The Coastal Plains Soil, Water, and Plant Research Center has been monitoring spatial yield in a test field since 1985, using a conventional corn-wheat-soybean rotation most of that time. Observations of variation in soil and crop response that correlate with yield variation suggest that crop water relations may be the key feature that causes spatial variability in yield for the Southeastern Coastal Plain. Experience with mechanistic modeling indicates that for normal weather years, the final yield is particularly sensitive to variations in soil water, presumably because the surface soil is sandy and rooting volume is limited. These conclusions, plus difficulties encountered in scheduling irrigation under a center pivot on typically variable soils, led the USDA-ARS to desi~n and build a site-specific center pivot capable of differentially irrigating lOO-m areas. A 3-tower commercial center pivot was modified by adding 39 9.2-m manifolds in 13 sections, 3 to a section. The manifolds and nozzles were sized lx, 2x, and 4x, so that octal combinations would provide up to 7x the minimum application depth for a given outer tower speed. At 50% speed, the application depths are 0 to 12.5 mm in 1.8-mm increments. A prograrnrnable controller was attached near the pivot end of the boom, so that it was proximal to but avoided the pivot control panel when the system rotated. The individual manifolds were controlled by a program residing in the progranunable controller, which obtained pivot position and other information via radio modem link with the pivot control panel. Water and nitrogen application has been accomplished using this system on a replicated field experiment. Experience gained during this phase will guide modification of a similar pivot for site-specific water, nutrient, and pesticide management on a typically variable Coastal Plain field.

During the past decade, there has been increasing interest in applying water and chemicals to crops based on need or yield potential rather than applying uniformly to the entire field. While ground-driven variable-rate chemical application equipment is now being used, most irrigation systems continue to apply nominally uniform water depths. Our objective was to make variable-rate irrigation applications possible by developing a digitally controlled metering device. The device consists of a reservoir that is alternately filled and emptied at a rate determined by a digital pulse from an external source and requires pressurized sources of water and air. The flow rate can be altered by changing the cycle duration and frequency, by changing air and water pressure, or by exchanging the reservoir with one of different volume. Tests with prototypes indicate reproducible flow rates for a range of operating pressures and discharge cycle durations. Various sprinklers or nozzles may be attached to the outlet if specific distribution patterns are desired. Additionally, the metering device can be used in a wide variety of applications with a variety of fluids or gases for variable-rate flow or injection of a fluid into either another fluid or gas.

Spatial yields since 1985 in a corn-wheat-soybean rotation at Florence, S.C., show little correlation of yielddata with expected yields for soil map units. Research suggests that spatial yield variability for the southeastern CoastalPlain may be caused primarily by water relations. This causes difficulties in scheduling irrigation for conventional centerpivot irrigation systems, which are not capable of applying variable depths of water to small areas of variation within thetotal system. Thus, the objectives of this work were to design and construct a site-specific center pivot irrigation systemthat could independently apply variable rates of water and chemicals to 100-m2 areas within the irrigation system. Acommercial center pivot system was modified by adding three 9.1-m manifolds in each of 13 segments along the truss.Nozzles were spaced 1.5 m apart along each manifold, and both manifolds and nozzles were sized to provide 1x, 2x, and4x nominal application rate at a given tower speed. All combinations of the three manifolds provided up to 7x nominaldepth, which was 12.7 mm, in 1.8-mm increments when the outer tower traveled at 50% of full speed. A programmable,computer-controlled management system was installed near the pivot on the moving portion of the center pivot system.This controller obtained the position from the center pivot controller via a radio frequency modem and switched on theappropriate valves to obtain the application rate for a specific area. During 1995 and 1996, the system applied water andN fertilizer in a fixed-boundary field experiment. Measurements and observations of water and N application uniformitieswere acceptable; however, more extensive evaluation will be required before definitive conclusions can be reachedregarding N application. Surface temperatures measured with an integral infrared thermometer system producedencouraging results that may be useful in management of water and nutrients. Using experience gained with this system,a second commercial center pivot system is being modified for site-specific water, nutrient, and pesticide management ona field with soil variation (irregular boundaries) typical of the Coastal Plain.