Modeling for predicting soil wetting radius under point source surface trickle irrigation

Abstract

Irrigation practices that are profligate in their use of water have come under closer scrutiny by water managers and the public.  Trickle irrigation has the propensity to increase water use efficiency only if the system is designed to meet the soil and plant conditions.  Information on moisture distribution patterns under point source trickle emitters is a pre-requisite for the design and operation of trickle irrigation systems.  This will ensure precise placement of water and fertilizer in the active root zone.  For many practical situations, detailed information on matric potential or water content distribution within the wetted volume is not necessary and prediction of the boundaries and shape of the wetted soil volume suffice.  Simple models are more convenient for system design than the dynamic models.  Therefore, the objective of this study was to develop a simple heuristic model that can help to determine the wetting radius from surface point drip irrigation using infiltration properties of the soil.  The parameters of the model are easily measurable and available.  The expression for determining the radius of water entry at the surface (r_w), the depth of wetting front (d) for a particular discharge as a function of time and the radius of wetted bulb at the selected depth (R_w) could be estimated  The model validation was attained in two stages: a) theoretically by matching the volume of water contained in the bulb constructed using developed methodology with the amount of water supplied and b) by conducting an experiment at the instructional farm soils of Junagadh Agricultural University, Junagadh, with three different emitter discharges (0.002 m³ h^-1, 0.004 m³ h^-1 and 0.008 m³ h^-1) ) to compare the computed values with the field observations.  Results indicated that wetted bulb had circular top area with radius (r_w) which increased both with the increase in Q and elapsed time t.  If time t was fixed, radius was directly proportional to Q^1/2.  The depth of wetting front was found to be invariant with emitter flow rate provided the emitter discharge was less than the infiltration capacity of the soil and an impervious stratum exists at the bottom of the soil.  The relative agreement between computed values with the experimental data was evaluated quantitatively using goodness of fit and efficiency coefficient.  High efficiency coefficient and goodness of fit were observed.  The computed volume of water contained within the wetting bulb matched well with the amount of water supplied.

Keywords: trickle irrigation, wetted radius, simple heuristic model