Study region: 15 catchments in the Chinese Loess Plateau (CLP) Study focus: In catchment-scale hydrological processes, both precipitation and potential evapotranspiration play dominant roles, while all relevant factors from the land surface to the atmosphere are coupled together to play a regulating role. The Budyko-based and orthogonality-based graphical attribution methods (BGA and OGA, respectively) visually separate the direct effect of dryness index (& empty;) and the regulating effect of land-atmosphere system. This study identified the limitations of OGA in its path section and accuracy, explored its essential difference from the wellestablished BGA, and proposed a more appropriate graphical attribution method in the waterenergy partitioning (WEP) space. Finally, the proposed method was applied to attribute runoff change for 15 CLP catchments. New hydrological insights for the region: The orthogonality-induced linearization in OGA may fail to capture nonlinear Budyko relationships, potentially leading to violations of the water and energy boundaries. By mapping Budyko curves in the WEP space, we elucidated the inconsistencies between OGA and BGA in both decomposition orders and paths, and proposed the two-path BGA as a more appropriate attribution framework in the WEP space. The improved method uncovered that, on average, the direct effect of & empty; and the regulating effect of land-atmosphere system contributed 18 % and 82 %, respectively, to runoff change for 15 CLP catchments. This study contributes to the advancement of hydrological change attribution methods.

Study region: The loess tableland region, China. Study focus: A decade-long (2014-2023) investigation assessed soil water content (SWC) dynamics across four rainfed farmlands (Alfalfa field, AF; Fertilized cropland, FC; Unfertilized cropland, UC; Fallow field, FF) to clarify post-drought soil water replenishment. Using the Standardized Precipitation Index (SPI), three extreme drought-rewetting events (WE1: 93.6 mm, WE2: 47.4 mm, WE3: 131.2 mm) representing precipitation gradients were identified. New hydrological insights for the region: Soil water response varied significantly with land use and rainfall intensity. Recharge depths differed notably: during WE1, the depths reached 100 cm for AF, FF, and UC, but only 50 cm for FC, with corresponding Delta SWC increments of 5.3%, 6.4%, 8.1%, and 3.7%, respectively. In WE2, the depths for AF, FF, and UC reached 120 cm versus FC's 100 cm. During WE3, FF achieved 260 cm, while AF and FC remained at 100 cm. Precipitation storage efficiency (PSE) peaked in FF (WE1, 48.2%) and AF (WE2, 80.9%; WE3, 56.3%), showing a nonlinear relationship with precipitation linked to antecedent soil water content (ASWC). Daily precipitation and duration governed wetting duration and front uniformity; FC exhibited the longest duration (12 days) under low-intensity, long-duration rainfall. These findings elucidate soil water dynamics during extreme climate events, aiding disaster forecasting in rain-fed agricultural regions.

Large-scale vegetation restoration on the Loess Plateau (LP) has significantly increased regional evapotranspiration (ET), raising concerns about water security. However, the net impact on meteorological drought, which balances moisture supply (ET) and atmospheric demand (PET), has remained poorly quantified. To address this, we employed a counterfactual modelling framework for 2001-2022, using the Standardized Evapotranspiration Deficit Index (SEDI) to compare a "Baseline" (actual greening) scenario against a "Fixed Vegetation" (no greening) scenario. A two-step attribution analysis was then used to quantify the contributions from vegetation structure, CO2 physiology, vegetation physiology, and climate drivers. Results show that vegetation restoration was the primary driver of the 23.2 mm decade-1 ET increase. Despite this, meteorological drought was substantially mitigated: the Baseline SEDI showed a wetting trend of 0.21 decade-1, nearly double the 0.11 decade-1 trend in the Fixed Vegetation scenario. This phenomenon can be explained by two mechanisms: (1) at the component level, vegetation's contribution to ET (17.83 mm decade-1) was almost twice its opposing contribution to PET (9.02 mm decade-1), leading to a net reduction in the atmospheric water deficit; and (2) at the driver level, the wetting trend in SEDI (0.21 decade-1) was mainly driven by favourable climate shifts (0.170 decade-1, 81.4% of the net trend) and reinforced by vegetation structural changes (0.104 decade-1, 49.5%), which together outweighed drying pressures from CO2 physiological (-26.3%) and vegetation physiological (-4.7%) effects. These findings demonstrate that human-led ecological restoration, while increasing total water consumption, has successfully alleviated atmospheric drought. This provides critical scientific evidence for the climate-resilience benefits of the 'Grain for Green' program and offers insights for sustainable water management in other water-limited regions.

