ATTENUATION WITH SOIL DEPTH OF DYNAMIC FLUCTUATIONS IN THE RATIO OF CHEMICALS TO IRRIGATION WATER IN CHEMIGATION

Abstract

Use of chemigation for the application of fertilizers and chemicals through the irrigation system has increasedworldwide. The concentration is usually controlled and maintained constant over the irrigation duration. Dynamicfluctuation in the concentration may, however, result in inaccurate application of the fertilizers and chemicals. This may, inturn, cause a reduction in crop yield when the application amounts are lower than the planned amounts, or present anenvironmental hazard when they are higher. These dynamic fluctuations in the chemical concentration of the irrigation waterdecay with soil depth due to the convection-dispersion nature of the transport process of infiltrating water during irrigation.An analytical solution, which employs the Fourier Transform method, is used here to estimate the decay of these dynamicfluctuations as water infiltrates from the soil surface to the root zone. This solution assumed constant irrigation flux and soilmoisture conditions, typical to high frequency irrigation. It assumes periodic fluctuations in the chemical concentration ofthe irrigation water. A general analytical solution was derived for these asymmetric dynamic periodic fluctuations. The effectsof fluctuation period, asymmetric time factor, and high and low concentration values within the irrigation period on the decayprocess were evaluated and results are presented for clay loam and sandy loam soils. Damping depth was chosen tocharacterize this decay process. It is the depth at which the amplitude of the chemical concentration decays by a factor exp(-1). 37% of its value at the soil surface Two (high and low) damping depths characterize the decay process of asymmetric timefluctuations. Damping depths for symmetric fluidations in clay loam were 21, 58, 138 mm for periods of 2, 10, 30 days, and78, 239, 682 mm in sandy loam for the same periods, showing the significance of soil texture and fluctuation periods. Theeffect of asymmetry was assessed for constant average concentration. High damping depths values of 92, 156, 235, and308 mm were computed for asymmetric factors 0.1, 0.25, 0.50, and 0.80, while the low damping depths were 322, 296, 239,and 145 mm, respectively, in sandy loam and 10 days period, showing a significant effect of asymmetry. This analysis is usefulfor assessing the level of accuracy required for controlling chemical concentration during chemigation, considering the soilbuffer effect.