A Watershed Year: Anatomy of the Iowa Floods of 2008

8. The Hydrologic Footprint of Annual Crops

8 The Hydrologic Footprint of Annual Crops Michael Burkart Given the question of agriculture’s potential contribution to the 2008 floods, one might ask what would have happened had the same weather patterns occurred before the 1830s, when native vegetation still dominated the landscape. Would extreme flooding have occurred then, and if so, would these earlier floods have resembled those of today? Would their extent and magnitude have been as great? While Iowa’s lands certainly flooded in past centuries, floods predating Iowa’s agricultural transformation would have differed greatly in character. Iowa today is among the most intensively managed landscapes in the world. This is largely due to its legacy of rich soil, a climate ideal for growing corn, and a culture and economy eager to exploit these resources. These traits have steadily pushed for the replacement of perennial plant cover with annual row crops. The hydrologic footprint of row crop agriculture has been steadily and substantially expanded in parallel with agronomic and engineering changes in the landscape. These changes have systematically eliminated water storage on the land, accelerated the flow of water from the land, expanded the number of streams, and increased the velocity of water moving though streams. 78 why here, why now? With agriculture’s need to remove water as quickly and efficiently as possible, Iowa’s rivers now carry more water than before, creating the threat of extreme stream flooding that is magnified in size and extent. No changes have had a more dramatic effect on our land and its hydrology than those associated with agriculture. Before examining these changes, let’s look at Iowa’s hydrology before the 1830swhenEuropeanAmericansstartedtransformingtheterritorytoaworking landscape. Prior to that time, perennial plant communities, primarily tallgrass prairies, covered most of Iowa and governed the consumption and flow of water through the landscape. The prairie’s complex web of dense, diverse grasses and forbs (broad-leaved flowering plants) grew quickly from a dormant state when water and temperatures were suitable, maximizing water consumption throughout the year. Consequently, little water escaped the prairie. Infiltration of water below the prairie root zone was limited, and runoff was generated only during extreme rainfall events or rapid snowmelts. In much of Iowa, runoff from prairies during these periods filled lakes, potholes, and marshes that also supported highly water-consumptive perennial aquatic vegetation. Extreme flood flows were probably rare. Perennial streams are those that flow year-round and are differentiated from ephemeral streams that flow only during and following rainfall or snowmelt. Runoff is that part of the stream discharge that flows over the land surface and can be observed only during and immediately following rainfall or snowmelt. Perennial streams carry water during long periods between rainfall episodes because groundwater continues to flow into the streams, providing what is called baseflow. In landscapes with a large capacity to store water in surface features, little runoff or baseflow is available for stream discharge. Since the 1830s, Iowa’s native perennial vegetation has systematically been replaced with annual row crops. Prairies, forests, and wetlands have been removed or drained so that by 2002, 62 percent of Iowa’s land surface was intensively managed to grow corn and soybeans (U.S. Department of Commerce 2004). While the native perennial vegetation did not allow much water to escape the terrestrial ecosystem, the extensive annual crop system now covering Iowa has been engineered to facilitate movement of water from the landscape. Annual crops inherently contribute to greater stream discharge because they store and consume water during only three or four months of the year; thus much of Iowa’s precipitation now falls on land without living plants.1 A comparison of the average monthly precipitation in Iowa and the growing period of corn and soybeans shows that these crops intercept precipitation only the hydrologic footprint of annual crops 79 four or five months each year (figure 8-1). In such a landscape, both runoff and baseflow contributions to streams increase (Schilling and Libra 2003), because neither infiltration nor runoff is abated by water-consuming plants between mid-September and mid-May. A number of factors contribute to today’s resulting hydrologic problems. Little overland runoff is impeded by minimal residue from soybeans, and corn residue provides poor water retention when compared to perennial vegetation. Most of Iowa’s natural ponds, lakes, wetlands, floodplains, etc., have been drained to increase land for annual crop production. This eliminates the possibility of storing...