Ok, I promised to follow up coverage of the historic northern Indiana flood with a little science. We’ll start first with hydrology, then move to climate in part 2. Human economies and cultures intersect with both of these.
Six inches or more of rain came down from Feb 19-21. This is more than we might typically get over the entire months of January and February. This also occurred after a large snow melt off, so the soils were already completely saturated.
Where does the water go? Temperatures were cool, so there was almost no evaporation happening. A small amount of water percolated down into the water table, but this is typically a slow process and doesn’t happen everywhere. The rest had to go somewhere. A large amount of water flows horizontally through the soil and can re-emerge at ditches and streams, where the water table meets the surface. The rest of the water flows over land.
There is a lag time between the rain event and the peak crest of a large stream or river, as water takes some time to make its way to the central channel. Through land use changes, we have shortened up this lag time and our waterways are now “flashier” than they would be in a more vegetated state; that is, they reach peak quicker and at a higher altitude (flood).
What changes have we made? Esteemed Indiana botantist Paul Rothrock writes,
Since the early nineteenth century the Indiana landscape has undergone a massive transformation. In the pre-settlement period, Indiana was an almost unbroken blanket of forests, prairies, and wetlands. Much of the land was cleared, plowed, or drained for lumber, the raising of crops, and a range of urban and industrial activities.
In the 1904 Marshall County soil survey, the authors notes:
The greater part of the county was originally covered with a heavy growth of timber, consisting principally of walnut, oak, and poplar. This timber, except that little that was used for building material, was either burned or destroyed in any possibly way to clear the land. As the country because more thickly settled and transportation facilities improved, the lumber business became an important industry in the development of the county. The period from 1860 to 1870 was the most prosperous for this industry.
These changes meant less vegetated cover in the “headwaters” (upper reaches) of our watershed. Each branch, leaf, and stem can absorb moisture. It also caused a massive drop in the soil organic carbon (SOC) which was released into the atmosphere. SOC is the principal determinant of a soil’s water-holding capacity. Wetlands that used to hold large amounts of water on the surface for weeks after a storm released water immediately. Instead of being held upstream and slowly released downward and outward, water now flows straight through or over the soils.
A network of open ditches were dug when Europeans colonized the area. They are maintained on a regular basis by county taxpayers to keep wetlands in agricultural production and low-lying roads open. The county maintains rights to these easements through public property. This ditch on our property was just “cleaned” last month. However, there do not appear to be any provisions for erosion control, re-vegetation, or advanced engineering like two-staged ditches. Sediment inevitably is re-deposited and taxpayers will pay to have this excavated again in a few years.
Lastly, impervious surfaces – like roofs and parking lots – move water even faster than agricultural lands. There is almost no infiltration and water is quickly moved to waterways (along with spilled oil, grime, chemicals, fertilizers, and salt).
Below is a 2016 land cover map of Marshall County (complements of IndianaMAP). Yellow is corn, green is soybeans. 75% of the county is in farmland, just as it was 114 years ago. The light/teal-green color is forest, which is more prominent in the Western half (water from here does not flow through the City of Plymouth). Dark blue is open water, pink is grasslands and pastures.
Gray is urban/industrial. Plymouth is clearly visible in the center. Much of the gray area has impervious surfaces, where water is quickly sent to communities downstream. Water flowed off of these surfaces, into ditches, and eventually to the city of Knox, where large flooding damage also occurred. Ripple effects.
The image below is a hydrograph of the Yellow River during this flood event. I drew a hypothetical green line over top of it to illustrate what a reference (natural) watershed would look like. The river rises slower, peaking at a later date and at a lower elevation. Hypothetically, flood damage is reduced and water quantity and quality is improved. (Caveat: the actual scale of this change, or the peak crest, is completely unknown and not necessarily even useful, other than for illustration).
It can be tempting for a largely urban population to point fingers at the shrinking number of farmers raising our food. But we have all inherited the industrial/European model of living on the land, and of course most of our families can trace our routes back to farmers. The economic/agricultural system we created rewards drained soils. Indeed, they are some of the most productive in the country when it comes to producing annual grains, and investment in drainage usually pays back quite handsomely. As I drove to work after the flood, I saw still more drain tile being installed.
Private drainage does not account for the very real downstream costs of water pollution and flood damage. This is similar to us non-farmers commuting great distances to our workplaces. We are imposing very real and sizable air pollution costs on others (what economists call “externalized costs”) while we privately benefit from the transportation. Additionally, many of us suburbanites and urbanites have ourselves inherited farmland and rent it out to farmers growing annual grains on drained land. How to we humans behave? Very often in our own narrow and short-term economic self-interest.
Even with all the land modification, it has been remarkable to see how long some of the water has been ponded up. I drove west on US 30 towards Valparaiso yesterday (March 6), 13 days post-rainfall. There were still huge fields full of water, one was lined by hundreds (maybe thousands) of Sandhill Cranes probing the mud for kernels of grains or perhaps some invertebrates. This, of course, was the old Kankakee wetland, the everglades of the north.
The photo below was taken March 5, 12 days post-rainfall. You are looking at about 4 acres of water. Let’s presume the average depth is about one foot. That’d be 4 acre-feet of water, or 1.3 million gallons of water. We currently use it as pasture, letting the cows convert the solar-powered grass into beef.
With a big outlay of cash, I suppose it could be drained (It has been farmed in the past). If so, that 1 million+ gallons of water would not be sitting in this field, but would be in the basements and living rooms of the residents of Knox, IN, or anywhere else downstream. That is ultimately the choice we have to all make. We can’t stop floods, and we don’t get to choose when the rains come, but we do get to decide how we raise food and use the land.