Minnesota Science
Vol. 44, No. 2
Rainmaking Is a Science in This Laboratory
by Larry Etkin
[INTRODUCTION: Rainmakers and Robots: All to the Aid of Water Quality Research
In the public arena, scientists now debate the potential for, superconductivity and the possibility of room temperature fusion. Such debates focus attention on the process of research. It's a mysterious activity to many. We hear about the breakthroughs and the insights, but hove does science actually get done? Here's a look inside some experiment station scientists' labs, where researchers seek sometimes novel solutions to practical problems.]
==========
At least one University of Minnesota laboratory can forecast rain with 100 percent accuracy, provided you don't go outdoors. It's the laboratory of Agricultural Experiment Station soil scientist Satish Gupta. In that laboratory, he and graduate student David Freebairn are so phenomenally accurate because they make the rain themselves.
The researchers are simulating rain to develop better soil management practices, to reduce groundwater contamination from agricultural chemicals. Their rainmaker uses about 2,000 hypodermically thin needles, spaced about five inches apart, to produce simulated raindrops. The researchers test the impact of these drops on various types of soil by raining them down onto samples 12 feet below.
"We can simulate some kinds of rain, but not all," notes Gupta. "The size of our simulated rain is determined by the size of the needles. We are stuck with a size of rain, but we can vary the intensity by adding more pressure." Simulated rainfall lets Gupta and Freebairn study how soils disintegrate under rain's impact, and how soil particles move when soil erodes. They are currently working on soils like those found in southeastern Minnesota.
"We're looking at how the water moves across the landscape. Is it really moving as runoff, or is it going through the soil and then going to the groundwater?" Gupta asks. The researchers' idea is that runoff from slopes concentrate at low points of a landscape and infiltrate from there.
Gupta says southeastern Minnesota soils are high in silt. Rain crashing down onto bare soil creates a kind of seal that enhances runoff. In the lab, he and Freebairn are trying to determine how that seal forms. "How does that soil disintegrate? How do the small particles fit in between the larger particles?" Gupta asks. "If we can understand the mechanism of surface seals, then we can use models to predict the proportion of runoff for a given rainstorm."
That predictive ability would help them determine whether contamination of groundwater is more from runoff water or more from infiltration. "Think of a watershed that is one acre in size, on which we have one inch of rain," suggests Gupta. "If the rain is distributed evenly over the whole watershed, it would probably soak into the soil to a depth of about five inches. But if all that water doesn't go into the soil, if that one-acre-inch of water collects in a one?twentieth acre pond down slope, you have perhaps 20 inches of standing water which has all the nutrients it brought down from the landscape."
"That water, with all those chemicals, is still going to infiltrate, though at that lower part of the landscape it's more likely that the 20 inches of water is going to saturate the soil to about 8 feet, and that's more likely to go to the groundwater."
"Water is going to go to the soil whether it goes uniformly through the landscape or it concentrates at one place, and then infiltrates. Our idea is to have uniform movement," he says. "The more of that water that goes directly into the soils, then the better the chances that nutrients and chemicals are going to be held back by the soils rather than carried down with runoff."
The bottom line for Gupta and Freebairn is finding soil management practices to both minimize erosion and keep agricultural inputs where they are applied, in the upper levels of the soils where the plants can use them.
PHOTO CUTLINE: Satish Gupta (left) and David Freeman track infiltration of
simulated rainfall on three soil surfaces.