Bogue Phalia Basin Study Area


In the Bogue Phalia basin (fig. 10), data for the ACT study were collected during water years 2006–10 from surface water, groundwater, the unsaturated zone, the streambed, and precipitation (table 2). Surface-water data were collected from several sites (fig. 11), including the outflows from the Bogue Phalia basin (site 07288650), and two catchments within the basin where focused studies were conducted—the Tommie Bayou catchment (site 07288636), and the catchment of an unnamed tributary to Clear Creek (site 333150090530400). Groundwater and unsaturated-zone data were collected from a focused study area near the basin headwaters (fig. 11B) to assess areal groundwater recharge and better understand the fluxes of water and chemicals to the underlying alluvial aquifer. Streambed and related data were collected from another focused area just upstream of the basin outlet (fig. 11C) to assess the hydrologic and chemical interactions between surface water and groundwater and to determine the direction of local groundwater flow. Precipitation data were collected at two sites in the Tommie Bayou catchment (fig. 11A) to determine the intensity and timing of rainfall, and atmospheric samples were collected at one of these sites to measure pesticides in air and rain. 


Environmental Setting


The Bogue Phalia basin covers 1,250 km² within the Yazoo River basin in northwestern Mississippi (fig. 10). The area is locally referred to as the “Delta.” The region has a moderate climate with temperatures ranging from a mean of 26 °C in July to a mean of 5 °C in January. Mean annual precipitation for the 13-year period of record (1996–2008) was 137 cm. Typically, less than 30 percent of the annual precipitation occurs during the May–August growing season (fig. 12). Evapotranspiration plays a large role in the water budget of this area, with an annual average of 120 to 149 cm of moisture returning to the atmosphere as a result of evaporation and transpiration. (Air temperature, precipitation, and evapotranspiration information presented here is based on data collected and archived by the Mississippi State University Extension Service, Stoneville station, 2011.)


Land Use


Agriculture is the principal land use in the Bogue Phalia basin, and 80 percent of the total land area is used for row crops including corn, soybeans, rice, and cotton. Growers in the area use a broad variety of fertilizers, herbicides, and pesticides, but manure application is uncommon. The uneven distribution of precipitation throughout the year (fig. 12) makes irrigation a necessity for profitable agriculture, and because of the flat topography and generally poorly drained soils, successful farming in the area typically requires some type of engineered drainage.


The study area is sparsely populated (less than 15 persons/km²) and residents live either in small communities or rural areas. The combined population of Bolivar, Carroll, Coahoma, Grenada, Holmes, Humphreys, Issaquena, Leflore, Panola, Quitman, Sharkey, Sunflower, Tallahatchie, Tunica, Warren, Washington, and Yazoo Counties—an area of more than 26,000 km²—is less than 400,000.


Physiography, Geology, and Soils


Physiographically, the Bogue Phalia basin is completely contained within the Mississippi River Alluvial Plain, an area that was the Mississippi River flood plain before levees were built. The Bogue Phalia basin overlies quaternary-age, unconsolidated alluvial materials (sand and gravel), which were deposited on an erosional surface composed of Tertiary‑age sand and clay units. 


The primary aquifer of interest underlying the Bogue Phalia basin is the Mississippi River Valley alluvial aquifer, often referred to simply as the “alluvial aquifer” (fig. 10). The alluvial aquifer is the primary source of groundwater for irrigation, but generally is not used for drinking water due to issues with encrustation, color, and taste associated with hardness and high iron content (Taylor and Thompson, 1971).


Soil types in the Bogue Phalia basin are broadly identified as the Forestdale, Sharkey, Alligator, Dundee, and Commerce series, which range from a very fine smectitic clay to a fine, silty loam, respectively. Soils gen­erally are shallow, very poorly to somewhat poorly drained, finely textured, and nearly level. Erosion from runoff occurs, but it is not a major issue as the slope of the land generally is very low (from 0 to 8 percent) and deep-tillage is no longer practiced regularly in the area. Available water capacity of the soils varies from very low to moderately high, and the organic matter content is moderate. The Forestdale, Sharkey, and Alligator series soils are naturally suited to row crops, such as rice and soybeans, although the Dundee and Commerce series soils are naturally suited to row crops, such as corn and cotton.


Hydrology


The average discharge at the surface-water outflow of the Bogue Phalia basin is 21.3 m³/s and ranges between 0.10 and 274 m³/s (U.S. Geological Survey, 2010, site 07288650). This discharge is from a combination of overland flow from precipitation, irrigation runoff, and—to a much lesser extent—subsurface drainage and groundwater discharge. Floods in the Bogue Phalia basin can be caused by heavy rains within the basin (including rain associated with nearby hurricanes), as well as rainfall events outside of the basin. For example, flooding in upstream basins can lead to flooding in the Delta reach of the Mississippi River that can breech levees and cause flooding in the Bogue Phalia basin. In fact, the worst recorded flood in the study basin, with an estimated occurrence interval exceeding 100 years, occurred in 1927 as the result of rainfall events in the Midwest. Major floods, such as those brought on by hurricane events can occur even during droughts. At base flow, the Bogue Phalia primarily contains drainage water from bank storage and discharge from shallow groundwater, and some reaches can become dry during the summer and fall. The record 7-day minimum streamflow recorded at the basin outlet (site 07288650) was 4.20 m³/s in early November 2007. 


