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This thesis is concerned with the problem of determining an optimal irrigation policy, that is, an optimal quantity and frequency of irrigation water application. The purpose is to present a solution to this problem using a continuous review model of an inventory system. Initially, the functions of the plant -water -soil system are discussed. This is followed by a review of several existing methods for maximizing crop yield or profit by determining an optimal irrigation policy. Next, the inventory problem is briefly examined. An analogy is drawn between the farmer's problem of determining an optimal irrigation policy and the businessman's problem of determining an optimal ordering policy. Subsequently, a continuous review model of the irrigation system is developed and an example of its use is given.

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Typescript. / Digitized by Kansas Correctional Industries

Arid and semi-arid countries are facing the exhaustion of their water resources and are being forced to use saline water (brackish groundwater and drainage water) for irrigated agriculture. The result is often disastrous as extensive productive regions become salinized. Nevertheless, there is potential to expand irrigated agriculture through the increasing use of saline waters for irrigation. / This study presents an analysis of the performance of a transient state, model for numerical simulation of water and solute transport, known as LEACHM-C. It is assessed for areas where saline water may be an option for crop production. The model estimates the salt and water balance of a soil profile given certain irrigation and crop rotation strategies. / First, the predictive capability of the model was successfully tested using one year of data from a field experiment in a dry region of India. / Second, potential usefulness of the LEACHM-C model as a tool in the planning of reclamation activities was examined for a semi-arid basin in Syria. (Abstract shortened by UMI.)

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This thesis presents the results of a computational study on salt transport and accumulation in crop root zone. The main objective of this study is to examine the impacts of past land use on the environment and to examine the effect of irrigation water on the rising of groundwater level and the subsequent salinity problem in rice growing area under given climatic conditions. A special focus has been such impacts in the Wakool irrigation area, NSW, Australia. To this end, a computational model for the assessment and prediction of salinisation in agricultural areas has been developed. This modelling system consists of a land surface scheme (ALSIS) for simulating unsaturated soil moisture and moisture flux, a groundwater flow model (MODFLOW) for estimating the spatial and temporal variations of groundwatertable, a surface flow model (DAFLOW) for calculating water flow in river networks, a module for calculating solute transport at unsaturated zone and a 3-D model (MOC3D) for simulating solute transport in groundwater as well as a module for calculating the spatial and temporal distributions of overland flow depth during wet seasons. The modelling system uses a finite difference linked technique to form a quasi three dimensional model. The land surface scheme is coupled with the groundwater flow model to account for the interactions between the saturated and unsaturated zones. On the land surface, the modelling system incorporates a surface runoff model and detailed treatments of surface energy balance, which is important in es-timating the evapotranspiration, a crucial quantity in calculating the moisture and moisture fluxes in the root zone. Vertical heterogeneity of soil hydraulic properties in the soil profile has been considered. The modelling system has the flexibility of using either Clapp and Hornberger (1978), Broadbridge and White (1988), van Genuchten (1980) or Brooks and Corey (1966) soil water retention models. Deep in the soil, the impact of groundwater table fluctuation on soil moisture and salinity in the unsaturated soil is also included. The calibration and validation for the system have been partially performed with observed groundwater levels in the Wakool irrigation area. The applications of the model to theWakool region are made in two steps. Firstly, a one-dimensional simulation to a selected site in the Wakool irrigation area is carried out to study the possible impact of ponded irrigation on salinisation and the general features of salt movement. Secondly, a more realistic three-dimensional simulation for the entire Wakool region is performed to study the spatial and temporal variations of root zone soil salinity under the influence of past land use from 1975 to 1994. To allow the assessment and prediction of the effects of ponded rice irrigation water (which contains salt) on soil salinity in the area, several hypothetical scenarios using different qualities of water for rice irrigation are tested. To facilitate comparative analysis of different scenarios, a base case is defined, for which irrigation water is assumed to be free of salt. The simulated results show that irrigation increases overall recharge to groundwater in the Wakool irrigation area. The use of ponded irrigation for rice growing has a substantial effect on salt accumulation in the root zone and the rising of groundwater level, indicating that irrigation at rice bay is a major budget item for controlling soil salinity problem in the local area.

A computer model simulating moisture migration in soil from a drip source considering root water extraction (RWE) was developed. The model was formulated using Continuous System Modeling Program (CSMP). / A two-dimensional non-linear unsaturated transient flow equation was solved using the principle of mass conservation and Darcy's law on soils of dwarf-apple orchards located in southwestern Quebec. A finite axisymmetric cylinder with homogeneous, isotropic and non-swelling soil was considered for the simulations. No flow conditions across the boundaries of the cylinder were fixed. The initial soil moisture contents in the soil profile observed in the field were input for the simulations. / The macroscopic approach was used to compute RWE as a function of (THETA), Z and t. The RWE was assumed to be equal to evapotranspiration (EP) which was estimated using temperatures and the solar radiation index of the location. / The moisture contents in the soil profile observed at the termination of emitter discharge were in close agreement with the simulated values. The soil moisture distribution was found to depend on the amount of water remaining in the soil and soil moisture retention characteristics. It is independent of the rate of emitter discharge, the depth of root zone and method of application.

Undertaken in conjunction with JUMP (Joint Universities Masters Programme in Hydrology and Water Resources). Bibliography: leaves 76-83. Establishes a methodology using genetic algorithms to find the optimum operating policy for variable speed pumps in a water supply network over a period of 24 hours.

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