Native prairie filter strips reduce runoff from hillslopes under annual row-crop systems in Iowa, USA
Highlights
► Runoff was reduced by 37% by the prairie strips compared to 100% row crop. ► 10% prairie strategically incorporated into watersheds enough to reduce runoff. ► Buffer effectiveness increased along the years. ► Greater runoff reductions in spring–fall by prairie strips compared to 100% crops.
Introduction
The conversion of native vegetation to agricultural production systems to yield diverse goods and services represents one of the most substantial human alterations of the Earth system. The impact of this conversion is well recognized within the scientific community and it interacts strongly with most other components of global environmental change (Ramankutty and Foley, 1999, Vitousek et al., 1997). Agriculture affects ecosystems through the use and release of limited resources that influence ecosystem function (e.g. nitrogen, phosphorus, and water), release of pesticides, and biodiversity loss (Tilman et al., 2001), all of which can alter the availability of diverse ecosystem services (MEA, 2005). In particular, agriculture has been one of the major drivers of increasing water scarcity, declining water quality, and loss of flood regulation capacity worldwide (Houet et al., 2010). Agricultural production, and its related hydrological changes, have greatly increased during the 20th century and are expected to continue in the 21st century (Gordon et al., 2008). These impacts of agriculture on diverse hydrologic services represent a major threat to the well-being of human populations in many regions across the globe (MEA, 2005).
The Corn Belt of the Midwestern US has experienced one of the most dramatic and complete landscape scale conversions from native perennial ecosystems to monoculture annual cropping systems. In this region, approximately 70% of the pre-European settlement prairies, savannas, riparian forests, and wetlands have been converted to annual crops (NASS, 2004), and the region now produces approximately 40% of the world’s total annual corn yield (USDA, 2005). However, the environmental consequences of these changes are increasingly becoming apparent, including documented increases in baseflow (Schilling and Libra, 2003, Zhang and Schilling, 2006), contamination of water supplies (Jaynes et al., 1999, Goolsby and Battaglin, 2001), diminished flood control (Knox, 1999), all of which have far-reaching social and economic consequences (Alexander et al., 2009, Schilling et al., 2008, Rabalais et al., 2010).
In contrast to annual cropping systems, perennial vegetation can have positive impacts on hydrologic regulation (defined as the combined effect of increased evapotranspiration, infiltration and interception of runoff). Perennial vegetation has greater rainfall interception (Bosch and Hewlett, 1982, Brye et al., 2000), greater water use (Brye et al., 2000, Livesley et al., 2004, Anderson et al., 2009), deeper and more extensive rooting system (Jackson et al., 1996, Asbjornsen et al., 2007, Asbjornsen et al., 2008), extended phenology (Asbjornsen et al., 2008), and greater diversity in species and functional groups, conferring advantages for productivity and resilience (Tilman et al., 2001). Moreover, perennial vegetation can improve soil structure and hydraulic properties by increasing the number and size of macropores (Yunusa et al., 2002, Seobi et al., 2005) and building organic matter (Liebig et al., 2005, Tufekcioglu et al., 2003), which combined contribute to increasing soil water infiltration and hydraulic conductivity (Anderson et al., 2009, Udawatta et al., 2006, Udawatta et al., 2008).
Reversing the process of agricultural expansion and intensification by restoring native prairie vegetation is not realistic given the goal to meet important societal needs for global food, fuel, and fiber (Tilman et al., 2001). Moreover, technology, knowledge and policy frameworks for effectively managing large-scale highly diverse perennial-based production systems are not yet available (Glover et al., 2007). A promising alternative approach involves the incorporation of relatively small amounts of perennial cover in strategic locations within agricultural landscapes (Asbjornsen et al. in review). Over the past decade, policies have targeted such conservation practices by, for example, promoting the establishment of riparian buffer systems, and grass waterways (Feng et al., 2004). However, achieving the most appropriate balance for maximizing hydrologic functions proportional to the amount of land removed from production will require a better understanding on the influence of spatial extent, position, and type of perennial vegetation within a watershed (Dosskey et al., 2002, Blanco-Canqui et al., 2006), about which little empirical field data exist.
