Spatial Variation Scales of Rainfall Characteristics and Bromide Leaching

Amount and intensity of rainfall are known as important characteristics that affect the leaching of surface-applied agri-chemicals. Besides these, the effect of the time interval between a fertilizer, pesticide or tracer application and subsequent rainfall on solute leaching is not well understood. Moreover, little is known about the spatial representativity of the solute concentration based on a relatively small soil sample in field-scale transport studies. To know the spatial representativity of a solute concentration sample at a time is crucial for analyzing solute leaching behavior over time as well as over space. The objectives of this study were to identify the impact of rainfall intensity and amount as well as the application time delay on solute transport in a well-drained Maury silt loam soil. Moreover, an experimental design and protocol had to be developed that exhibited spatial variability structure and representativity of bromide concentration. For this purpose, the variation scale of each of the factors investigated was chosen differently to apply frequency domain statistics. The study was conducted in a Maury silt loam soil at the University of Kentucky, College of Agriculture Experimental Farm Spindletop. Along a 64-m transect, 32 plots each 2-m long and 4-m wide were established. The three different treatments were spatially laid out in sinusoidal patterns at three respective wavelengths. Two different rainfall amounts were applied in blocks of eight consecutive plots, hence a wavelength of 32 m. These two different rainfall amounts were applied at four rates, spatially distributed in two waves each of 16 m length. Individual plots received the irrigation at specific times after the tracer had been applied. Four application delay times were chosen, hence the wavelength for this treatment was 8 m. Bromide concentration was measured for soil samples that were taken with a percussion auger at every 50 cm distance along the 64-m-transect. Auger cores were vertically separated in 10-cm-depth increments. The center of mass of bromide was used as a state variable reflecting the leaching behavior of bromide. The spatial range of representativity of bromide concentration measured in soil samples was in the order of 1 to 1.5 m. Due to the regular frequency of treatments applied across the field, peaks in power and cross spectra revealed the interaction of the respective treatment and bromide leaching depth. Rainfall amount showed the largest effect on bromide leaching. The effect of rainfall intensity on leaching became only obvious for the highest and lowest intensity but not for the two intermediate intensities. With increasing time delay between tracer application and subsequent rainfall, the leaching depth became shallower. Application of agri-chemicals should be delayed if there is a moderate or high chance of a rainfall event. Long and low-intensity rainfall events may cause deeper leaching than short and high-intensity rainfall events. The experimental design was efficient to study impact of transport-relevant rainfall characteristics of solute leaching and contributed to the understanding of leaching behavior under field conditions.