The generalized proportionality hypothesis (GPH) highlights the competitive relationships among hydrological components as precipitation (P) transforms into runoff (Q) and evapotranspiration (E), providing a novel perspective on E partitioning that differs from the traditional physical source-based approach. To achieve sequential partitioning of E into initial (Ei) and continuing (Ec) evapotranspiration under the GPH, a P-Q relationship-based Ei estimation method was proposed for the Model Parameter Estimation Experiment (MOPEX) catchments. On this basis, we analyzed the relationship between the GPH-based E components and the physical source-based ones separated by the Penman-Monteith-Mu algorithm. Additionally, we explored the differences between the calculated and inverse Budyko-WT model parameter (Ei/E) and discussed the implications for the Budyko framework. The results showed the following: (1) A significant linear P-Q relationship (p < 0.05) prevailed in the MOPEX catchments, providing a robust data foundation for Ei estimation. Across the MOPEX catchments, Ei and Ec contributed 73% and 27% of total E, respectively. (2) The combined proportion of evaporation from canopy interception and wet soil averaged about 25%, and it was much lower than that of Ei, indicating that it was difficult to establish a connection between Ei and the physical source-based E components. (3) The potential evapotranspiration (EP) satisfying the Budyko-WT model was strictly constrained by the GPH, while the inappropriate EP estimation method largely explained the discrepancy between the calculated and inverse Ei/E. This study deepens the knowledge of the sequential partitioning of E components, uncovers the discrepancies between different E partitioning frameworks, and provides new insights into the characterization of key variables in Budyko models.

Abstract. Potential evapotranspiration (EP) is one of input variables in the Budyko framework, yet the diverse estimation methods cause discrepancies in its values. This raises a question about whether there exists a kind of EP specially satisfying the Budyko framework. Based on the relationships among variables in the Budyko models and the deterministic value of EP with known mean annual precipitation and runoff, we uncover the characteristics of EP and its estimation method from hydrological observation in the Budyko framework. Accordingly, we introduce the concept of Budyko EP. The non-parametric and parametric Budyko equations correspond to the reference and the adjustable Budyko EP, respectively. For the Model Parameter Estimation Experiment catchments, the reference Budyko EP is higher in the central and southern contiguous United States and lower in the northeastern and northwestern regions. The linear conversion functions are established from the meteorological EP to the reference and optimized adjustable Budyko EP separately. When estimating actual evapotranspiration (E) by Budyko models with the same data resources, employing two conversion functions with the meteorological EP reduces the mean absolute error of E estimation by 33 % and 35 %, respectively, compared to using the optimized Budyko model parameter with the meteorological EP. Further investigation suggests that the complementary relationship for evapotranspiration is one factor affecting the expression of the region-specific conversion function. Future in-depth exploration of the spatiotemporal differences in conversion functions will advance E estimation and the applications of Budyko EP.

As a proxy for atmospheric evaporative demand, potential evapotranspiration (EP) is usually estimated by meteorologic elements such as radiation, temperature, and vapor pressure. We investigate the controlling factors of EP from the perspective of catchment water balance. Through analyzing the relationships and constraint conditions of the variables in the Budyko framework and the generalized proportionality hypothesis (GPH), we demonstrate that the mean annual EP depends on precipitation (P) and runoff (Q) and the information of EP is contained in the water balance process. Further, we propose the Budyko-based and GPH-based hydrological approaches for EP estimation and obtain the hydrological EP for the MOPEX catchments. Significant linear relationships exist between the hydrological EP and the commonly used meteorological EP, i.e., the Penman EP (EP-Pen), Priestley-Taylor EP (EP-PT), and Hargreaves-Samani EP (EP-HS). Specifically, four hydrological EP are more consistent with EP-Pen among three meteorological EP, and the Budyko-based hydrological EP are more closely related to meteorological EP than the GPH-based one. This study enriches the EP estimation methods and provides new insight into the catchment water balance from the connection between EP and hydrologic elements. (Supported by Project 41971049 of NSFC).

