Applicability of the unit response equation to assess salinity impacts of irrigation development in the Mallee region:supplementary analyses

[摘要] Executive Summary:The simpleunit response equation (URE) has been successfully implemented in SIMRAT to assess the impacts of irrigation developments on discharge to the River Murray in South Australia.The perceived limitations in the applicability of the URE in the Mallee region were rigorously investigated by Rassam et al.(2004)and mainly addressed 3 concerns:firstly, doubts about the underlying theory of the URE in relation to basic concepts such as linearity and superposition, and secondly, doubts regarding the capacity of this simple approach to produce accurate results comparable to those arising from more sophisticated models, and thirdly, concerns about model applicability in the real world where the underlying assumptions might be violated (different boundary conditions, complex geometries, and aquifer heterogeneity).Since the publication of CSIRO Technical Report 35/04 (rassam et al., 2004), further questions were raised regarding the applicability of the URE in the Mallee region; they mainly relate to the following issues:non-uniform transmissivity and aquifer heterogeneity under large irrigation developments, high sensitivity to base slope of the solution presented by Knight et al.(2005) for sloping base aquifers, and aquifer head response under reduced recharge conditions.These issues are addresssed in this report.Objectives of the current report are: *Investigate the temporal variation of aquifer transmissivity under large irrigation developments and its subsequent effect on discharge to rivers.*Compliment the study of aquifer heterogeneity conducted by Rassam et al. (2004) to include low conductivity barriers spanning in the veritcal direction.*Investigate the effect of aquifer base slope on the URE predictions.*Show how various parts of the aquifer may response differently to reductions in recharge.Key findings of the current report are: The temporal variation of aquifer transmissivity at Loxton was shown to increase by about 40% during a 100-year period; this only resulted in a 10% increase in discharge to the river, which is well within the expected range due to aquifer parameter uncertainty as outlined by Rassam et al. (2004).Low conductivity vertical barriers were found to speed up the flux response to the river when they are situated behind the recharge source; as their hydraulic conductivity becomes extrememly low, they function as no flow boundaries.Low conductivity vertical barriers located between the recharge source and the river slow down the flux response to the river; a modified form of the URE is proposed to estimate the discharge flux response under such conditions.The high sensitivity of flux response to the aquifer base slope is due to existing head gradients and not to the slope angle per se; for a sloping base aquifer with no head gradient, the basic URE is still applicable provided a suitable aquifer thickness that accounts for the slope effect is used; in the presence of head gradients, the criteria outlined by Rassam et al. (2004) should be followed.The analytical functions for predicting pressure heads presented by Rassam et al.(2004) may be used under reduced recharge conditions; when recharge is reduced in a high transmissivity aquifer, the water table between the recharge sourced and the river drops quickly while it continues to rise behind the recharge source long after recharge had beed reduced.This demonstrates that different parts of the aquifer may respond differently to recharge reduction.[flagship]

[发布机构] CSIRO

[学科分类] 地球科学(综合)