Hojjat Ahmadi; Mohammad Hemmati; Mehdi Motallebian
Abstract
Introduction: Coastal aquifers are major source of freshwater in many parts of the world. Saltwater intrusion is a serious environmental issue since 80% of the world’s population live along the coast and utilize local aquifers for their water supply.Under natural conditions, these coastal aquifers ...
Read More
Introduction: Coastal aquifers are major source of freshwater in many parts of the world. Saltwater intrusion is a serious environmental issue since 80% of the world’s population live along the coast and utilize local aquifers for their water supply.Under natural conditions, these coastal aquifers are recharged by rainfall events, and the recharged water flowing towards the ocean would prevent saltwater from encroaching into the freshwater region. However, over exploitation of coastal aquifers has resulted in reducing groundwater levels (hence reduced natural flow) and this has led to severe saltwater intrusion. Saltwater intrusion from the sea into below the freshwater of aquifer impairs the quality of these resources. Cause ofthe complexity of saltwater intrusion issues and generally they cannot be solved analytically, so numerical methods can be useful tools for simulation and prediction of salt water intrusion.
Materials and Methods: CTRAN/W is a finite element software product that can be used to model the movement of contaminants through porous materials such as soil and rock. The comprehensive formulation of CTRAN/W makes it possible to analyze problems varying from simple particle tracking in response to the movement of water, to complex processes involving diffusion, dispersion, adsorption, radioactive decay and density dependencies. SEAWAT is a three-dimensional variable density groundwater flow and transport model developed by the USGS based on MODFLOW and MT3DMS. SEAWAT is based on MODFLOW and MT3DMS. SEAWAT includes two additional packages: Variable-Density Flow (VDF) and Viscosity (VSC).In this study, the precision of CTRAN / W and SEAWAT models to simulation and prediction of saltwater wedge were investigated in three states: a) steady state salt-wedge data observed underdifferenthydraulic gradient conditions; b) transient salt-wedge data observed underintruding-wedge conditions; and c) transient salt-wedge data observed under receding-wedge conditions. Both models were initially calibrated and then the models were performed for the above conditions. The simulation results of the two models with the experimental results of Goswami and Clement (2007) have been compared. For comparing the measured data and simulated data, statistical indicators were used: root-mean-square error (RMSE), a measure of Nash-Sutcliffe (CE), the Correlation Coefficient (R^2), the ratio of difference (r) and the General Standard Deviation (GSD).
Results and Discussion: In this study, the precision of CTRAN / W and SEAWAT models to predict saltwater wedge wasinvestigated. At first step, both models were calibrated and the best values for longitudinal and transverse dispersion were obtained 0.5 and 0.05, respectively.Then simulation was performed with both models for all three modes(a- steady state salt-wedge data observed underdifferenthydraulic gradient conditions; b- transient salt-wedge data observed underintruding-wedge conditions; and c- transient salt-wedge data observed under receding-wedge conditions). The results showed thatCTRAN/W and SEAWAT models have high precision for simulation of position and movement of saltwater wedge in steady state with average of root mean square error (RMSE) equal to 1.05 and 1 cm, respectively and Both models have a higher estimate than the actual value for a steady state. As well as for transient state under the underintruding-wedge conditionsCTRAN/W and SEAWAT models have high precisionwith average of root mean square error (RMSE) equal to 0.65 and 0.44 cm, respectively and other statistical indicators were acceptable. The results of prediction of position and movement of saltwater wedgeunder receding-wedge conditionswith average of root mean square error (RMSE) equal to 0.54 and 0.56 cm, respectively provided acceptable estimates of both models. Finally, in order to determine the accuracy of the models in estimating the flow rate from the source of fresh water to the source of salt water, a comparison was made between the results of the models and the laboratory data, which showed that The CTRAN/W revealed appropriate estimation of amount of transferring discharge from freshwater reservoir to saltwater reservoir in compared with SEAWAT model. In general, according to statistical indicators, the results of both models were acceptable
Conclusion: The results showed thatCTRAN/W and SEAWAT models have high precision for simulation and prediction of position and movement of saltwater wedge with average of root mean square error equal to 0.67 and 0.58 cm (less than 10% of the average of measured data), respectively. The CTRAN/W revealed appropriate estimation of amount of transferring discharge from freshwater reservoir to saltwater reservoir in compared with SEAWAT model. In general, according to statistical indicators, the results of both models were acceptable.
vahid Rezaverdinejad; M. Hemmati; H. Ahmadi; A. Shahidi; B. Ababaei
Abstract
In this study, the FAO agro-hydrological model was investigated and evaluated to predict of yield production, soil water and solute balance by winter wheat field data under water and salt stresses. For this purpose, a field experimental was conducted with three salinity levels of irrigation water include: ...
Read More
In this study, the FAO agro-hydrological model was investigated and evaluated to predict of yield production, soil water and solute balance by winter wheat field data under water and salt stresses. For this purpose, a field experimental was conducted with three salinity levels of irrigation water include: S1, S2 and S3 corresponding to 1.4, 4.5 and 9.6 dS/m, respectively, and four irrigation depth levels include: I1, I2, I3 and I4 corresponding to 50, 75, 100 and 125% of crop water requirement, respectively, for two varieties of winter wheat: Roshan and Ghods, with three replications in an experimental farm of Birjand University for 1384-85 period. Based on results, the mean relative error of the model in yield prediction for Roshan and Ghods were obtained 9.2 and 26.1%, respectively. The maximum error of yield prediction in both of the Roshan and Ghods varieties, were obtained for S1I1, S2I1 and S3I1 treatments. The relative error of Roshan yield prediction for S1I1, S2I1 and S3I1 were calculated 20.0, 28.1 and 26.6%, respectively and for Ghods variety were calculated 61, 94.5 and 99.9%, respectively, that indicated a significant over estimate error under higher water stress. The mean relative error of model for all treatments, in prediction of soil water depletion and electrical conductivity of soil saturation extract, were calculated 7.1 and 5.8%, respectively, that indicated proper accuracy of model in prediction of soil water content and soil salinity.
H. Ahmadi; V.R. Verdinejad; M. Hemati
Abstract
The main objective of this research was to find the stable side slope of irrigation canals, considering type of subgrade materials as well as canal depth. For this purpose two methods of limit equilibrium and finite element methods were used for stability analysis. In respect to the geotechnical properties ...
Read More
The main objective of this research was to find the stable side slope of irrigation canals, considering type of subgrade materials as well as canal depth. For this purpose two methods of limit equilibrium and finite element methods were used for stability analysis. In respect to the geotechnical properties of subgrade materials, the Unified Soil Classification System as well as the technical properties of each soil group such as cohesion, angle of internal friction and unit weight was used based on the recommendations given in the literature. To consider operation conditions, three cases namely: steady seepage, rapid drawdown and end of construction of canal were taken in to consideration. The results of investigation showed that stability of side slopes in canals is a function of type of subgrade material in respect to its cohesiveness, operation conditions and depth. In non-cohesive materials, the safety factor is not related to the canal depth and is normally less. Based on the mentioned functions safety factor of side slopes of canals with depth up to 8 meters were calculated. For non-cohesive soils stable slopes were about 1:0.6 and for cohesive soils it was determinate around 1:3.
