Case study 3 – the Des Moines River watershed:

The Des Moines River watershed in the state of Iowa and the heart of the American Corn Belt has highly productive agricultural soils, a beneficial climate, and extensive tile drainage systems. Iowa State University, located in this watershed, has extensively monitored and studied the impacts of the conversion and use of these soils in intensive agriculture. These studies provide a more complete understanding of how vegetative cover, infrastructure, hydrological function, nutrient cycling, and water quality interrelate. They help make clear how these changes to the natural landscape affect both water flow and impact water quality.

Iowa is a leader in corn and soybean production where farmers use a substantial amount of fertilizer to achieve high yields. It also has a highly drained landscape.

In fact, Zulauf and Brown (2019) noted that Iowa is the state with the highest percentage of cropland with tile drainage (53%) (see also, National Agricultural

Statistics Service, 2019a). The draining of wetlands, the associated tile draining systems, and the development on the floodplain have each played a role in increased flooding in the city of Des Moines, with major flooding occurring in 1993. The combination of a highly cropped, fertilized, and drained landscape has resulted in impaired water quality, with drinking water for the major city in the watershed, Des Moines, regularly exceeding the drinking water standard for nitrates (Des Moines Water Works, 2020). Des Moines Water Works had to install an expensive water purification system that cost several million dollars in order to meet these standards (Des Moines Water Works, 2020). In 2015 the equipment was in operation 177 days costing $1.5 million (Des Moines Water Works, 2016).

Calls to reduce the loadings of nitrates into the Des Moines and Raccoon Rivers initially focused on decreasing nitrogen fertilizer application rates within the watershed. Though seemingly the feasible approach, simply focusing on fertilizer use failed to account for all sources of nitrates. An examination of nitrogen fertilizer use within the Raccoon River watershed between 1989 and 2003 showed little change in the amount applied. Further the increased fertilizer uptake by crops indicated increased nitrogen use efficiency raised doubts that increased fertilizer applications were the source of the additional nitrate (Hatfield et al., 1999). Manure applications were considered by some as the source of the nitrate, but an examination of cattle and hog production over the period in question revealed the number of animals had declined by 63% and 20%, indicating the nitrogen available for application had decreased by 25% (Hatfield et al., 1999).

So why was the nitrate concentration in the river continuing to rise while the total nitrogen application decreased? Pause for a moment and think about the scientific principles and concepts that we have been examining. What do we know and what has not been examined?

We know that mass is conserved, neither disappearing nor being created. And we know water flows downhill from the uplands to the rivers where it infiltrates soils downward until it reaches the water table. Finally, remember different plants have different transpiration rates. In an examination of how agricultural practices affects carbon, nitrogen, and phosphorus cycling, Hatfieldet al. (2009) found that crop rotations (crops sequentially grown on the same plot of land) had been steadily changing over time. As the amount of land in small grains (e.g., wheat and oats) and grasses (e.g. hay) declined, nitrate concentrations in water systems increased. This relationship between the crop rotations and nitrate concentrations can be understood by examining the timing of fertilizer applications, precipitation patterns, and comparing water use patterns and movement associated with small grains and hay with those of the corn and soybeans that replaced them.

In Iowa, April, May, and June have the three of the highest average monthly precipitation rates (rssWeather.com,2020) and are the months when spring fertilizer is typically applied. Now consider water use by these crops. Hay and small grains are actively transpiring between April and mid-June reducing the Guide to Understanding the Principles of Environmental Management 134

water movement to base flow. During the same period corn and soybeans have limited water use, leaving more water to move downward through the soil profile contributing base flow. Replacing small grains and hay with corn and soybeans, allows nitrates from recently applied fertilizer to be flushed out of the soil with the water and transported with the base flow to the river. The timing of fertilizer applications, and precipitation and temperature patterns contribute more to explaining nitrate concentrations in the river than changes in fertilizer use (see also Jayasingheet al., 2012).

7.2.4.1 Take home message

The Des Moines Watershed containing the Raccoon River Basin demonstrates the interconnectedness of the landscape, vegetative cover, water movement, and nutrient cycling, in other words how changing carbon, nitrogen, and phosphorus balances can produce undesired results. The substantial modifications of the Iowa landscape–the loss of wetlands and their water storage function, the conversion of native vegetation and wetlands to cropland, fertilizer and manure applications, and changing crop rotations – have altered the ecosystem services provided.

Water pathways through the landscape (and transpired into the atmosphere) have been altered. The reduced ability of the landscape to hold water has intensified the watersheds’response to major precipitation events resulting in greater flood damage. The alterations of water movement combined with nitrogen applications on cropland have resulted in elevated nitrate concentrations in the rivers. The changing crop rotations altered plant water uptake during the period when fertilizers were applied, leaving more water to percolate through the soil and transport rN to the base flow which then carried it into the waterway. This case study shows that the interrelationships between changing land use, water movement, crop production, nutrient availability, and water quality are complex and need to be carefully examined before assigning relationships between cause and effect.

Although all of the above variables discussed play a role in the increased nitrate concentrations, this case study was presented to stress the use of a tool as well as our core principles. Many in the Des Moines River watershed jumped to what was a feasible but incorrect conclusion that increased fertilizer or manure applications were driving the increasing nitrate concentrations in the Raccoon River. By observing and measuring the amount of nitrogen applied to crops over time, this hypothesis was rejected. When an analysis taking into account water availability and movement was completed, the role of changing crop rotations in increasing the water available to transport nitrates through the soil column was documented.

The lesson here is not only do we need to apply scientific principles but we also need to test our hypotheses by observing and measuring outcomes. If we reject our hypothesis, we need to reexamine the system on the basis of fundamental science.

7.2.5 Case study 4–eutrophication in western