There are some other high-caliber tools such as eddy covariance and its second cousin by marriage, the surface renewal method. Eddy covariance has an (undeserved) reputation as a “gold standard” method for direct measure of the water vapor content in short-lived turbulent motion above the canopy. Aside from a severe burden for equipment cost, installation, and data processing
the measurements aren’t that good: the assumptions that the terrain is flat, the canopy uniform, the tower tall, the energy balanced closed . . . these are rarely met except in idealized circumstances.
Surface renewal is a clever method: when NASA scientists were dreaming up how to measure the fluxes of heat on the moon, they developed a technique using just a simple thermometer to measure the turbulent exchange of warm air. Converting this over to measuring ET though is not without its challenges: it requires an estimate of shortwave and longwave radiation to estimate the total energy to be dissipated, and subtracts out the surface renewal to arrive finally at the ET number. So in a very real sense, the method depends closely on how well the radiation budget is measured.
Vineyards and other specialty crops in microclimates are often located on slopes, since these conditions often yield the highest quality yield, but it is under these conditions that the methods are presented with challenges.
There are a variety of other methods for estimating ET using remote sensing snapshots of canopy cover and canopy temperature, but these suffer from the same problem: without a measure of the radiation budget, there are poor bounds on the total amount of heat loss and ET. It’s as though I offered you half the contents of my pocket: half of what?
And finally there are a few methods that use weather stations or crop temperature monitors for irrigation management. The weather station method measures one component of radiation (incoming sunlight) but none of the other components. Nor does it measure canopy cover or crop stress. Crop temperature monitors also measure one component of radiation (upwelling thermal radiation), but none of the others, and tends to be either a qualitative measure of crop feel, or simply trigger an alert above a threshold. Either way, these are at best correlated with ET and not quantitative measurements.
So even though we live in a world with these tools available, most of agriculture still uses the tools created by the pioneers of the 20th century: a shovel to see if it’s wet down there. Why? Everyone has a shovel! Obviously the world is still turning and crops are still growing. The problem is that water is increasingly a commodity, which can be bought and sold and otherwise shows up in the P&L for a farm budget, and has asset value attached to the land. What’s more, manipulating the water budget can increase yields, influence quality, and force timing. A shovel makes it difficult to scale to 20, 100, 1000 sites and also contributes to lack of accessibility and transparency across an organization. Having the ability to quickly check the ET measured across your operation without having to go into the field is quite the time saver.