Review Report

Title: Modelling groundwater recharge, actual evaporation and transpiration in semi-arid sites of the Lake Chad Basin: The role of soil and vegetation on groundwater recharge

Author(s): Christoph Neukum et al.

MS No.: hess-2022-319

General Comments

In this research, the estimation of Evapo-transpiration (ET) and groundwater recharge in Chad Lake Basin (CLB) has been done using unsaturated zone studies and modelling approach. In this regards, the authors collected soil samples from six boreholes and measured grain sizes (soil texture), water content and chloride concentrations. In addition they used climatic data (precipitation amount and Cl content) and vegetation cover characteristics to calculate the ET by a dual-crop coefficient method (Kc=Kcb+Ke). Hydrus-1D software was used to model unsaturated flow and transport and then to simulate the groundwater recharge and separated evaporation and transpiration values.

In my opinion, the structure of the manuscript is fairly appropriate as it generally represents a good example of unsaturated zone modelling. Although the results are highly site-specific, the collected data and modelling approach could be interesting for the readers of the HESS Journal.

Specific comments    

• More explanations are needed about the criteria for selecting the sites (soil profiles) in CLB as they are so close and limited. Regarding the extensive area of the CLB, are the selected sites representative of the region? Is it possible for upscaling the results from these limited sites to the whole CLB? What is the recommended strategy for upscaling results in CLB as a whole?
• Why the bulk densities were not measured in the field? (Line 169)
• Regarding the uncertainties inherited with the modeling approaches especially in unsaturated zone with more limited and unknown data, how do you confirm the modeling results on simulated ET and groundwater recharge values?
• In the case of groundwater recharge you need to verify the modelling results by presenting the groundwater hydrographs and show any consistency between the recharge time series and water table fluctuations and then confirm the reliability of the method and results.
• Please explain in the text why you used the both flow and transport modelling for estimation of ET and groundwater? Regarding the higher uncertainties in transport models, the basis for implementing transport model needs to be clarified as it was possible to estimate both ET and groundwater recharge by a flow model, only.     

Technical corrections

Line 1: in the title “actual evaporation and transpiration” is better to be replaced as “actual evapo-transpiration”.

Line 74: check the English “Pedotransfer functions (PTF) bridge available and needed data and are frequently used to”.

Line 108-109: The sentence is redundant, better to be deleted.

Line 121: ST1 has not shown on Fig.1.

Line 331: The figure caption (Fig. S1.) needs more clarification. You need to explain the abbreviations.

Line 390: “actual evapotranspiration is comparable to or higher than ET0”!! Eta>ET0 is possible?

Line 625: Figure 2 is not suitable. It’s better to be displayed as separate time series.

Line 63: Please add a box at the end of flowchart in Figure 4 to show that what are you looking for with this methodology?

Line 656: The notations used in table 1 (ST1, ST2 and ST3) are not consistent with Figure 1 (S1, S2, S3).

This paper aims at evaluating groundwater recharge in a semi-arid area, the Lake Chad Basin, which is an important and difficult task. The authors use soil water contents and chloride concentrations measured in the unsaturated zone and a 1D-model to simulate water flows and chloride contents. The approach Is repeated at six locations over the catchment and allows for the estimation of ET and groundwater recharge along 15 years. This work shows that the interannual variability of groundwater recharge is first controlled by soil texture and vegetation, with lower recharge variability in coarse soils with grass cover. It also nicely shows different chloride retention in soils.

The paper is an interesting case study of unsaturated zone flux modelling but some improvements and clarifications in the manuscript are required. Following are my comments :

1. In the abstract, the authors say that it is a generalized approach. However in the example given here, it actually seems very localized and site-specific. The results are different between each soil, which suggest that we would need a large number of soil profiles to estimate recharge over the catchment. Are the results obtained generalizable? Are the soils and vegetation types studied here covering all expected soils and vegetation types of the LCB? How do the authors extrapolate the local recharge estimation to an average recharge rate?

2. The introduction should be clarified. In particular, a clear presentation of the objective should arrive early in the introduction as a number of different methods are detailed, but their advantages and limits in regards of the objectives of the present study are not clear.

To better organize the introduction, I would recommend to first present recharge estimates and the factor controlling it in semi-arid regions (l.78 to 90), then focus on the case of the LCB (l.30-48) and highlight what is missing and requires further work (objective of the present paper). In a second part of the introduction, I recommend to gather all descriptions of the existing methods to evaluate the unknown variables on the LCB (recharge, evaporation and transpiration), with their potential and limits of application in the case of scares-data catchments such as the LCB. In particular, the benefit of using both chloride and water contents should be pointed.

3. Extreme precipitation events are very important recharge processes in semi-arid regions, which is not taken into account here. Instead of applying the same precipitation rate all days of a month, how would the result be different if irregular precipitation rates were applied with extreme precipitation events?

4. What is the depth of the water table at each soil location ? Information such as the thickness of the unsaturated zone at each site are missing. It seems to me that the study is restricted to the first meters of the unsaturated zone, while in this area it can reach up to 30m. I am wondering if the depth of the unsaturated zone investigated here is sufficent to get representative estimates of recharge in the unsaturated zone. I guess the underlying assumption is that there is no ET below the a few meters. If I am correct, the assumption should be clearly stated and discussed. Furthermore, even if water contents and chloride concentrations data are not available deeper, simulations could be run at greater depth.

5. Please give possible explanations for the discrepancies between simulated and modeled chloride dynamics for ST1 and ST2.

6. Results on chloride accumulation and retention in soils are very interesting and additionnal calculations would be interesting. For each profile, what is the mass and mean residence time of chloride stored in soils? What is the concentration of chloride at the bottom of the unsaturated zone? How does it correlate to concentrations measured in groundwater?

7. Is there groundwater data (both chloride concentrations and water levels) that could be used to validate the recharge anf chloride fluxes estimates?

8. The value of chloride wet deposition of 1.8 ± 0.5 kg ha-1 could also be compared with the value of 1 ± 0.5 kg ha-1. estimated in Bouchez et al., 2019. And the recharge estimated in the present study could also be compared to the recharge estimates in Bouchez et al., 2019 at different locations in the catchement (16 to 240 mm/year).