After all that Rain, How can it be Dry?

Posted: July 11, 2011

Remember earlier this spring when you weren’t dressed until your umbrella was in hand? Now you almost need earplugs to quiet the noise of crunching grass. How long does it take to move from flood to drought anyway?

For a big plant in a small pot on a hot windy day the answer may be less than 4 hours. For a reservoir near the end of a long river the answer may be 2 years. For a hand-dug well the answer may be one or two months.
Drought occurs first as a meteorological drought. The weather man with lots of statistics can identify a period of low rainfall in just a few weeks. The farmer and the golf course superintendent look for signs of water stress in their corn or grass and declare an agricultural drought when they see symptoms such as leaf rolling or wilting in their plants. This happens because the plant is unable to get water from the soil fast enough to keep up with transpiration. High temperature, dry air, wind, and sunshine can cause drought symptoms in plants even though their roots are in damp soil. Moderate temperature, still humid air, and cloudiness allow a plant to look OK even though the soil is dry.
If we look deeper we find out that kind of soil and length of roots are also important factors in determining when an agricultural drought starts. Plants in sandy soil have to be watered frequently because sand holds very little water. The same plants in a loam soil can go longer without watering because the loam holds more water than the sand. If the plant has short roots like radishes it will still need frequent watering but if it has long roots like alfalfa (up to 10 feet long) the plant can go a very long time before running out of water. The idea that sand holds less water than loam is the concept of water holding capacity. Sands only hold about 0.5 inches per foot of soil while loams hold up to 2.5 inches of water per foot of soil. Plants wilt when the available water is used up and perk up when the water is replaced by rain or irrigation.
We have reduced a very complicated system to two simple ideas. First, plants use water fastest under low humidity, windy, hot, and sunny conditions. Under these conditions they empty their root zone of water rapidly. Second, the amount of water held in the root zone varies from low for sandy soils to high for loam soils. When the available water is gone the plants wilt and growth slows or ends until water is added to the soil.
When to water your plants:
You can make practical use of this information. Put a cake pan in your garden or flower bed. Add 1.25 inches of water to it. When it is empty (be sure it evaporated and wasn’t consumed or spilled) add one inch of irrigation water to the garden. Refill the pan to 1.25 inches and start over. The extra quarter inch is needed because evaporation is faster than transpiration. Rain will be accounted for automatically, except for rains that increase the level above the 1.25 inch mark. In that case dump the pan and start over with 1.25 inches of water.
You can decrease the amount of water in your pan to account for shallow roots or sandy soils. When irrigating remember that water added slowly by drip irrigation or from a jug with a small hole in the bottom wets the soil more deeply than the same amount sprinkled on. Also using mulch increases the efficiency of water use because it reduces the evaporation of water from the wet soil surface.
Tom McCarty is the Penn State Cooperative Extension Water Quality Educator serving the Southeast Region. Penn State Extension in Cumberland County is located at 310 Allen Road, Suite 601, Carlisle, PA  17013, phone 717-240-6500, e-mail Penn State is committed to affirmative action, equal opportunity, and the diversity of its workforce.