General comments

This paper presents measurements of topsoil hydraulic properties made in four treatments of a long-term field experiment in California (with/without cover crops, conventional till/no-till) and uses these properties to simulate short-term soil water storage changes following subsurface drip irrigation.

The authors conclude that the no-till + cover crop system improves soil water storage (and presumably also the supply of water to the crop). However, this conclusion does not seem too well supported by the data as many of the differences between the treatments appear to be small. Many times in the paper, the authors report p values >0.05 as being statistically significant, which is not standard practice. I am speculating now, but I can imagine the authors may have been worried that negative results and conclusions would not be considered publishable. In fact, I think that this kind of publication bias may be quite common. If so, it would be unfortunate because it would mean that our consensus view on the efficacy of different management practices to improve soil health could be somewhat biased.

The small number of replicates (4) is an important limitation of this study. Forward modelling based on such limited replication, without the benefit of field monitoring data to calibrate and validate the model is very uncertain. These uncertainties are not discussed by the authors. Field measurements of soil water contents/potentials during and after irrigation would have helped to strengthen the study.

More details on the soil properties and the tillage practices implemented at the site are also needed (points 6-8 under “Specific comments”). The routines in the model related to root water uptake and soil evaporation also need to be better described (see points 10 and 11 under “Specific comments”)

The paper is generally well written and easy to read. There are a number of grammatical errors in addition to the ones noted below under “Technical corrections”, but these will be easy to correct.

Specific comments

1. Line 18, and lines 20-22: these statements appear to be contradictory. First, the authors write that the differences in water storage are marginal, but then they conclude that NT and CC systems improve water retention at the field scale. This should be clarified.
2. Line 44: The authors should write this in a less categorical way, by replacing “have been shown” to “may”, because it is definitely not always the case that reduced or no-till systems increase carbon sequestration in soil (see Meurer et al 2018). This is probably mostly because under some agro-environmental conditions, crop growth is poorer under no-till (Pittelkow et al., 2015), which reduces the carbon inputs to soil.
3. Lines 47-48: Yield certainly is sometimes compromised by no-till (see point 2 above, Pittlekow et al., 2015). Please re-phrase this to acknowledge this fact.
4. Line 60: 900 mm annual rainfall?
5. Lines 61-62: Perhaps you should explain why no-till can exacerbate compaction problems (because the soil is not loosened, but it is still trafficked) … and also that this can impact yields negatively (Pittelkow et al., 2015)?
6. Lines 78-79: it’s only a more descriptive term if no-till really was adopted at the site. This is not fully clear to me. At line 75, you write “reduced disturbance”. It’s important to be clear and explicit about the tillage system. Is it “reduced tillage” or “no-till”? The tillage operations should be described in more detail for both ST and NT, giving information on the time of year, the implements used, and the depths to which they are operating.
7. Lines 81-83: it’s important to mention that the tractor traffic was controlled in this way, but what about harvesters? Presumably this kind of traffic was not controlled in the same way?
8. Lines 84-87: The authors must give more information on the basic soil properties at the site: at the minimum, information is needed on the particle size distribution and organic carbon contents (the latter specified for each treatment, as they presumably differ after 20+ years).
9. Lines 128-129: Why did you calculate unsaturated K at -10 kPa, and not say, at -33 kPa (field capacity)? You should also say how you did this: by fitting to the HYPROP data presumably?
10. Line 174: What does “… a radius of maximum uptake intensity at 0 cm” mean? Not all readers will be familiar with this 3D water uptake model, so this should be explained better.
11. Lines 175-182: this entire section is unclear to me. The authors mention evapotranspiration, but it’s not clear how soil evaporation and transpiration are dealt with individually. For example, the name Feddes is mentioned on line 172, so I presume that the Feddes model is used to calculate actual transpiration from the potential uptake rate, but this should be explicitly stated. But how is soil evaporation reduced below the potential rate (which I presume is given by equation 4) when the soil surface dries out?
12. Line 227 and elsewhere in the paper: the authors refer to results with p values greater than 0.05 as “statistically significant” (or similar phrases). The authors should reserve such a description for results with p<0.05. Instead, write “… a tendency for ..” or something similar.
13. Line 237: “… healthy organic matter cycling” is vague. This should be written more specifically.
14. Lines 246: Jarvis (2007) did not discuss these processes. It would be better to cite the recent review on soil structure dynamics by Meurer et al (2020) here instead.
15. Figure 7. Is the y-axis on the the Kunsat plot correct? 10 to the power of 0.004 is only 1.009. Isn’t the variation in K larger than 1 to 1.009 cm/d?

Technical corrections

1. Lines 45, 47 and 52: terms like “soil fertility” and “environmental quality” are rather vague. Please replace these with terms specific to what was measured in these studies you cite.
2. Line 122: add “of water tensions” after “range”
3. Line 124: Write … “We define field capacity …” (this definition is only conventional in the U.S., not worldwide)
4. Line 125: delete the minus signs prior to 33 and 1500 (you refer to suction)
5. Lines 130-131: this can be deleted, as it is defined in connection to equation 1.
6. Line 163: delete “that of”
7. Line 165: “the van Genuchten-Mualem hydraulic model (van Genuchten, 1980)”
8. Line 171: add… “(S_r in in equation 3)” after “root water uptake”
9. Line 175: insert “An .. ” before “… atmospheric ..”
10. Lines 208-210: this sentence can be deleted (it’s repeating the methods). The authors should refer to figure 4 here, but in a different way … “Figure 4 shows one example of …”
11. Line 227: Replace “A unique …” by “One …”
12. Lines 238-239: this is unclear. Is there text missing here?
13. Line 260: section 0?
14. Line 327: insert “of drainage“ after “days”
15. Line 364: replace “steady-state” by “equilibrium”

References

Meurer, K., Haddaway, N., Bolinder, M., Kätterer, T. 2018. Tillage intensity affects total SOC stocks in boreo-temperate regions only in the topsoil - a systematic review using an ESM approach. Earth-Science Reviews, 177, 613-622.

