WATER MANGEMENT TO IMPROVE WATER QUALITY AND INCREASE CROP YIELDS
Tidewater Research Station (TRS) Ag-lands, Plymouth, NC
J.W. Gilliam, R.W. Skaggs, R.O. Evans, and G.M. Chescheir
Objectives
SITE DESCRIPTION
A 13.8 hectare agricultural field has been instrumented to study the movement of nutrients and pesticides in the soil, groundwater and surface runoff. The site is located on the Tidewater Research Station, near Plymouth, in the North Carolina Lower Coastal Plain approximately 200 km east of Raleigh. The field, which was cleared for agriculture in 1975, is nearly flat. The research site is bounded on all four sides by drainage ditches approximately 2.0 m deep (Figure 1). Plastic drainage tubes, 101 mm in diameter, were installed in 1985 at a spacing of 22.9 m on center, and a depth of 0.8 to 1.1 m below the surface. The effectiveness of these drains was limited by low hydraulic conductivity of soil at the depth of the drains. A new set of plastic drains was installed in 1990 and 1991 at deeper depths (1.2 to 1.4 m) to improve the operation of the drainage system. The new drains are located midway between the old tubes resulting in a new drain spacing of 11.4 m. Valves were installed on the old drain lines allowing the system to operate with either a 22.9 m or a 11.4 m spacing.
The site is divided into eight, 1.7 hectare, experimental plots. These plots are delineated by the area drained by six adjacent subsurface drains (Figure 2). Each experimental plot has an underground vault and instrument house. Each underground vault intercepts the drainage outflow from the six adjacent subsurface drains as well as the surface runoff from two field catch basins.
Each vault contains four cylindrical PVC outlet tanks that are 0.61 m in diameter (Figure 3). The center and guard tanks are 1.8 m in height and the two surface runoff tanks are 0.9 m in height. These four holding tanks intercept water from the field as follows: the center tank receives water from the two middle subsurface drains, each surface runoff tank receives water from the one of the surface runoff collectors, and the guard tank receives water from the four outside subsurface drains (guard drains). The function of the guard drains is to prevent water table management treatments in one plot from influencing soil water conditions in adjacent plots. The two outside drains on each side of the center drains function to hydraulically isolate the area drained by the center drains from the influence of adjacent experimental plots.
All four holding tanks are equipped with sump pumps and control floats that automatically pump water from the holding tanks to the drainage ditch outlet. Drainage and runoff rates and volumes are measured in two ways. 1. The water level in each holding tank is continuously measured with a potentiometer mounted on a Stevens water level recorder. The output is recorded directly by the computer which determines flow rate by calculating the rate of rise in the tank during an inflow cycle. The rates are stored and integrated to determine cumulative flow volumes, which are also stored as part of the data file. 2. Flowrates are measured directly by a Signet Industrial Paddlewheel Flowmeter installed in the outlet line from each of the drainage outlet tanks. The computers record the flowrate and the time of pumping at 10 sec intervals. Flowmeters and cumulative flow volumes are processed by the computers and available in graphical form for on-site monitoring.
Three water table management treatments: conventional drainage, controlled drainage and subirrigation can be implemented using the subsurface drains. In conventional drainage, the pumps are set so that the water level in the holding tanks is always below the drain tubes in the field. In controlled drainage, pump controls are set to remove water when the water level in the holding tanks exceeds the set point or control elevation, which is higher than the drain. The pumps come on when the water level exceeds the set point and go off when the tank water level falls to the set point. No water is pumped in to maintain the control water level elevation in controlled drainage. This emulates field conditions where a weir in a drainage ditch serves to block drainage until the water level in the ditch rises to the weir elevation. In subirrigation, the water level in the holding tank is maintained at a set point above the field drain outlet. Irrigation water from a shallow irrigation wells is pumped in to replace water lost from the outlet tanks via subirrigation; when rainfall occurs, drainage water is pumped out of the tanks to maintain the subirrigation set point.
Drainage outflow and subirrigation data are collected and processed by personal computers located in climate controlled rooms in equipment houses 2 and 5. The computer in house 2 collects data from plots 1, 2 and 3 while the computer in house 5 collects data from plots 4, 5, 6, 7,and 8. Water surface elevations in the outlet tanks, flow rates and weather data are processed by the computers, which can be accessed by telephone from our lab in Raleigh to evaluate experimental conditions and to download data.
Each equipment house has a refrigerator to preserve samples. Each refrigerator contains three large sample containers (carboys), one for subsurface drainage and two for surface runoff samples. Flexible 12 mm diameter tubing is connected to the discharge pipes from the subsurface drainage tank and the surface runoff tank. This flexible tubing passes through the wall of the refrigerators and discharges into the sample containers. Every time the drainage tank pump or the surface runoff tank pump operate, a portion of the discharge flows into the refrigerated sample containers. The proportional samples are taken to the soil science water quality laboratory for analysis at least weekly and more frequently for rainfall events after fertilization.
Each experimental plot has two 100 mm diameter water table wells equipped with automated water level monitoring equipment. One well is located midway between the two control drains and the other well is located midway between the old control drain and the new guard drain. Both wells are equipped with a float, weight, and pulley system connected to a potentiometer and an Omni Data Pod mounted on a Stevens Recorder platform. Thus water table depth is continuously recorded at two points in each plot. A deep irriga- tion well, 91 m deep, and two shallow irrigation wells, 23 m deep, are located on the eastern edge of the field.
Surface runoff is collected from two in-field surface runoff plots approximately 6.1 m (4 crop rows) wide by 30.5 m long (Figure 2). The north plots were graded at 0.6 to 0.8 % slope and the south plots were left with their natural slope which is approximately flat. The plots are surrounded by 30 cm tall berms to isolate them from the rest of the field. Gutter collectors made of PVC were installed at the end of each plot to collect runoff and route it through underground PVC pipe to the vaults. The soil surface within 30 cm of the collectors is stabilized by a porous plastic sheet.
A series of five to six wells, referred to as a well nest, are installed in lines parallel to the drain tubes. Wells in each nest are spaced approximately 0.45 m horizontally and are at depths of 0.6 m, 0.9 m, 1.2 m, 1.8 m, 2.4 m, and 3.0 m, or 3.7 m. They are constructed of 50.8 mm diameter polyvinyl chloride (PVC) well casing, with a slotted screen for the bottom 150 mm. Each well in the nest is screened in a distinct soil layer. The number, location, and depth of the wells are designed to provide detailed information on the movement of fertilizer nutrients in the flow domain. The wells are installed with granular filter pack around the screen and a bentonite clay seal around the well casing.
The research site is equiped with a complete Campbell CR-10 weather station as well as two additional recording rain gages. Weather parameters measured at the research site are rainfall, air and soil temperature, wind speed and direction, and solar radiation.
The soil on the research site is classified as Portsmouth sandy loam (Typic Umbraquult; fine-loamy, siliceous, thermic). It is a very poorly drained soil that formed in loamy fluvial and marine sediments. The Ap horizon is a black fine sandy loam 0.3 m thick with an organic content in the 3 to 5% range. Various layers of fine sandy loam extend down to a sandy clay loam located at 0.5 to 0.9 m. The sandy clay loam is underlain by a horizon comprised of alternating thin layers of sand or loamy sand and silt. Depending on location in the field, this horizon is underlain at 1.0 to 1.2 m by grey sand. Coarse sand intermixed with pockets of sand clay loam is found from 1.2 to 2.4 m, a tight marine clay deposit, approximately 6.1 m thick, that restricts vertical seepage from the profile.