An investigation of surface and subsurface flow characteristics during an alfalfa irrigation event

The goal of this study was to minimize the tail discharge and estimate deep percolation in a flood irrigated alfalfa check through theoretical and experimental investigation of surface and subsurface flow characteristics, respectively. For this purpose, two models were developed -- one for the development of a cutoff irrigation strategy by monitoring wetting front advancement in a flood irrigated alfalfa check and the other for determining soil hydraulic properties using an inverse solution technique. Moreover, field tests were conducted to verify the cutoff irrigation strategy and obtain soil moisture data to implement the inverse solution technique.;The model based cutoff irrigation strategy used a sensor controlled cellular communication system. For this purpose, a low cost water arrival sensor was designed and developed to monitor the wetting front advance along a check. The task of the model was to predict the wetting front advance rate so that the irrigation cutoff time (i.e., wetting front arrival time at the cutoff point) could be predicted. In order to accomplish this task, tests were conducted during 18 irrigation events (5 conventional plus 13 cutoff) in four alfalfa checks (all 15 m X 220 m) in 2008 and 2009 alfalfa growing seasons. The set of water arrival sensors detected the wetting front advance rate and the model that relates wetting front advance rate to the arrival of wetting front at the cutoff location in the field, determined exactly when to turn off the inflow. This information was transmitted to single or multiple cell-phone(s) of irrigator(s) through a novel cellular communication system developed to communicate the information collected and analyzed by the data acquisition system wirelessly in the form of short text messages (SMS). The experimental data fitted the wetting front advance rate model very well (R2 > 0.90 for all of the cases) and the proposed cutoff irrigation strategy produced zero tail water drainage. The cutoff times estimated by the model were found to be quite accurate in all cases tested.;To achieve the second objective, an inverse solution technique was developed in this study to determine saturated soil hydraulic conductivity ( Ks) and the conductance of plant to water uptake ( C) under field conditions using a 1-D water transport model. The soil moisture contents (theta) were measured at 12 locations and at several depths at each location within the root zone in two different alfalfa checks using access tubes (each 1.5 m deep) and a neutron moisture probe. The inverse solution process utilized the measured soil moisture conditions in the alfalfa checks between two successive irrigations. In this study, the soil water retention curve was assumed to be of the form described by van Genuchten. The moisture contents at upper (0.225 m) and lower (1.2 m) depths provided the boundary conditions (BCs) needed for the model. The moisture content readings immediately following the irrigation event (i.e., t = 0) were used as the initial condition (IC). The model was solved for assumed model parameters values (K s and C) and the resulting model predicted soil moisture data were used to develop a third order orthogonal response surface in these model parameters. This approach leads to a response surface corresponding to each depth and time that was not a part of either BCs or IC. Moreover, a multiple linear regression technique was used to empirically model experimental theta to create a smooth surface so that the experimental error would be minimized. The response surface solutions were then optimized against the empirical surface to seek the optimum values of K s and C. The moisture contents predicted by the inverse solution technique were found to be unique, robust and compared very well with the experimentally observed data (R2>0.95 and slope = 1:1 for most of the cases).