Potential Evapotranspiration

This is a screen shot. MSU is Michigan's Agricultural university and they have a site dedicated to Integrated Pest Management. This weather site is a subsection of that IPM site. As you can see there are "wheels" to change variables and locations.

 

Potential Evapotranspiration is a simple idea. "How much water would evaporate out of a cake-pan if you left it in the sun?"

Clearly, a multitude of variables come into play. 

• Air temperature
• The dew-point
• Windspeed at ground level
• Solar loading
• Day length

being some of the main variables.

The very simplest way to measure PE is to actually put a cake pan in the middle of a sunny yard and weigh it at sunrise and again at sunset. That, however, is labor intensive and subject to error due to thirsty animals availing themselves to the water.

Consequently, scientists developed several different formulas to calculate the PE from the variables listed above. I suspect that the data is accessed hourly and integrated to produce the daily PE number.

The person who uses this information still has the responsibility to "map" the raw PE number to his circumstances. For example, an orchard might have both northern and southern exposures. The orchard might have some areas that are clay-based and retain water well while others are sandy and prone to drought. The orchard might have newly planted dwarf trees with roots that don't strike deeply into the soil and it might also have mature, semi-dwarf trees that are more deeply rooted.

So the orchardist will have one, calculated number to work with but will still have to use his brain to develop individual irrigation plans for each portion of his orchard.

Other states

I believe that most other states offer similar data. You just have to do a little bit of digging. For instance, Texas A&M offers this site and This one.

The smart money will look at the three stations that are nearest your growing location and will use rain-gauges at your growing location to judge actual rainfall.

The thing about rainfall is that it can vary wildly in a half mile and unless the weather station is on your property you have to take your own measurements.

Next gen technologies?

The next generation seems to be sensors that measure actual soil moisture. At one time these were blocks of gypsum with electrodes embedded in them. The resistance changed as the moisture content changed.

The weakness of soil moisture sensing technology is that if you are trickle irrigating with individual emitters on sandy soil, the wetted volumes are deep-and-narrow and likely to be invisible to the sensors.