How thirsty is the atmosphere? Using PET to quantify atmospheric demand and monitor crop health.

05/31/17
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Rainfall is clearly the key variable when monitoring crop production, but what are the ramifications of the warming atmosphere?  Even a slight increase in temperatures during the growing season is cause for careful monitoring.  More and more I am seeing reports when ‘near normal’ rainfall is equated to ‘all is well,’ yet there is a fundamental flaw in this thinking.

The evaporative demand for water – meaning the atmosphere’s ‘thirst’ - is a way of explaining and quantifying the substantial difference between a calm, humid, warm day and a windy, dry, warm day.  desert-1.pngHuman senses perceive this quite clearly: above about 70% humidity, we feel the mugginess and the simple chore of drying clothes on the line outside, well, the clothes can take all day to dry.  Contrast this to a day with the same temperature, low humidity, an even a slight breeze.  Not only does it feel comfortable but our clothes will dry in a couple hours. That’s evaporative demand working for you.

The “thirst of the atmosphere,” is calculated by an equation that combines the daily temperatures with humidity, wind, and solar radiation (or as climate scientists say, the FAO-56 Penman-Monteith formulation).   This calculated drying power of the atmosphere (its thirst) is physically an estimate of the amount of water that would evaporate from the soil and be transpired by plants if the soil were well watered.  We call this reference evapotranspiration (Eto) or potential evapotranspiration (PET).

PET provides a highly interpretable quantification of the ambient stress on plants – on the crop.  PET is calculated utilizing the variables measured at a meteorological station: daily minimum and maximum temperature and humidity, wind speed/run, solar radiation. The result of the calculation is expressed in millimeters per day – mm.  Rainfall is also measured in mm and so this then provides a mechanism to dive deep into drought stress and the impact on yield.

An elegant and simple index – the ratio of P (Precipitation) to PET, provides a highly interpretable first indication of plant health: is it water stressed or not?  Obviously if the ratio is 1.0 then P is equal to PET and if the temperatures are also suitable for the plant, then all is likely fine.  Even in the absence of integrating and tracking soil moisture, the ratio of P/PET relates total rainfall over any time period (often 7 or more days – rolling forward 1 day at a time, for example) with a measure of environment demand.  If this ratio is lower than 1.0, irrespective of the amount of P, the plants will start to struggle to meet their water requirement.   As this ratio drops, for example a P/PET ratio over 30 days of 0.50, the interpretation can be made that any crop experiencing this deficit of precipitation relative to the ambient thirst of the environment will be under significant stress.

We are now observing many manifestations of our warmer atmosphere on a regular basis.  Not only is the variability significantly increased, but extreme events are occurring with increasing frequency.  Monitoring production areas has become much more important as ‘flash droughts’ or ‘pocket droughts’ can emerge over even a few weeks and these can have a significant impact on yield. 

As to the drying power of the environment.  Here in semi-arid Colorado we can put our jeans out on the line to dry at dawn and they will be dry by morning coffee break.  In muggy humid parts of the world (think Washington DC or NYC on a muggy July day!), at the same air temperature, well, your jeans may take multiple days to dry.  At the same time, a vegetable garden in Colorado during a typical July needs water nearly every day or every other, preferably soaker hose or drip irrigation since ‘through the air’ sprinklers literally lose valuable water to the atmosphere.  At 90F and 22% humidity, the plants are pumping water as fast as they can – and sometimes they cannot keep up, the leaves droop and yield is compromised.

Topics: Agronomic Insight

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