Using ArcGIS hydrologic modeling and LiDAR digital elevation data to evaluate surface runoff interception performance of riparian vegetative filter strip buffers in central Iowa

Introduction

Water is necessary for the survival of most living things and is important for domestic, industrial, and agricultural purposes by mankind. Today, there is enormous concern about the quantity and quality of fresh water because of its scarcity due to overuse and pollution. The issue of water quality is currently of greatest concern for the world as polluted water is causing alarming death rates for aquatic organisms, human health hazards, and the aesthetic qualities of many water bodies. Water pollution throughout the world is affecting food chains and food webs and is a growing problem in our environments. Due to the increasing hazardous consequences related to water quality, awareness to conserve water resources is spreading globally. With respect to growing public concern and awareness to reduce water pollution, the US Congress enacted the 1972 Clean Water Act (CWA), with a motive to protect and enhance the surface water quality in the United States. As a requirement of CWA 303 (d), the US Environmental Protection Agency (USEPA) has identified more than 40,000 water bodies nationally that exceed the maximum pollutant limits of CWA water quality standards (USEPA 2013).

The two primary types of pollution that enter the water environment are point and nonpoint source (NPS). Agricultural production and NPS (diffuse) pollution are very closely related. In farming areas, NPS pollution includes pesticides, fertilizers, animal manure, and soil washed into streams during rainfall-runoff events. All of these various pollutants can degrade the surrounding environment. However, one way of controlling the loss of agrochemicals and soil sediments into receiving water bodies from farmland can be accomplished by planting tall, close-growing stiff grasses or other perennial vegetation in a linear area known as a vegetative filter strip (VFS) buffer. These VFS buffers are bands of planted or indigenous vegetation situated downslope of cropland or animal production facilities to prevent erosion and filter nutrients, sediments, and other pollutants from agricultural runoff before it can reach the nearby water sources (Dillaha et al. 1989). According to the USDA Natural Resources Conservation Service (NRCS), VFS buffers are vegetated land areas of either planted or indigenous vegetation used for minimizing the amount of sediments and contaminants entering a nearby water body carried by runoff from agricultural land or animal production facilities. These best management practices (BMPs) are considered to be an effective measure in reducing the sediment delivery from overland flow by retarding the runoff velocity and filtering sediment (Van Dijk et al. 1996).

Many efforts have been made to minimize NPS pollution from cropland and to reduce off-site impacts by reducing erosion and surface runoff within fields. When flowing across the VFS, surface runoff undergoes changes in composition and volume, entering the watercourse relatively cleaner than when it left the field (Abu-Zreig et al. 2004). The VFS buffer acts as a barrier to the movement of the suspended particles and decreases the velocity of flow in the runoff, which in turn promotes settling of the suspended particles. The sediment of sizes typically greater than 40 μm can be captured easily. However, the remaining small size primary particles and aggregates are difficult to remove by filtering because there is still the presence of some relatively low turbulent energy in water sufficient to keep these sediments in suspension (Gharabaghi et al. 2001). Dosskey et al. (2002) concluded that the efficiency of VFS decreases with increasing flow concentration. This BMP also requires timely maintenance to maintain its effectiveness over time. Several studies have been conducted to assess the effectiveness of VFS buffers in reducing sediments and nutrients from runoff. The effectiveness of a VFS buffer depends on the length, types of vegetation, age, level of development, and most importantly, flow interception capacity of the VFS buffer. The quantification of a surface flow interception coefficient for a VFS buffer will help to quantify the amount of sediments and chemicals removed from runoff.

The effectiveness of a VFS is determined by several factors such as the VFS length, slope, and vegetation species, as well as the sediment size distribution and chemical concentration in the runoff. The length of the VFS is considered to be an important factor affecting sediment removal efficiency. Several studies have shown that increasing the flow length beyond 10 m has very little effect in increasing the efficiency of a VFS (Gharabaghi et al. 2001; Lee et al. 2003; Abu-Zreig et al. 2004). A study conducted by Ree (1949) on grass filters of lengths 1, 4 to 5, and 10 m showed a filtering efficiency of 50% to 60%, 60% to 90%, and 90% to 99%, respectively. Gharabaghi et al. (2001) studied the sediment removal efficiency of a VFS on varying lengths of 2.44 m to 19.52 m for a 1.22 m wide field with a slope of 5.1% to 7.2% and concluded that the first 5 m were significant in removing suspended solids and aggregates greater than 40 μm in runoff. The experiment conducted by Abu-Zreig et al. (2004) in 20 fields with filter lengths of 2, 5, 10, and 15 m and slopes of 2.3% to 5% concluded that there is no significant increase in sediment removal efficiency with greater than a 10 m VFS length. The area ratio (AR) of the cropland drainage area to the VFS area is one of the important factors that affects the efficiency of VFS. Greater AR allows a larger volume of flow through smaller sections of VFS, thus lowering the efficiency of VFS in filtering the pollutants and sediments from runoff. Past studies by Arora et al. (2003) and Leeds et al. (1993) suggest that AR between 1:1 and 8:1 can achieve excellent sediment retention. According to Leeds et al. (1993), AR should be maintained less than 50:1 for good sediment retention. However, Arora et al. (2003) found that while higher AR values tended to relate to lower sediment removal efficiency, there wasn't a significant difference in VFS performance.