Presently, the most reliable field-based information available on effects of perennial cover on agricultural watershed hydrology comes from research on riparian and grass buffer systems with various studies reviewing their effects (Castelle et al., 1994, Liu et al., 2008, Zhang et al., 2010). While the buffer literature is extensive, little research has been done assessing perennial vegetation higher up in the landscape. A few field and plot level studies (Udawatta et al., 2002, Blanco-Canqui et al., 2006, Jiang et al., 2007) as well as modeling efforts (Geza et al., 2009) have begun to address the strategic placement of perennial vegetation, but most works are plot studies with controlled flow paths. Thus, there is a need to better understand the in-field performance of vegetative filters where flow is not controlled in some manner (Baker et al., 2006). The effectiveness of vegetative filters will vary significantly, depending upon the area of the filter that overland flow will encounter and the flow conditions in a filter, e.g. concentration of flow (Helmers et al., 2008).
Research is needed to determine how the amount and placement of perennial vegetation within agricultural watersheds can affect hydrological regulation. This would help determine the proper design of conservation practices that strategically places perennial vegetation in the landscape. In this study we incorporated perennial vegetation filter strips that varied by the area and location in the uplands of 12 zero-order watersheds that typically only flowed following snowmelt or following sizable rain events (ephemeral systems). The objective of our study was to assess the effects of strategic placement of native prairie vegetation (NPV) that varied by the landscape position and % of overall watershed cover on: (1) total runoff export from the experimental watersheds, and (2) the effects of annual and seasonal variation in rainfall on watershed response. Additionally, we sought to (3) determine the optimal size and location of native prairie vegetation for achieving maximum hydrologic benefits. Our central hypothesis was that strategic incorporation of small amounts of NPV into annual cropping systems would result in runoff reduction due to the greater hydrological regulation using NPV compared to annual crops. We further expected that differences between treatments would be greater during periods when annual crops were less active (e.g., early spring, late summer) and for smaller rainfall events, where the regulation capacity of NPV strips compared to the annual crops would likely be maximized.
Section snippets
Site description
The study was conducted at the Neal Smith National Wildlife Refuge (NSNWR, 41°33′N, 93°16′W), a 3000 ha area managed by the U.S. National Fish and Wildlife Service, located in the Walnut Creek watershed in Jasper County, Iowa (Fig. 1). The NSNWR comprises part of the southern Iowa drift plain (Major Land Resource Area 108C) (USDA Natural Resources Conservation Service, 2006), which consists of steep rolling hills of Wisconsin-age loess on pre-Illinoian till (Prior, 1991). The landscape is well
Rainfall
A total of 149 rainfall events were analyzed during the study period, where a rainfall event was defined as rainfall that occurs after a rainless interval of at least 12 h duration. According to our experience this inter-event time is a good compromise between the independence of widely-spaced events and their increasingly variable intra-event characteristics (Dunkerley, 2008). Surface runoff occurred in at least one watershed for 129 of the rainfall events.
Precipitation in the NSNWR was highly
Discussion
In this work, we demonstrated through the use of different watershed response measurements (runoff rates and volume) and other variables (runoff peak, runoff coefficient, time to first peak and time to onset of runoff), that the conversion of small areas of cropland to native prairie can produce significant ecosystem service benefits in terms of hydrologic regulation. Restitution of runoff dynamics in agricultural watersheds towards conditions present under native prairie vegetation can have
Conclusions
Our results indicate that small amounts of NPV (<20% NPV) strategically incorporated into corn–soybean watersheds in the Midwest found in dissected glacial (pre-Wisconsinan) terrain, can be used to effectively reduce runoff. The differences among the watersheds were attributed mainly to NPV amount, position, and establishment time. The differences in runoff reductions were greater in spring and fall (crops dormant season) due to the different perennial and annual phenology. Soil water
Acknowledgements
Funding for this project was provided by the Leopold Center for Sustainable Agriculture, Iowa State University College of Agriculture and Life Sciences, USDA Forest Service Northern Research Station, Iowa Department of Agriculture and Land Stewardship, USDA North Central Region SARE program, and USDA-AFRI Managed Ecosystems program. We would like to thank Pauline Drobney and the staff at the Neal Smith National Wildlife Refuge for their support of this project. We would also like to thank the
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