If a Raindrop Falls....

I have a brother who cannot seem to want enough rain. In one email he asked the question “What is the best rain?” In an even earlier email he intimated that but for people we would not know flooding. The title of this post is then a play on the long known philosophical thought experiment about the falling tree in the forest if no one is around to hear does it make a sound. So, forming my brothers question and statement another way, would rain drops fall if no one was around to feel/see them? As geology teaches us, rain and wind have been forming the landscape for eons, well before man, well before the dinosaurs, and even before continental drift. It has left limestone, which is formed from creatures of the sea, well above sea level (I can take you to a cave in the east bluff of Devil’s Lake that shows this), it has made mountains into monadnocks, created river deltas, and helped form our soil. The determination of what is the best rain is related to a variety of factors—rainfall intensity, capacity of the soil, type of soil, land cover/use. As we know from grade school, all the water we will ever have has been on the earth, it just keeps recycling by the hydrologic cycle. Of the water on the earth, reportedly only about 3% is fresh water, and of that, one-third (or 1% of total) is surface and groundwater. Therefore, not only is the hydrologic cycle important, but so is how we choose to use our water, and how we develop our land uses. Perhaps over time I can provide more on this connection, but for the time being this post is related to one aspect of storm water runoff. 

Overland Flow

Most, if not all, readers of this blog have been outside in an intense rain event where we see water runoff (for relatives, think of the family picnic in Stevens Point a few years back), and my brother’s email post reminded me of what I learned in a college hydrology course. That runoff we typically see has a name and is referred to Horton overland flow, named for Robert E. Horton, who the text book (yes, I still have the text book) notes provided the foundation for modern quantitative hydrology (1978, Dunne, Thomas and Luna Leopold, Water in Environmental Planning, W.H. Freeman, San Francisco). Horton overland flow, as noted in the book (p. 259) occurs anywhere rainfall intensity exceeds the infiltration capacity of the soil. Further, once rainfall exceeds the infiltration capacity of the soil “runoff rises to a sharp peak at the end of the rainfall, followed by a rapid decline as soon as rainfall intensity decreases.” Of course, not all land surfaces have a direct slope to a stream, there are pockets or depressions that hold water and that water later infiltrates or evaporates. A rain garden is a man-made depression to hold water and allow it to infiltrate or evapo-transpirate. Since the mid 1980’s we have seen detention ponds which hold back storm water and release it based on the calculated development rate by the use of what are called weir structures. But, even detention ponds have problems. More scouring of streams can occur simply because there is more storm water than there would be under predevelopment conditions. In any rain even you can walk out on to your driveway or street and you will experience Horton overland flow. Why? Because the intensity of the rain exceeds the capacity of the land surface to infiltrate it. More impervious surfaces lead to more storm water. Detention ponds release a greater amount of water than what originally have been present, due to impervious surfaces, and hence you get more scouring and disruption of the natural stream system. More scouring leads to more soil laden water that will be deposited somewhere.

Rain Garden

One would think that semi-arid land would have a great amount of capacity for water, after all, it is not like the soil is saturated from many prior storms, since semi-arid is almost desert like. But, what is known is that Horton overland flow occurs most frequently on areas devoid of vegetation or possess only a thin cover. Cultivated fields also have a high rate of flow, even though the fields may be planted the area between the rows leave soil exposed and without proper cover it loses its better infiltration capacity. Lawns of course also see Horton overland flow, partly due to compaction. Of course, infiltration of rain is greater in humid regions due to its often better vegetative cover. A good vegetative land cover, such as forest cover, has the least amount of overland flow. A few of the reasons for this are: the detritus of the leaves and decaying branches provide a nice humus layer that assists in water take-up; the leaves break up the intensity of the rainfall; lesser compaction of the soil; and perhaps even more earth worm or other micro-fauna activity which produce macro-pores. Dunne and Leopold even go on to say that in these areas Horton overland flow may not occur.

Prairie Plant Root Depth compared to
Bluegrass

Don’t be fooled by the presence of what we planners call macro-pores. In 1998, in attempting to devise a more suitable storm water management system, the city for which I work engaged a consultant and actually tested varied soils, and the same soils but under different conditions. What we found was that macro-pores greatly assist in infiltration of rain and storm water. We found that earthworms and prairie plants provide great macro-pores. Prairie plants are able to do this as their root system is much deeper, some up to 14 or 15 feet, compared to a standard 3 or 4 inches for common lawn grasses. The long roots assist in providing openings in the soil. For example, purple coneflower is known as a clay buster due to its root structure, and its effect on that type of soil. The natural vegetation for the area of Wisconsin in which I am located was prairie and oak openings (oak savannah), but do not get the impression that there were was a high density of trees. It is thought that there was hundreds of feet between trees, leaving much area for prairie grasses. The soil present in pre-settlement conditions was not unlike the soil today, but for less topsoil due to erosion.

Rainfall Map for part of June 2008

What may be key is the intensity of the rainfall. The red of the radar means a more intense rainfall than a green or yellow. Intense rainfalls this past week in Chicago caused flooding, and of course in 2008 the Wisconsin Dells area saw two back-to-back 100 year storm events separated only by a few days which led to the Lake Delton Dam breach and the flooding of the interstate highway. Soil was inundated, runoff filled depressions, and the overflow. With intense rains, so much comes so quickly that the soil capacity is quickly utilized or the soil has little time to absorb a heavy rain.

Lake Delton, WI

To answer the question posed by my brother, the best rain is dependent upon a variety of factors, including what you hope to get from the rain. It would seem that a rain that can infiltrate and produce little in terms of overland flow would be the best. The infiltrating rain would then be available for transpiration by plants, evaporation by sun and wind, or recharge of the ground water. (Groundwater also feeds, lakes, streams and wetlands.) Therefore, most people would not consider a good rain event to be a radar reporting red or purple storm event. If one wants a great deal of overland flow to fill streams and lakes quickly, than radar red is a good rain. In the end, perhaps it is appropriate on this day in which Laudato Si was released that we borrow a line from St. Francis of Assisi and his Canticle of the Sun: "Be praised, my Lord, through Sister Water; she is very useful, and humble, and precious, and pure." We just need to realize that it is our job to tend and keep well our water and earth.

Images courtesy of Google