NUMERICAL MODELLING OF UNSATURATED FLOW TOWARDS PLANT ROOT

Abstract

Roots of the agriculture crop mostly lie in the unsaturated zone. The zone extends from ground surface to the water table. The soil moisture status in the root zone has an important bearing onthe evapotranspiration (ET) of crops. Transpiration defined as the process by which soil moisture is absorbed by the plant roots and evaporated into the atmosphere, is the major component of evapotranspiration in the vegetated areas. While transpiring water, plant roots function like water pumps moving water from the soil and into the atmosphere. The hydraulic system that conveys water from the soil through the plant roots and into the air acts as a continuum, which is known as soilplant- atmosphere-continuum (SPAC). The components of water conveyance through SPAC are the unsaturated flow to the soil-root interface, flow through the plant and finally transfer of water from the plant to atmosphere. The present study involves development of a microscopic numerical model for simulating two-dimensional (r-z plane) axi-symmetric unsteady state flow towards the plant root in the unsaturated zone. The model essentially comprises a numerical solution of head form Richard's equation in axi-symmetric polar co-ordinate system by finite difference method. Apart from simulating the soil moisture distribution, the model also simulates water movement through the plant and from plant to the atmosphere. This is accomplished by assigning boundary condition at the root-soil interface, derived from the meteorological condition prevailing over the plant leaf surface and a plant conductivity parameter {plant transmissivity) defined herein. At the ground surface ponding up of water as well as direct evaporation are accounted for by assigning a boundary condition that varies from Neuman to Dirichlet type and viceversa. The unsaturated component ofthe present model has been successfully validated by simulating Hall's (1955) observed water profile data. The plant transmissivity parameter assimilated in the model, has been estimated for wheat crop under soil and meteorological conditions prevailing at the experimental farm of Water Resources Development Training Center (WRDTC) located in the campus of University of Roorkee, Roorkee (India). The crop growth period is divided into four stages i.e., initial stage, developing stage, middle stage and flowering stage. The plant transmissivity for each crop growth stage is estimated by arriving at the best possible match between the field observed and the model computed evapotranspiration. The corroboration of estimates of plant transmissivities is achieved by projecting evapotranspiration from wheat crop under soil and hydro-meteorological conditions different from those for which the model was calibrated. Errors in the evapotranspiration estimates due to errors in plant transmissivity and saturated hydraulic conductivity are also studied. The proposed model after due calibration, can be employed to arrive at irrigation schedules for a pre-assigned maximum permissible soil moisture depletion in the root zone. This has been demonstrated by designing irrigation schedule for wheat crop at WRDTC farms constraining the maximum soil moisture depletion to 55% and assigning the pre-computed plant transmissivity values