Kentucky farm soils are known for their spatial variability that is typical for the Karst landscapes and caused by soil erosion processes leaving the land surface with a heterogeneous mosaic of infiltration capacity. Most irrigation systems in Kentucky and in the Southeastern U.S. are pivot systems that allow a spatially uniform rate of water supply. This technical limitation leads to either optimum water application, over- or under-application of water. The latter two imply not only economic losses for the farmer but also environmental harm through sub-optimal use of resources or even loss of resources through surface runoff or leaching. Little is known yet how to design an irrigation map and how to manage irrigation on the go while considering local soil properties, crop growth stage, past management, weather forecast, and technical precision of VRI system. In this project, the technicalities of variable-rate irrigation will be explored and used to compose a manual with guidance for growers on the various steps of a sound variable rate irrigation management.

The objective of this project is to develop a protocol that includes:
- processing of site-specific soil information,
- crop growth development through different types of remote sensing,
- database management,
- access to precise local weather information, and
- knowing the sensitivity of the irrigation system and its limitations.

- Evaluation of the efficiency of VRI over uniform rate,
- Manual with guidance on VRI for growers.

One way of securing high soybean yields in a sustainable way is to irrigate. This strategy is especially relevant four double crop soybeans that are planted after wheat harvest when the soil water storage is depleted to some extend by the wheat. To meet the plant's water needs irrigation water has to be applied efficiently. For this study, a center pivot system was used that was recently upgraded to a variable-rate irrigation (VRI) system was tested for the first time in combination with a new soil water content measurement system. One objective was to validate the application of two different irrigation rates. The second objective was to identify soil water content increase in the soil profile during a rainfall or irrigation event, and to evaluate the agreement between soil water content change and amount of precipitated or irrigated water. Third, the Root Zone Water Quality Model 2 (RZWQM2) which is an agro-ecosystem model that can simulate plant growth and soil processes was used to simulate profile soil water dynamics and crop growth. Two spatially different irrigation rates of 0.5 and 0.25 inches were used in eight different zones in a farmer’s field at Hillview Farms, Princeton, Caldwell County, Kentucky. The eight zones were located in two different soil types within the same field, i.e., silt loam and silty clay. The irrigated amount that was caught in a tipping bucket rain gauge was slightly below the intended amount. After calibration, the simulation model was able to describe soil water content time series in different soil layers. Accuracy of the model is comparable to the results obtained in other studies in the literature. For the silty clay soil, 0.5 inches of irrigation increased yield by approximately 7 bu/ac whereas for the silt loam soil, the lower irrigation rate did not cause a yield decrease. In drier years, this result would be expected top differ.

Through the findings of this research project and the manual that explains the technical steps of variable rate irrigation (VRI), growers are enabled to improve crop water use efficiency, and avoid negative environmental impact of over-irrigation. Both result benefits in economic return of irrigated soybean production.