## 4 Discussion  
### How to Influence the Soil Water Content in the Soil Profile  
Root distribution is crucial for accurately modeling hydrological processes and vegetation growth, particularly for deep-rooted species [29; 70]. The sensitivity experiments highlighted the importance of soil stratification and root distribution in accurately simulating the soil water content. Similarly, a previous study also found the importance of vertical discretization in the soil column for the soil water content [66]. In the former experiment, the depth of the infiltration wetting front was less than 5 m in both Scheme 1 and Scheme 2. However, Scheme 2 had higher infiltration and lower surface runoff, leading to a higher soil water content in the upper 5 m. Consequently, the annual transpiration under Scheme 2 (239.7-301.4 mm) was consistently higher than that of Scheme 1, with increases ranging from 3.8% to 16.3%. In the deeper 5-15 m, the soil water content was lower because of deep water uptake, with Scheme 1 showing relatively lower values because of greater water uptake. Our findings suggest that the improved model performed well in simulating the soil water content when the root zone was uniformly divided into 0.1 m soil layers.  
In the latter experiment, the differences in the soil water content were primarily driven by variability in root water uptake (Figure 9). As parameter \(a\) increased, water uptake in the deeper layers (5-12.4 m) declined, while water uptake within the upper 5 m became a non-linear function of parameter \(a\). Lower values of parameter \(a\) lead to greater water uptake in deeper soil layers. Previous studies have shown that _R. pseudoacacia_ exhibits an exponential root distribution pattern [34; 35; 42; 59]. In this study, 90% of the fine roots were concentrated within the upper 2 m of soil, indicating that a relatively large parameter value should be used in model simulations. The annual average transpiration increased from 260.2 mm to 268.6 mm as parameter \(a\) rose to 0.5. However, when \(a\) exceeded 0.5, the annual average transpiration began to decline, reaching a minimum of 243.9 mm (Figure 9e).  
A previous study found that about 81% of the water lost through transpiration by _R. pseudoacacia_ was sourced from the top 0-2 m of soil during the growing season [35]. Therefore, accurately simulating the soil water content in the upper 5 m is sufficient to capture the major influence of root water uptake. Although the improved Noah-MP model extended the hydrological active soil profile to \(\sim\)130 m, the physical realism of water movement at such depths requires further discussion. Groundwater tables in the Loess Plateau are typically very deep--often exceeding 50 m--making direct groundwater uptake by plant roots highly unlikely. As such, our simulations focused on the unsaturated zone in the upper 20 m, which serves as a long-term water reservoir recharged by episodic infiltration events. Compared to the default drainage scheme, the improved model showed larger differences in soil moisture in the deep 100-130 m layers. Under the default scheme, excessive subsurface runoff leads to a persistently unsaturated profile, highlighting the absence of a groundwater table in the simulation. A previous study that incorporated actual soil thickness into CLM4.5 at the Changwu station found that only monthly precipitation events exceeding 230 mm could recharge soil water deeper than 75 m in deep-rooted apple trees [67]. In our simulations, only five months during the study period had precipitation exceeding 200 mm, suggesting that groundwater recharge is rare. Therefore, the contribution of groundwater via capillary rise is negligible in this region and was not considered in the model. Future work should integrate in situ observations of deep soil moisture across the entire profile and investigate the coupling between capillary fluxes, water table dynamics, and deep soil water availability to improve the physical representation of deep soil processes in land surface models.