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Università di Palermo, Dipartimento ITAF, Viale delle Scienze 90128, Palermo, Italy
gcrescim{at}unipa.it
In response to Dr. Coppola, we would point out first that SWAP is not a model with specific chemistry, and therefore can be used to predict the total salt concentration, without making any distinction between cations and anions.
It was clearly stated in our paper that we applied SWAP to predict the total amount of salts, expressed in terms of ECsat, and not the transport of specific ions: "The objective of this paper was to evaluate the ability of SWAP to accurately predict both 
and ECsat in four profiles of a Sicilian cracking soil" (Crescimanno and Garofalo, 2005, p. 1945).
We disagree with Dr. Coppola's comment: "the measured ECsat results from several physicochemical processes, most of them (such as pure retardation, sorption/desorption, degradation, etc.) are quite important, and they were not accounted for by the authors," because:
In a recent field-scale flow and transport analysis of mixed Na-Ca solutions, Russo et al. (2004) found that: "enhanced soil solution-soil matrix interactions, induced by soil alkalinity and dilution of the soil solution, may ....decrease both the magnitude and the effective retardation factor." These recent findings thereby demonstrate that the effect of retardation at the field-scale could be less significant than sometimes expected.
Papers by Smets et al. (1997), Kelleners et al. (1999), and Tedeschi and Menenti (2002) have also reported applications of SWAP with reference to solutions containing different chemical species using the same form of the advection–dispersion equation (ADE) given by van Dam et al. (1997) as Eq. 3.14.
Instead, we clearly stated in our paper that the process of Na-Ca exchange, not taken into account in the model, could affect prediction of ECsat: "SWAP predicts solute concentration by using the ADE, assuming a constant dispersivity (Ldis) and neglecting the process of cationic exchange. Due to these as well as to other simplifying assumptions, the accuracy of the predicted ECsat in clay soils needs to be checked by comparison with measured ECsat values" (Crescimanno and Garofalo, 2005, p. 1944). In the Results section, we discussed the possibility that some inaccuracies in the predicted ECsat were determined by chemical processes not taken into accounted by the model: "...no systematic errors were associated with the predicted ECsat. However, visual observation of the predicted ECsat (Fig. 6) showed that in some cases the predicted ECsat did not match the temporal evolution of the measurements. It was therefore possible that local non equilibrium conditions existed during the simulation period. This would invalidate some of the assumptions of the ADE equation and cause inaccurate prediction of ECsat. However, the Durbin-Watson statistic (DW) (Table 4) proved that the random errors in the estimated ECsat were independent, excluding internal dependence of errors" (Crescimanno and Garofalo, 2005, p. 1951).
It was therefore clearly stated in our paper that not taking into account the process of cation exchange did not significantly affect prediction of ECsat because we considered a nonsodic soil, irrigated with water having a SAR close to soil ESP: "However, our results were obtained for a nonsodic soil, under a condition of SAR of irrigation water close to the soil ESP, and consequently sodium in the solution and in the exchange complex should be almost in equilibrium. Under this condition, the simplifying assumption that the salts are not adsorbed to soil solids could be considered acceptable. Prediction of ECsat provided by SWAP should be carefully checked when irrigation is performed on sodic soils, or when sodication can be the consequence of using irrigation waters with SAR higher than soil ESP" (Crescimanno and Garofalo, 2005, p. 1952).
Concerning Ldis, which was obtained by calibration, we stated in the conclusions (Crescimanno and Garofalo, 2005, p. 1952): "The narrow range of variation of the calibrated Ldis in four soils having different hydraulic parameters, with an average value of 20 cm for the four considered profiles, seemed to indicate that the calibrated Ldis was a lumped parameter representing the irregularities in the flow pathways. The possibility of using the same Ldis value to predict ECsat for additional soil profiles located in the same area will be further checked." In a following paper (Crescimanno and Garofalo, 2006), we will show that using the Ldis value of 20 cm, accurate predictions of ECsat were obtained for three additional soil profiles without calibration. These results will therefore lead to the conclusion that "the calibration procedure previously adopted to determine Ldis provided an effective dispersion coefficient which reflected the complexities of the flow pathways and heterogeneity in local fluid velocities in the flow direction" (Crescimanno and Garofalo, 2006).
For all these reasons, we cannot agree with Dr. Coppola's comment: "the relatively good agreement between measured and simulated ECsat is largely expected in a calibration context." Instead, the good agreement between measured and predicted ECsat proved that for the conditions examined in our paper, the approach we used was a relatively accurate representation of the physicochemical processes occurring under field conditions.
REFERENCES
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