Several studies have suggested that infiltration is the primary mechanism responsible for trapping the suspended solids and applied chemicals (Ree 1949; Meyer et al. 1995; Gharabaghi et al. 2001). The submergence of vegetation also can result in a decrease in Manning's coefficient (n), which in turn significantly decreases the efficiency of a VFS (Ree 1949; Van Dijk et al. 1996). The flow retardation and infiltration were more efficient with older grass species (Van Dijk et al. 1996) since this denser vegetation provided more resistance to flow velocity, resulting in an increased contact duration between runoff and vegetation. Consequently, this led to less erosive power and transport capacity of the runoff, resulting in an increased VFS sediment trapping efficiency.

Sediment size distribution is also an important factor that determines the efficiency of a VFS. Studies have concluded that smaller-sized sediments require a longer settling time, therefore requiring a longer vegetative filter length (Meyer et al. 1995; Gharabaghi et al. 2001). In a study conducted by Abu-Zreig (2001), trapping efficiencies of 0% and 47% were observed over filter lengths of 1 m and 15 m, respectively, for clay particles. Lee et al. (2003) conducted an experiment to study the effectiveness of a multispecies riparian vegetative buffer in removing NPS pollutants from cropland runoff. The experiment involved installing three plots where each of the cropland source areas was matched with a no buffer (control), a 7.1 m switchgrass (Panicum virgatum) buffer, and a 16.3 m switchgrass/woody plant buffer. Sediment removal efficiencies of 95% and 97% were observed for switchgrass and switchgrass/woody plant buffers, respectively. The increased sediment removal efficiency of the switchgrass/woody plant buffer was determined to be the additional vegetative buffer length that increased infiltration. The ratio of sediment transported through the “control” plot to sediment transported through the switchgrass buffer was 13:1. Particle size distribution in surface runoff changed as runoff passed through the VFS buffer. In this case, large particles were deposited before small particles, and more than 90% of the sediment in surface runoff from the buffered plots was in the <0.05 mm size fraction. During the infiltration of nutrients, suspended fine particles with adsorbed chemicals also entered the soil profile, thus decreasing the surface runoff and sediment transport capacity.

The performance of VFS buffers in removing pollutants from runoff also largely depends upon the type of flow. Factors like concentrated flow or a nonuniform distribution of flow limit the performance of a VFS. Generally, a uniform flow distribution (sheet flow) helps to achieve higher pollutant removal efficiencies. Undulating surfaces and slopes >6% cause concentrated flow, erosion, and decreased sediment removal efficiency of the VFS buffer. When the flow is concentrated, the velocity of runoff becomes too high to be effectively treated by a VFS. Dosskey et al. (2002) found that the concentration of surface runoff from agricultural fields can significantly restrict the efficacy of riparian buffers to remove pollutants. Riparian buffer evaluation plots on four farms were used to study the influence of surface runoff on sediment trapping efficiency. A numerical model using a regression equation based on the proportion of buffer area to contributing field runoff area (buffer area ratio) was used for evaluating the sediment trapping efficiency. The model yielded sediment trapping efficiencies of 99%, 67%, 59%, and 41% for uniform flow conditions and 43%, 15%, 23%, and 34% for nonuniform flow conditions for the four fields, respectively.

Site conditions play a key role in the performance of a VFS buffer and should be considered when assessing a VFS. Gilliam et al. (1993) observed that pesticides are less adsorbed in the shallow vadose zone. The direction and rate of chemical movement is greatly dependent on whether the underlying layer is permeable or impermeable. If permeable, chemicals can flow in a vertical direction and leaching is more pervasive. However, if the layer is impermeable, this would contribute to the lateral flow of shallow ground water and hence will result in polluting the surface water.

The primary objective of this study was to evaluate the performance efficiency of currently installed riparian VFS buffers by modeling and analyzing the flow accumulation from the field in the VFS using geographical information systems (GIS) and light detection and ranging (LiDAR)-derived high-resolution digital elevation model (DEM) data. The DEM data sets used in this study were generated by resampling an airborne sensor-derived LiDAR 1 × 1 m DEM to a 5 × 5 m LiDAR DEM to minimize the file size of the data sets and possibly improve the computer computational proficiency in handling the data sets. Also, this study included the development of the coefficient of flow intercept (CFI) equation, which was applied to three selected Rock Creek watershed research sites.