Meurer, K., et al. 2020. A framework for modelling soil structure dynamics induced by biological activity. Global Change Biology, 26, 5382–5403.

Pittelkow, C., Linquist, B., Lundy, M., Liang, X., van Groenigen, K., Lee, J., van Gestel, N., Six, J., Venterea, R., van Kessel, C. 2015. When does no-till yield more? A global meta-analysis. Field Crops Research, 183, 156-168.

The manuscripts explore the effect of long-term reduced tillage and cover crops on soil hydraulic properties. Based on laboratory samples from a long-term experiment, they derived the water retention curve (WRC) parameters and the K(h). Using these observations, they simulate in Hydrus 2D, the effect of an irrigation even and analyze the dynamic of the water storage that follows.

The fact that the samples came from a long-term experiment with a sound statistical design is a strength of the research. The analysis of the observed hydraulic parameters is also statistically sound. The use of a simulation to access some more dynamic information on the soil profiles is interesting. While I find it less valuable than direct field observations, it remains based on laboratory measurements (K(h) and WRC). As such, it helps to assess the dynamics of water storage in the soil which is a valuable insight for the interpretation even if not confirmed by field measurements. The authors also discuss their results at the light of other works and seem well-informed of the work done in the area.

In general the manuscript is well written, the figures are clear and well-designed with good caption. The errors bars are used to display uncertainty whenever possible. The abstract would gain to be a bit reworked so that the message and the findings of the work came through better. Also in several places, a bit more discussion is needed or further interpretation and hypothesis will deepen the discussion. Also, a discussion about the limitation of the modeling approach is needed.

General comments

The advantage of the simulation and its links with the observation doesn’t come through easily as we read. It’s difficult to see straight from the start why the simulations are needed and what they bring to the story. Maybe it can help to highlight, at the end of the introduction, the limitation of traditional PAW and field capacity to better highlight the need for simulation?

As mention by the authors, tillage can temporarily impact soil hydraulic parameters and soil properties. Potentially doesn’t the growth of the (cover) crops also have an impact on WRC and K(h)? Given the study provides topsoil and subsoil samples, can you discuss more the expected dynamics of soil hydraulic properties expected from each depth through the growing season? A more global discussion on the effect of the sampling depth would be great.

The simulations bring insights into the dynamic water storage of the soil columns. However, I think it should be pointed out that these simulations are not backed up by observations of water content in the soil profile but only based on laboratory derived WRC and K(h).

Specific comments

L14: “an improved structure” -> put it more explicitly: larger proportion of smaller pores?

L15: rephrase more simply: "The conventional measurement of water content at field capacity (water content at -33 kPa suction) and the associated plant available water (PAW) showed that NT and CC plots had lower water content at field capacity and lower PAW compared to standard-till (ST) and plots without cover crop (NO)" -> "NT and CC plots had lower water content at field capacity (-33 kPa suction) and lower plant available water (PAW) compared to standard-till (ST) and plots without cover crop (NO)"

L18: "higher profile-level water storage"? -> what does 'profile-level' mean? maybe rephrase more simply? (after reading the paper, I realized it was referring to the simulated provides but I didn’t get it at first read, maybe drop ‘profile-level’? You’ve already said it was in simulations)

fig1: missing north arrow

L60: < 9000 mm annual rainfall is drier? -> 900 in original paper

L65: 'individual and interactive impact'? -> 'individual impact and its interaction

L74: please also give GPS coordinates

L79: I though no-till was more absolute than reduced tillage. In reduced tillage, as you explain, there is a reduction of tillage operation, but no-till really means no-till no? If its a reduced tillage, maybe RT would be a better acronym. Also, what is the tillage depth? Does the reduced tillage involve a non-inversion tillage? What tools was used for the tillage (rotovator, harrow, chisel plow, moldboard plow) more explanation is needed here even if more details are published in Mitchell et al. previous papers.

L97: “months after tillage” -> how many months about?

fig2a: different colors of layers but no legend, also why a 3D schematic if the modeling is only in 2D, this is confusing, I would do a 2D schematic if only 2D modeling

fig2b: why no flux on the side? If it’s in the field, it should be free flow no? Unless you simulate your cylindrical lysimeter

L188: why the spin-off has only 2.5 irrigation compared to the real simulation?

L196: equivalent water depth (change?) -> not sure I understand this, is it the water in the entire profile or at a specific depth? what does the 'water depth' means?

L205: seems sound. Do you also provide the .R file and the data as open-source attachment to the article?

L238: 'all the treatments had higher relative... compared to ... had the highest proportion' -> rephrase

L260: 'discussed at section 0' -> 'section 3.1?

fig7: why is NT-CC for Ks so much higher while there not such difference for K100cm? Discuss

L300: "Our results showed that while this was the case with ST, it was not the case for NT" -> do you have an explanation/hypothesis?

L301: 'For the subsurface layer of NT treatments, θ FC was significantly lower for the NT-CC compared with NT-NO treatments.' -> could this be that the roots of the CC are reaching deeper down? what is your hypothesis? discuss more

fig10: are these curves for one single plot/simulation? or averaged over all 4 plots for each treatment? Make this clear in the caption.

L344: "(P < 0.15)" -> lower p for p-value (typo)

fig11: could it be plotted the same way as figure 8 with both top and subsoil in the same subplot? that will make comparison easier

figA4 and A5 shows the distribution for a set of 4 plots but you simulate 4*4 in total right? are these plots average per treatment or just one pick among the 4 replicates? Make this clear in caption