Major modifications, such as the building of levees, deforestation, and the draining of wetlands have been made to the natural hydrology of the Bogue Phalia basin to promote agriculture. Installation of irrigation wells also has had a major effect on the hydrology of the basin. The amount of water typically pumped from the alluvial aquifer in Mississippi alone is approximately 3.9 billion gal/d during the growing season (Maupin and Barber, 2005). Many Delta streams have lost connection with the shallow alluvial aquifer due to declines in the water table from the high irrigation withdrawal rates. In 2005, a large number of irrigation wells were installed to pump water into the Big Sunflower River (to which the Bogue Phalia is a tributary) to supplement flow that had declined markedly as a result of irrigation. 


Sites 07288636 and 333150090530400 (fig. 11A) are located at the surface-water outflows of the two focus catchments. More than 90 percent of the land in both of these catchments is used for row crops, and surface-water outflow from both is comprised primarily of overland flow. The Tommie Bayou at site 07288636, located 44.8 km upstream of site 07288650, has a drainage area of 15.3 km². The observed range of flow at the site is 0.0 to 25.0 m³/s, and the average annual discharge is 0.18 m³/s. Elevated discharge at site 07288636 is often due to the release of water from rice fields within the catchment; however, during the dry season and in the absence of overland runoff, the stream can become stagnant. Site 333150090530400, located 16.3 km upstream of site 07288650, drains an area of 2.2 km². Observed flow at the site ranges from 0.0 to 2.4 m³/s, and the average annual discharge is 0.06 m³/s. The stream at site 333150090530400 is dry much of the time. 


Data Collection


For site 07288650, flow data were available from the USGS gaging station, and water samples for chemical analyses were collected 67 times during the 2006–10 study period, including during both base flow and storm conditions. In addition to samples for chemical analyses, which were collected manually, a continuous water-quality monitor measured temperature and specific conductance. Two additional continuous temperature and specific conductance monitoring probes were installed in streambed piezometers at a depths of 1 and 2 m near site 07288650. 


Flow at site 07288636 was determined by discharge measurements made with an acoustic flow sensor. Collection of water samples occurred during both base flow and storm conditions. Base flow samples were collected manually, and storm samples were collected when an autosampler was triggered by rising stage in the stream. A continuous water-quality monitor measured temperature, specific conductance, pH, and dissolved oxygen. This site also was equipped with a precipitation gage and with samplers for the collection of pesticides in air and rain. 


Flow at site 333150090530400 was estimated from stage. For the time period between October 1, 2006, and October 10, 2007, two culverts were installed below the gage to direct flow under a road. From October 11, 2007, through November 6, 2007, there were no culverts at the site. On November 7, 2007, a single, larger culvert was installed below the gage to replace the two, smaller culverts. Infrequent water samples were manually collected during base flow and storms. Storm samples were collected by an autosampler triggered by rising stage in the stream. A continuous water-quality monitor measured temperature, specific conductance, and pH. A precipitation gage was maintained at this site. 


To assess the quality of groundwater in the alluvial aquifer at the areal recharge study area (fig. 11A), which had relatively permeable soils, water samples were collected and analyzed for various chemical constituents from a shallow (10 m deep) water-table well and an abandoned irrigation well (36.6 m deep). The two wells were sampled nine times from 2006 to 2008 for major ion chemistry, nutrients, and field parameters (depth to water, pH, water temperature, specific conductance, dissolved oxygen, and turbidity). Soil moisture probes also were installed at the site.


At the groundwater/surface-water interaction and flow system study area (fig. 11B), 13 locations in and near the stream were sampled to assess the hydraulic gradient between the stream and the subsurface using installed drive-point samplers and piezometers. Samples were collected from 2 m below the sediment-water interface in the channel, and from 9.8 to 12.2 m below land surface on the eastern and western sides of the channel. All piezometers were instrumented with pressure transducers, which measured groundwater level and temperature at 15-minute intervals. Temperature dataloggers were installed at fixed depths within the in-stream piezometers and recorded water temperature every 15 minutes. Water level and temperature were recorded continuously in four near-stream piezometers, five piezometers in the streambed, and surface water. Nests of soil-moisture probes and shallow wells also were installed adjacent to the stream to determine water quality and the direction of local groundwater flow in the alluvial aquifer.


First posted February 16, 2012