strive to maintain soil moisture level of 4-5 ft deep. But to keep it simple, consider this:

Let's say you measured soil moisture level at the end of Sunday to be 1 inch. And say, you can only tolerate half-inch depletion.

Monday ET = 0.15 inches --> 0.15'' of top soil lost moisture --> ok
Tuesday ET = 0.20 inches --> 0.35'' of top soil is dry --> ok
Wednes  ET = 0.15 inches --> 0.50'' of top soil is dry --> ok
Thurs     ET =  0.20 inches --> 0.70'' of top soil is dry --> NOT ok --> IRRIGATE NOW.

You get the idea..

How do we get daily ET numbers? It's calculated from daily satellite and weather data (temp, humidity, wind) and the type of crop (ground cover) at the farm. I find that different sources provide slightly different numbers. This ET number can be different for different parts of your farm based on soil type differences.

Alternative to doing this calculation is to install ground sensors to measure soil moisture up to several feet (like we did at Carpenter ranch). Based on the readings from these sensors, you can make irrigation decisions.

What exactly is evapotranspiration?

"Evapo what?." That's my reaction when I first heard that word.

Simply put, it is the amount of water vapor released into atmosphere by soil and plants on any given day.

Evapotranspiration = Evaporation from Soil + Transpiration from Plants

Evaporation: On average, water is evaporated across the earth's surface at the rate of 1/8th inch each day. At tropics, more evaporation occurs. At cooler areas, less so.

Transpiration: Transpiration is vaporization of the liquid water in the plant. Plants have small openings (called "stoma") in the underside of leaves that release water vapor into atmosphere. Read my previous post for more info on this.

If the crop is smaller (like strawberries), water is predominantly lost by the soil evaporation.
If the crop is alfalfa or well-developed almond orchard with full canopy, then transpiration becomes the main component of evapotranspiration.

Transpiration: How do plants release water into the air?

This is how transpiration happens:

• When sunlight falls on a leaf, its radiant energy excites the water molecules in the leaf. 
• The H2O molecules heat up, and they start bouncing around and eventually come up to the surface of the leaf. 
• When they absorb even more energy, bonds between adjacent H2O molecules break, turn into water vapor and evaporate into the atmosphere. 

Transpiration also cools plants, enables flow of nutrients and water from roots to shoots. The more water transpires from leaves, the more water is sucked in from the ground via roots by capillary action.

Potassium.. Where art thou?

Potassium is the third major plant and crop food, after nitrogen and phosphorus. Potash is important for agriculture because it improves water retention (because it regulates opening and closing of pores on leaves) and disease resistance of crop too.

Anyway, despite applying potassium annually at Carpenter the leaf tests and visual symptoms indicate that we continue to be deficient in this important nutrient. We have had decent crops and each crop removes a lot of potassium. So we need to increase the amount we apply to compensate for the amount being removed.

Our plan is to purchase two truckloads of potassium fertilizer this year for Carpenter (instead of the usual one), and one for Whitmore (half for the Candycots, the other half for the almonds).  We will begin to apply it as soon as the soil is dry enough to do so. If it stops raining for an extended period of time (10 days) we will be able to apply all of the potassium at both ranches before the end of December.