Estimations of permissible irrigation rates from soil properties in high frequency irrigation

Abstract

The effect of irrigation dynamics and soil physical properties on the permissible rate of application was analyzed using the analytic solution of Richard’s equation for periodic flux type boundary conditions typical to high frequency irrigation. Dynamics of irrigation regime was defined by its instantaneous application rate, R 0, its frequency, and the duty cycle which is the ratio of irrigation duration and irrigation interval (period). Soil properties were saturation, hydraulic conductivity, diffusivity, and Gardner’s soil type coefficient. Fourier Transform was used to derive a closed form analytical expression for the maximal permissible value of R 0, which would not result in water logging and saturation at soil surface. The analytical expression ties the three irrigation parameters with the above three soil parameters together. Prevention of aeration stress by restricting the moisture content at the soil surface, to become less than the minimal air pore volume (drainable porosity), was also used as an upper constraint of moisture at the surface soil. The effect of irrigation frequency and duty cycle on the permissible R 0 values was analyzed and computed regarding three soil types: coarse sand, sand, and sandy loam. Under short duration periods of water application resulting from either small values of duty cycle or short irrigation periods (high frequency), or both, the soil surface would not become saturated even for very high R 0 values. The maximal application depth V max, depends on both the duty cycle and the frequency. For a given soil, V max remains essentially constant per irrigation periods of 1 h or less, typical to pulsed irrigation and independent of the duty cycle. For periods longer than 1 day for coarse soils, V max increases with the duty cycle while for sandy loam soils, the increase in V max becomes insignificant for duty cycles larger than 0.3. The computed values of maximal R 0 and V max based on the water logging concept are much higher than the ones used in irrigation design and should be considered as upper permissible limits only. The permissible values of R 0 and V max are much lower if the aeration stress concept is adopted and found in the range used in practice. This analysis may be useful for the design of precision irrigation for high frequency microdrip, high frequency trickle irrigation, as well as for trunk diameter measurement (TDM) irrigation methods.