FOOLING OURSELVES – Page 2
By focusing on individual storms, the project produces much more detailed and accurate estimates of soil erosion than the statewide average annual estimates produced by the National Resources Inventory.
The picture that emerges is alarming.
In 2007, the project estimated that storms resulted in rates of soil erosion in some townships ranging up to more than 64 tons per acre per year. That figure is 12 times greater than the statewide average annual erosion rate of 5.2 tons per acre per year estimated by the Inventory. The project estimates that agricultural fields in 440 townships encompassing 10.1 million acres may have suffered erosion at rates greater than the NRI statewide average and that eight townships encompassing 184,000 acres experienced utterly disastrous average erosion rates exceeding 50 tons per acre (Figure 1).
Figure 1: Millions of acres in Iowa eroded at more than 5 tons per acre — the so-called “sustainable” rate — in 2007.
Source: Iowa Daily Erosion Project
The Iowa Daily Erosion Project is a collaboration of scientists at Iowa State University, USDA’s National Soil Erosion Research Lab, USDA ‘s National Laboratory for Agriculture and the Environment and the University of Iowa (http://wepp.mesonet.agron.iastate.edu/). The project is designed to produce daily estimates for rainfall, runoff and soil erosion for the State of Iowa.
The project was developed because important factors that determine rates of erosion and runoff from agricultural fields, such as soil type, slope steepness and length, crops planted and conservation practices vary greatly across the landscape. Moreover, localized heavy rainstorms commonly occur in Iowa and in other Corn Belt states.
As a result, localized soil erosion losses and runoff volumes can be extreme. Most estimates of soil erosion and runoff, however, are based on rainfall amounts that are averaged over many years. Such long-term averages miss the effect of the highly variable and extreme rainfall events that cause the most damage from erosion and runoff in agricultural watersheds.
The Iowa Daily Erosion Project uses the Water Erosion Prediction Project (WEPP) model for estimating soil erosion and runoff from agricultural fields, including cropland planted to row crops or hay and pasture. Key information regarding topography, soils, crop rotations and management practices are taken from USDA’s Natural Resources Conservation Service, 1997 National Resources Inventory (NRI). That information is coupled with rainfall amounts and other weather data provided by NEXRAD precipitation radar and the Iowa Environmental Mesonet (http://mesonet.agron.iastate.edu/).
The project uses information from 17,848 NRI sample sites in Iowa that include agricultural land. USDA provides information about NRI sample points within each 36 square mile township, but does not reveal the precise locations of NRI sample points within those townships. The rainfall data is provided in 15-minute intervals every day in the Hydrologic Rainfall Analysis Project (HRAP) projection grid. Each grid cell is about 2.5 miles by 2.5 miles in area.
Because the 6 by 6 mile township grid does not line up precisely with the 2.5 by 2.5 mile HRAP grid, the project uses a statistical procedure to estimate the erosion and runoff that likely occurred during a storm. Each HRAP grid cell is assigned to the township that contains its center point. Then all of the possible combinations of rainfall amounts and information from NRI points in a township are modeled in WEPP. The result is a large distribution of possible rates of erosion and runoff volumes for each township each day. The project reports that the average of all those possible erosion and runoff amounts as well as the maximum and minimum erosion and runoff prediction for each township each day.
As is the case with all the models currently used in conservation planning or national assessments, the model results are estimates of the erosion and runoff that likely occurred based on the combination of rainfall intensity and land characteristics in a particular township. The minimum estimate of erosion and runoff is best thought of as the best-case scenario — the erosion and runoff resulting from the least amount of rainfall falling on the least vulnerable or best protected agricultural land in the township. The maximum erosion and runoff estimate is the worst-case scenario resulting from the most intensive rainfall falling on the most vulnerable or least protected land in the township. Both minimum and maximum erosion and runoff events likely occurred somewhere in the township during the same storm. The reported averages give an indication of the general risk of erosion and runoff across all agricultural land in a township during a particular storm.
The biggest problem that confronts the Iowa Daily Erosion Project is the lack of current, comprehensive and site-specific information about the presence or absence of conservation practices on the Iowa landscape. The lack of such information hampers all efforts to get an accurate and up-to-date picture of the health of Iowa’s soil, waterways, and watersheds.
Surveys of conservation tillage completed by the Conservation Technology Information Center (CTIC), however, suggest that there has been relatively little increase in the percent of crop fields on which farmers practice conservation tillage.5 The CTIC survey found that conservation tillage was used on 37 percent of crop acres in 1998. That percentage had grown to 42 percent in 2008. This is a welcome but not dramatic change from the situation in 1997 – the year used by IDEP to make its estimates of soil erosion following storms in Iowa.
Source: R. Cruse, D. Flanagan, J. Frankenberger, B. Gelder, D. Herzmann, D. James, W. Krakewski, M Kraszewski, J. Laflen, J. Opsomer, and D. Todey. 2006. Daily estimates of rainfall, water runoff, and soil erosion in Iowa, Journal of Soil and Water Conservation 61(4):191-199.
Statewide erosion averages — by necessity — provide a very poor picture of what is actually happening across large areas of Iowa or any other state or region. The statewide average erosion for agricultural land in 2007, according to IDEP data, was only 4.7 tons per acre — less than the amount reported by the 2007 Inventory.
The statewide average was low despite the likelihood that agricultural land in 440 townships encompassing more than 10 million acres eroded at more than the “sustainable” rate, and land in 220 townships encompassing almost 6 million acres likely eroded at twice the “sustainable” rate (Figure 2).
Figure 2: Millions of Iowa acres eroded faster than the “sustainable” rate in 2007.
Iowa and its neighboring states experience multiple erosive rainfall events each year. Adding up the erosion that likely occurred in each storm reveals that erosion frequently exceeds the “sustainable” level — often by several times — in many townships.
Figure 3: Townships (36 square miles) where average erosion exceeded the “sustainable” rate
|Year||> 5 Tons||> 10 Tons||>20 Tons||> 25 Tons||> 50 Tons|
Source: Iowa Daily Erosion Project
As Figure 3 shows, average soil erosion exceeded the “sustainable” rate in some townships every year except 2006. Agricultural land in six townships (encompassing about 138,000 acres) risked absolutely disastrous average erosion rates exceeding 100 tons per acre.
From early spring to the beginning of July, intense rainstorms, many of which are localized, cause extensive rill and gully erosion on Iowa farm fields. Such large erosion events are random. They don’t happen every year, but when they do, they are significant. Even when experts declare average soil loss to be less than the soil loss tolerance or “T” value, some places are losing significant amounts of soil.
Only soils with dense vegetative cover are completely protected from soil erosion, and corn-bean ground is virtually bare from November through the end of June, when the crop canopy begins to close up. Even with no-till, terraces, grassed waterways, and all the other best management practices (BMPs), we can’t escape this fact.
High-intensity rainstorms are predicted to increase in frequency because of climate change. Iowa must thus confront the potential for even more soil erosion if it is to attempt to reduce flooding and begin to improve water quality in the state’s lakes and streams.
Laura L. Jackson, Professor of Biology
University of Northern Iowa, Cedar Falls