“We know that crops use water based on their growth stage, so we see that water requirements start off low at early season, increase at mid-season, and then drop off later in late-season.

“Crop water-use changes with the developmental stage of the crop. In the initial stage, we must support increased vegetative growth, including the canopy and the root system. Then when we move into reproduction, the plant requires a lot more energy than when growing a leaf. Also, environmental conditions normally change during the growing season, with more heat and drought and less rainfall as it progresses.”

The peanut water-use curve currently being used was developed in the 1970s, says Rowland.

“We’ve been working to update these water-use curves, and we’re finding that they’re different based on the peanut cultivars being grown today. We can increase our efficiency by understanding how different our water-use requirements are for the various cultivars.”

The University of Georgia has translated this water-use curve into weekly water use by peanuts, considering each week after planting, with a peak at about .30 inches per day at about mid-season.

Looking at the water required by the most commonly grown row crops in the Southeast, peanuts need from 20 to 27 inches per growing season, cotton 27 to 51 inches, corn 20 to 31 inches, and soybeans 18 to 27 inches.

“It seems kind of simple — we can approximate how much water is needed at different points in the season, and we can apply it at that time. But we have to remember that water use differs by peanut cultivar, and it also differs by the maturity of the cultivar.

“So if you’ve got a 150-day peanut versus a 125-day peanut, the longer maturity cultivar will require more water just by the fact that it has a different maturity date.

“We also add soil conditions and weather conditions, and this is where it can become complicated. The primary factor is stress. Whenever stress is added into the equation, we’ve got a whole new ballgame.”

From a physiological standpoint, drought causes stress at the cell level, leaf level and plant level, explains Rowland.

“One of the first things that happens is that we lose the turgor or the rigidity of that cell. It automatically stops growing with decreased cell division, and the metabolism is slowed.

“That leads to growth stopping at the leaf and plant levels. That’s why we see leaves drop off — that’s how a plant reacts to the stress. As water use drops, photosynthesis decreases.

“Then we begin to see that the crop changes the allocation of its energy. As it comes under stress, it won’t put resources into reproduction.”