Good Rain, Bad Rain?

It's been a long, hot summer this year--short on blog posts and extra long on perspiration. The Denver area had less precipitation this summer (until now) than in the drought year of 2002. But that's all moot at this point, 'cause we've been getting rain the last week or so. Hurray! The monsoons are here!

It's not quite so simple though. Similar to the witches in Wizard of Oz, there is such a thing as "good rain" and "bad rain". In the land of the arid west, rain often turns into a destructive force, leaving landscapes with little usable moisture, while eroding precious soil and washing pollutants into rivers and streams. Typical summer days here involve warm morning temperatures rising to blistering early afternoons, relieved by late afternoon clouds and pounding deluges that seldom last longer than a few minutes. The air temperature easily drops 1o degrees when the sun is stopped from its unrelenting glare and when the violent air currents accompanying the moisture clash with hot dry air, the resulting winds can cool things down by another 10 degrees. It's a temporary respite for humans, but the rain usually comes so fast and furiously, plants aren't able to benefit from the moisture.

So "bad rain" is precipitation that comes faster than the landscape can absorb the water. Soils without much organic material, so prevalent in arid areas, don't hold moisture well. Further compounding the problem, clay soils are fine textured, compacted and become super-saturated quickly. In sandy soils the water runs through the root zone faster than the plants can use it and clay soils cause water to collect on the surface and run off before penetrating the root zone. Too much rain, too fast, just runs down the gutter carrying accumulated animal waste, lawn chemicals, plant debris and dirt into the storm sewers where it causes problems for water quality for downstream users--including humans and wildlife. Landscapes can be designed to take our precipitation patterns into consideration, but this sometimes involves manipulating grades in ways contrary to common engineering practices.

On the other hand, "good rain" is the sort we seldom receive--the misty, slow accumulation over a period of hours or even days. When the moisture is applied slowly, capillary action allows the water to move further down into the soil profile encouraging roots to grow deeper which protects them from the high temperatures and evaporation at the soil surface. It also allows oxygen to remain between soil particles so roots can continue to grow. When soil becomes super saturated water displaces the air and roots are unable to grow until enough oxygen becomes available again.

Rate of precipitation is just one more hardship plants face in our region. Unfortunately, the problems created by our natural patterns of rain is further perpetuated by common irrigation practices. Pop-up sprinkler heads--the ones that spray small droplets of water into the air--apply water at a rate much faster than the clay soils in this area can absorb. A lawn may require in a week as much water as can be put on by these sprinklers in an hour, but the soil becomes saturated after 5-7 minutes when applied by spray heads. Any water applied after that point will run off or evaporate unless the soil is given time to allow the water to percolate lower into the soil--making space for the next water molecules. A typical bluegrass lawn needs around one inch of water per week during hot periods, but in order for the turf to be able to use the water, spray heads should be run 5-7 minutes for three cycles in one day for 2-3 days per week. This practice assures the water stays in the root zone so the plants can use it.

Thanks to GoodJoe for use of the summer storm cloud photo.