| 22 Jun 2026
Structural limits and ill-posedness of soil water storage balance method for diagnosing root water uptake
Abstract. Depth-resolved root water uptake (RWU) can be inferred through soil water storage balance (SWSB) from soil water content observations, yet their fundamental identifiability and robustness remain unclear. Using controlled numerical experiments with prescribed contrasting RWU profiles, we systematically evaluate the performance of SWSB-based RWU inversion under varying spatiotemporal aggregation and upper boundary conditions. Our results show that while accurate estimates can be obtained under idealized, error-free datasets, even modest uncertainties in soil hydraulic parameters (±10 %) and soil water content measurements (±1 %) lead to error amplification in RWU estimates by around 60-fold. Furthermore, the sensitivity analysis shows that SWSB-based RWU inversion is highly sensitive to soil water content, as it directly influences storage change (ΔV) and regulates the relationships between soil water content (θ) and pressure head (h), as well as saturated (ks) and unsaturated (k) hydraulic conductivities. These findings highlight SWSB-based RWU inversion is fundamentally ill-posed and becomes non-identifiable under realistic uncertainties. Overall, this study delineates the conditions under which SWSB-based RWU inversion can be reliably applied and emphasizes the need for additional independent constraints.
The manuscript provides a systematic and rigorous evaluation of the soil water storage balance method for determining root water uptake (RWU) profiles. Using controlled numerical experiments and global sensitivity analysis, the authors demonstrate that while the method is conceptually appealing due to its simplicity, it is fundamentally ill-posed and lacks robustness. The manuscript is a valuable contribution to the field, offering an important critique of a common hydrological tool. I recommend acceptance after moderate revisions.
General comments
The manuscript is well structured and while the introduction, discussion and conclusion reads very well and clear, the presentation of materials and methods and results need improvement.
My biggest concern is that the forward simulation is not well motivated: It is not clear why the authors choose a forward time step of 1 day or 7 days. The types of soil-water-content sensors are not discussed in the manuscript, but the temporal resolution of most sensor types is between seconds and minutes, so there is no such restriction.
It would strengthen the manuscript to describe experimental set ups relevant for SWSB based RWU.
Section 2.1 should be carefully revised, and presentation of results should be improved (see detailed comments).
Detailed comments
Section 2.1: Please state the units of the relevant quantities explicitly.
L77: Please explain why this assumption is necessary. What would be the consequences if this assumption were not made?
Equation (2): Why is (Q) introduced here? It does not appear to be used subsequently.
Equations (3)–(4): There appears to be an error in these equations, possibly an extra Laplace symbol on the right-hand side. Please check the equations carefully.
L90: The term “drainage flux” may not be appropriate here, since depending on the bottom boundary condition, water could also move upward through the bottom boundary. “Bottom boundary flux” or simply (Q) may be more appropriate. In addition, using (D) for this flux is potentially confusing because (D) is commonly used to denote a diffusion or dispersion coefficient.
Equation (7): Both the cumulative flux and the flux itself are denoted by (D), which is confusing, particularly because they have different units. Please use different symbols for these quantities.
Equations (8)–(9): The variable (k) is not introduced. Please define it. Furthermore, when calculating an effective hydraulic conductivity for flow perpendicular to a sequence of layers, the harmonic mean should be used rather than the arithmetic mean.
Equations (14)–(15): Please explain how (R) can be calculated from measurements. In particular, if (D := D(\theta)), how is (\Delta V) obtained from the measured data?
L124: (D) is referred to here as “interlayer flux,” whereas it was previously described as a drainage flux. Please use consistent terminology throughout.
Figure 1: The red arrow indicating the right-to-left direction appears to be missing. Please check the figure.
Equation (17): Please state the units.
L179: Please rephrase this sentence. No analysis appears to be presented in Section 2.2.2, so the current wording is misleading.
Figure 2: Panels (c) and (d) appear to be superfluous, as their information could be conveyed adequately in the text. The remaining panels are currently too small to interpret easily. I recommend enlarging them and, if necessary, presenting the results over more than one figure.
L236: Is this a typo? Should the labels be 2j, 2p, 2v, and 2bb? More generally, I recommend organizing the subpanels in a more meaningful and intuitive order, potentially across more than one figure. The individual figures and subpanels should also be described in greater detail in the text so that the reader can understand the key findings without having to infer the intended comparison from the figure alone.