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University of New South Wales (UNSW) 2003

A computational model for the assessment and prediction of salinisation in irrigated areas

Xu, Peng

Titre : A computational model for the assessment and prediction of salinisation in irrigated areas

Auteur : Xu, Peng

Université de soutenance : University of New South Wales (UNSW)

Grade : Doctor of Philosophy (PhD) 2003

Résumé
This thesis presents the results of a computational study on salt transport andaccumulation in crop root zone. The main objective of this study is to examine theimpacts of past land use on the environment and to examine the effect of irrigationwater on the rising of groundwater level and the subsequent salinity problem in ricegrowing area under given climatic conditions. A special focus has been such impactsin the Wakool irrigation area, NSW, Australia. To this end, a computationalmodel for the assessment and prediction of salinisation in agricultural areas hasbeen developed. This modelling system consists of a land surface scheme (ALSIS)for simulating unsaturated soil moisture and moisture flux, a groundwater flowmodel (MODFLOW) for estimating the spatial and temporal variations of groundwatertable,a surface flow model (DAFLOW) for calculating water flow in rivernetworks, a module for calculating solute transport at unsaturated zone and a 3-Dmodel (MOC3D) for simulating solute transport in groundwater as well as a modulefor calculating the spatial and temporal distributions of overland flow depth duringwet seasons.The modelling system uses a finite difference linked technique to form a quasithree dimensional model. The land surface scheme is coupled with the groundwaterflow model to account for the interactions between the saturated and unsaturatedzones. On the land surface, the modelling system incorporates a surface runoffmodel and detailed treatments of surface energy balance, which is important in estimating the evapotranspiration, a crucial quantity in calculating the moisture andmoisture fluxes in the root zone. Vertical heterogeneity of soil hydraulic propertiesin the soil profile has been considered. The modelling system has the flexibilityof using either Clapp and Hornberger (1978), Broadbridge and White (1988), vanGenuchten (1980) or Brooks and Corey (1966) soil water retention models. Deep inthe soil, the impact of groundwater table fluctuation on soil moisture and salinityin the unsaturated soil is also included.The calibration and validation for the system have been partially performedwith observed groundwater levels in the Wakool irrigation area. The applicationsof the model to theWakool region are made in two steps. Firstly, a one-dimensionalsimulation to a selected site in the Wakool irrigation area is carried out to studythe possible impact of ponded irrigation on salinisation and the general featuresof salt movement. Secondly, a more realistic three-dimensional simulation for theentire Wakool region is performed to study the spatial and temporal variations ofroot 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 irrigationwater (which contains salt) on soil salinity in the area, several hypothetical scenariosusing different qualities of water for rice irrigation are tested. To facilitatecomparative analysis of different scenarios, a base case is defined, for which irrigationwater is assumed to be free of salt. The simulated results show that irrigationincreases overall recharge to groundwater in the Wakool irrigation area. The use ofponded irrigation for rice growing has a substantial effect on salt accumulation inthe root zone and the rising of groundwater level, indicating that irrigation at ricebay is a major budget item for controlling soil salinity problem in the local area.

Mots clés : Wakool Land and Water Management Plan (N.S.W.) ; Irrigation — Mathematical models ; Salinization — Mathematical models.

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Page publiée le 30 novembre 2014, mise à jour le 16 juillet 2017