Modeling the effects of riparian buffer zone and contour strips on stream water quality

Abstract

The use of contour and riparian buffer strips planted with perennial vegetation has been found to improve surface water quality by reducing NO₃-N and sediment outflow from cropland to a river. Modeling such a system to compare alternative layout and different strip sizes often faces challenges in flow routing scheme. The hillslope scheme in the Soil and Water Assessment Tool (SWAT) offers the flexibility of allowing the flow from a crop area to be routed through a buffer and/or contour strip, in which a thin sheet flow represents more closely the natural condition of a watershed. In this study, the SWAT model was applied to the Walnut Creek watershed and the hillslope option was used to examine the effectiveness of contour and riparian buffer strips in reducing NO₃-N outflows from crop fields to the river. Numerical experiments were conducted to identify potential subbasins in the watershed that have high water quality impact, and to examine the effects of strip size and location on NO₃-N reduction in the subbasins under various meteorological conditions (dry, average and wet). Variable sizes of contour and riparian buffer strips (10%, 20%, 30% and 50%, respectively, of a subbasin area) planted with perennial switchgrass were used to simulate the effects of strip size on stream water quality. Simulation results showed that a filter strip having 10–50% of the subbasin area could lead to 55–90% NO₃-N reduction in the subbasin during an average rainfall year. Strips occupying 10–20% of the subbasin area were found to be more efficient in reducing NO₃-N when placed along the contour than that when placed along the river. Varying the area and location of the contour and buffer strip affects NO₃-N outflow and crop yields as well since it takes the land out of production. The size of the filter strip has economic implications in deciding how much land area to dedicate to prevent NO₃-N loss to a desired limit or vice versa. The results of this study can assist in cost-benefit analysis and decision-making in best management practices for environmental protection.

Introduction

Nutrient, sediment and pesticide outflows from agricultural watersheds are often attributed as non-point source pollutants to streams and natural waterways, resulting in depleted dissolved oxygen, and higher level of NO₃-N and pesticide than the permitted standard (Humenik et al., 1987, Burgoa and Wauchope, 1995). Water quality of rivers and streams in Iowa and the Midwest, where the landscape is dominated by agriculture, is experiencing a higher level of NO₃-N causing hypoxic conditions in rivers that flow into the Gulf of Mexico threatening the marine ecosystems (US EPA, 1992, Rabalais and Turner, 2006, Mitsch et al., 2001). Keeney and DeLuca (1993) found that NO₃-N concentrations in Des Moines River water in central Iowa were above 10 mg L⁻¹ for an average of 14 days per year, generally in spring. Libra (1998) has reported an average annual export of NO₃-N from Iowa in surface water ranging approximately from 225,000 to 245,000 tons, which is about 25% of the NO₃-N that the Mississippi River delivers to the Gulf of Mexico, despite Iowa occupying less than 5% of its drainage area.

Nitrogen fertilizers, livestock manure application, nitrogen fixation by legumes and mineralization of soil nitrogen are the primary sources of NO₃-N in agricultural watersheds. Part of the NO₃-N are utilized by crops and other plants and excess of it become available to be carried by the surface and groundwater flow into the river and other water bodies as pollutants. Ecologically engineered solutions and Best Management Practices (BMPs) that comprise crop rotation, no till cultivation, application of filter strips along a river and along the contour in a crop field, field border and wetlands are often employed to reduce and or capture the nutrients and sediments from getting into the stream. Performance of such ecologically engineered systems has been studied by Mitsch and Mander (1997), Hernandez and Mitsch (2007), Meier et al. (2005), Lin et al. (2004) and Anbumozhi et al. (2005). It is important to numerically simulate the effects on NO₃-N outflow due to alternative land-use/management scenarios containing the ecological solutions. A wide range of numerical models have been developed to study non-point source pollution, however most of these models are designed to assess the pollutant outflow at a field scale. The Soil and Water Assessment Tool (SWAT) (Arnold et al., 1995) is a more comprehensive watershed scale model that can simulate the hydrological processes along with nutrient, sediment and pesticides in a watershed and river network. It can work on small to large scale watershed with key features such as continuous time simulation over longer periods, high level of spatial details, various levels of watershed subdivisions, and efficient computation and capability to directly simulate the likely water quality at the outlet of a watershed due to existing or changed land-use scenarios.

Vache et al. (2002) have applied SWAT to the Walnut Creek watershed, Ames, Iowa, and have reported that a significant reduction (54–75%) in NO₃-N occurred when the BMP's were employed in conjunction with wider riparian buffer strip. Chaplot et al. (2004) has applied SWAT to model the effect of reduced application in agriculture and found that lessening the nitrogen (N) application rate by 20%, 40% and 50% decreased the mean NO₃-N loads by 22%, 50% and 95%, respectively. Field experiments by Dillaha et al. (1989) showed that a filter strip with a width of 9.1 and 4.6 m removed an average of 84% and 70% of suspended solids, 79% and 61% of phosphorus (P), and 73% and 54% of N, respectively. They also found that occasional release of the nutrient from the VFS were even higher than the incoming one underscoring the fact that removal efficiency can be low due to nutrient saturation in the filter strip. Their observations indicated that on-farm VFS may not perform as good as experimental one or as the one simulated by numerical models. In a field experiment on small plots, Lee et al. (1999) found that 6- and 3-m filter strips removed 42% and 25% of NO₃-N, respectively. Syversen (2005) studied the effect of buffer strip in a field experiment under Nordic climate and found that 10 and 5 m wide buffer zones reduced the phosphorus, nitrogen and sediment by 60–89%, 37–81% and 81–91%, respectively. Santhi and Srinivasan (2002) used SWAT model to simulate filter strips using trap efficiency for sediments and nutrients based on strip's width. The selection of the coefficient that would replicate the trapping efficiency of a buffer strip is critical to the results of previous studies and could be under- or over-predicted depending on the local conditions (Barlund et al., 2007). However, the trapping efficiency may be different depending upon the type of vegetation and watershed parameters such as slope and soil type. Hence the challenge in application of SWAT model remains when applied to simulate the riparian buffer and contour strips.

The objective of this study is to investigate the effectiveness of contour and riparian buffer strips having perennial plant cover in reducing nutrient (NO₃-N) loading to streams in an agricultural watershed. SWAT2003 and its hillslope scheme are applied for this purpose in which the type of vegetation is specified to avoid the potential problem of selecting incorrect trapping efficiency. Hillslope scheme of SWAT allows the routing of overland flow from one unit through another. When the flow from a crop area carrying nutrients passes through the filter strip, perennial plant will be able to use up some of the nutrients and net outflow of nutrients will be reduced. When the area of the filter strip is large and velocity of surface flow through it is low, the perennial plants in the filter strip will be able to use up more nutrients. This in turn takes the land out of production and will have economical impact. Compensation for farmers for not growing crop is a direct cost of environmental protection and a tradeoff needs to be examined. It can be based on the relative efficacy of increasing the area under filter strip and its effect on NO₃-N outflow reduction. In this study, SWAT simulations of the Walnut Creek watershed were conducted to identify high impact subbasins based on total and per unit area NO₃-N yield, to compare the response of the two types of high impact subbasins to selected management practices, and to evaluate the reduction of NO₃-N load due to varying the area of filter strip. Numerical experiments on different scenarios were carried out to examine the effectiveness of filter strips on water quality improvement under various weather conditions and to determine more effective location for the placement of filter strips, i.e. contour strip or riparian buffer strip.