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This section of the web site features Management Updates written by the turf specialists of the UMass Extension Turf Program. The messages cover regional problems, are geared toward regional conditions, and are posted frequently during the growing season.

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Torrential Rain vs. Turf

Jul 10 2015

After an alarmingly dry start to the growing season, the pattern has certainly shifted in favor of more regular and soaking rain events. We have had some heat, but not extreme or long-lived heat, which has kept evaporative demand reasonable. These factors, which came together just in time, have put most turf in fine shape heading into the summer. Most water-related reports as of late have concerned too much moisture, as opposed to problems caused by too little.

With our typical humid summers in the New England region comes the promise of increased thunderstorm activity. In a lot of years one might comment that any form of precipitation is welcome in July and August, but precipitation brought by thunderstorms is often less than ideal. While thunderstorms definitely have the potential to deliver large amounts of rain, the rain tends to come too hard and fast, not to mention the possibility of problems from associated phenomena like hail or high winds. The harder and faster the rain, the less likely it is that appreciable moisture will percolate into the soil to benefit turf and other plants, and the more likely that issues can arise in the form of runoff and erosion.

An immensely beneficial force in mitigating problems from torrential rainfall is dense, healthy turfgrass cover. A lot of it has to do with energy, which the rainfall in gully-washing storms has no shortage of. With a dense turf, most drops hit the canopy of leaf blades first, dissipating much of the energy before the moisture reaches the soil surface. This interception goes a long way towards mitigating what is called splash erosion. Think about a bare soil or an area of thin turf, where droplets are able smash into the surface with substantial energy. Splash erosion occurs when the force of impact from a water droplet catapults soil particles in all directions. The most visible splash erosion problem involves soil that is splattered on to buildings, fences, vehicles, etc following heavy rain (Fig. 1). Regular episodes of splash erosion on exposed soil over time can lead to formation of surface crusts which inhibit water infiltration and promote runoff.

Another key issue with regard to torrential rainfall concerns water and gravity. The movement of water down slope is desirable to some degree in managed areas to prevent pooling, ponding, and other soggy spots, but there is an optimal level in terms of the amount and speed of drainage. This is because runoff can carry soil, nutrients, pesticide residues, and other materials out of the system, robbing the system of resources and increasing the potential for negative environmental impact. The ‘army’ of turfgrass shoots in a stand of turf serve as a break or a barricade to disrupt the momentum and reduce the energy of flowing water, thereby promoting infiltration into the soil and decreasing the chance that erosion will occur or materials will be carried off site. Infiltration and percolation, of course, are vastly preferable to run off that moves quickly and directly into storm drains, surface water, and environmentally sensitive areas.

A healthy turf also provides a sort of fortification, in the form of a deep and extensive root system. When soil becomes saturated to a certain degree during a storm, the soil and water can begin to move in combination in response to gravity and momentum, carrying soil down slope and leaving channels and gullies (Fig. 2). An expansive root system serves as a structure that supports and reinforces volumes of soil, making the soil less prone to being dislodged and moving under heavy rainfall. A healthy turfgrass root system also contains microorganisms and biological compounds that function in binding soil particles together, further helping to keep sites intact.

Despite the utility of turf in mitigating runoff and erosion, it is useful to recognize that turf might not be the most appropriate or effective cover in all situations. In some extreme situations, such as below downspout outlets (Fig. 3), for example, stone, splash blocks or other manufactured materials are often a better long term option.

Fig. 1: Heavy rain hitting exposed soil can cause splash erosion, the effects of which are often visible on structures.      Fig. 2: Soil without adequate turf cover and an associated root sytem is more susceptible to erosion, especially on slopes (note manhole covers to the left and right for scale)..      Fig. 3:  Areas subject to extreme pressure from falling water might warrant non-turf solutions.

Submitted by: Jason Lanier