A. Castillo, F. Castelli, and D. Entekhabi, Gravitational and capillary soil moisture dynamics for distributed hydrologic models
Review by Andrew J. Guswa, email@example.com
In this paper, the authors present a proof-of-concept for using a zero-D dual-porosity soil-moisture model in lieu of the 1D Richards equation to represent hydrologic fluxes in the vadose zone. The new representation is implemented in the MOBIDIC model. The authors compare predictions of soil-moisture (integrated over the top 50-cm) from a calibrated version of the MOBIDIC model to predictions from a calibrated SHAW model for two sites: one semi-arid (Arizona) and one sub-humid (Mississippi). The paper shows that predictions of the temporal dynamics of average soil-moisture can be adequately captured by either model.
In my original review of this manuscript, I found that the two case studies provided a reasonable demonstration of the ability of the simplified model to represent the soil-moisture dynamics as predicted by the SHAW model. I had two primary comments:
1. that the authors also include a performance comparison with a bucket-type model of the vadose zone to demonstrate the conditions under which the dual-porosity model is warranted (also suggested by reviewer 3)
2. that the authors clarify the description of the equations that represent the soil-moisture dynamics in the MOBIDIC model
The authors have elected not to pursue my first suggestion. I find this puzzling as the effort required would not be much, and the results have the potential to greatly increase the scientific impact of the work. Instead, the authors articulate that the comparison is not made “because of the advantages and merits of the dual-pore structure as discussed,” such as that hydrologic processes can act “separately on the dual reservoirs.” This may indeed be an advantage, but my point is that it has not been demonstrated as such – only claimed.
Thus, I still think the paper would be strengthened with a comparison against a bucket-type model. Even if the bucket and the dual-porosity models gave the same results for soil-moisture dynamics, I think that would be valuable information. In such as case, the argument for the dual-porosity model could be made on the basis of other characteristics, such as modeling for transport and mixing – for example for stable isotopes.
I also found the last sentence of the added paragraph on p. 7, which claims that bucket models do not capture hysteresis, a bit odd. I do not see bucket models as fundamentally different from the dual-porosity model in this regard. The inclusion of hysteretic or non-unique behavior simply requires an inclusion of history-dependence, which could be incorporated in either type of model.
With respect to my second suggestion, I found the paper to be improved with respect to the articulation of the equations, but I still think the clarity could be improved. In particular, my recommendation is to present the balance equations for the water stores, Wg and Wc in particular, followed by the expressions for the flux terms. E.g.,
dWg/dt = I1 + I2 – Rr + QL,up – QL,down – Qper – Qas
By including only the expressions for the flux terms, I think clarity suffers. The authors have added an expression for the updated storage in the gravity reservoir (Wgu), which helps with respect to Qper. The expression for QL, however, needs to be revisited – should that depend on Wg or Wgu? Also, one of the Qas terms in eq 7 should be Qper.
Lastly, a minor point, the authors refer to the dual-reservoir model in MOBIDIC as being 1D. In fact, it is a dual-reservoir model (zero-D). There is only one soil layer – there is no spatial discretization in any dimension.