Playing with numbers, evapotranspiration edition

I’ve used equation 52 in Crop Evapotranspiration: Guidelines for computing crop water requirements (FAO Irrigation and Drainage Paper 56) to calculate the predicted ET_o for San Digeo, California, and for Madison, Wisconsin.

I’ve also obtained the average ET_o for San Diego (Torrey Pines, station # 173) from CIMIS and the average ET_o for Madison from UW Extension Ag Weather. 

Plotting the calculated ET_o from equation 52 (predicted ET_o) against the ET_o average, one sees a pretty close relationship between the two values.

[][3]{.asset-img-link}But there is not such a close relationship between the temperature-based growth potential of PACE Turf and the ET_o.

I’ve suggested that the growth potential can be used to estimate turfgrass nitrogen use, for any grass, anywhere. Doug Soldat has pointed out to me that ET, or the consumptive water use, can be used as an estimate of how much nutrient uptake there may be. Nutrients (including nitrogen) go into the roots with water, and Bill Kreuser has written about this in Rethinking fall fertilization. 

The ET is one way to look at nutrient uptake and nutrient demand. And it is important to remember that nutrients do go into the roots with water, so there is going to be a huge, dominating role of consumptive water use (ET) in how much of each nutrient gets used by the grass.

Even so, I think the GP is more useful as a predictor of nutrient requirement extended across grass species and locations. These calculations don’t show the whole story, but they are a first step at looking into this. And, it is interesting to see just how well equation 52, which requires only temperature and latitude data, predicts the average ET.