Study region: 15 catchments in the Chinese Loess Plateau (CLP) Study focus: The concept of the water year has been widely used in analyzing the coupled water- energy balance at the annual scale. However, a comprehensive quantitative investigation about its determination and usability in catchment water balance analysis remains absent. Leveraging long-term hydrometeorological observations and terrestrial water storage (TWS) data, we ascertained the commencement month of the water year for 15 catchments in the CLP and explored the feasibility of the Budyko framework at the water year scale concerning evapotranspiration (ET) estimation and its change attribution. New hydrological insights for the region: (1) TWS recharge commenced predominately in July, with the minimal TWS variation (Delta S) consistently observed for years starting from July. Consequently, July marked the start of the water year for CLP catchments. (2) Disregarding Delta S at the calendar year scale caused significant errors in estimating ET (MAE = 51.16 mm yr-1 ), while the MAE decreased by 35.40% at the water year scale. (3) The relative contributions of precipitation (P), potential evapotranspiration (ET0), and w to ET change deviated significantly from the actual values when ignoring Delta S at the calendar year scale. Conversely, these contributions aligned closely with actual values at the water year scale. Overall, the improved accuracy in ET estimation and enhanced consistency in ET change attribution results collectively justify the water year temporal framework.

Study region: The loess tableland-gully region, China. Study focus: The Grain for Green Program (GFGP) has been implemented on the Loess Plateau for over 20 years, and the changes of soil water resources are widely concerned. Based on measurements of soil water content (0-6 m) in 2003, 2011, and 2024, this study investigates the changing trend and recharge characteristics of soil water under different land uses. New hydrological insights for the region: Soil water recharge depth reached to 2.4-2.6 m in forestlands, and 4.2-4.6 m in both wheat fields and grassland in a wet year with a 6-year return period. Soil water content (SWC) was consistently in a severe deficit status in the root zone below the recharge depth in forestland after 14 years old, but did not show a continuous decreasing trend, which reflected a balance between forest water consumption and precipitation infiltration. As the orchards were converted to cropland or grassland, the SWC gradually increased with rainfall infiltration and was back to be close to the field capacity in 3-6 m soil depths after 4-6 years. The GFGP has not caused a severe soil water crisis under a reasonable land use structure. The limited and variable precipitation should be maximally intercepted and infiltrated into the soil, so both the rainfed agricultural production and the forest-grassland vegetation can achieve sustainable development in the Loess Plateau under the carrying capacity of precipitation resources.

Land-surface evapotranspiration (ET) is a major component of the hydrologic cycle. It is a very attractive approach to estimate land surface ET by means of complementary relationship (CR). After 60 years of continuous exploration, the CR has developed from linear relationships to the present nonlinear ones. There are usually four boundary conditions (BCs) for the nonlinear CR, among which the first-order one in completely wet environments (dy/dxx=1) has been a debatable issue, including both the difference in values of dy/dxx=1, and the divergence in definitions of the independent variable x. It has always been a problem how to consider the advection effect in CR. The effect degree of advection from outside the region varies in the ET process at different spatial scales. In this paper, x denotes the ratio of equilibrium ET (ETe) to apparent potential ET (ETpa), y denotes the ratio of ET to ETpa, and x=1 is set as the benchmark with ETe as the lower limit of ETpa. According to the characteristics of ET processes at different spatial scales, we extend the value range of dy/dxx=1, and take dy/dxx=1=k (k≥0) to establish the generalized BC. The generalized CR model for ET is then proposed by using an exponential function, expressed as y=EXP[k/d(1-1/x^d)] (denoted by GCR-EXP; d>0), where k and d are model parameters. k is equal to 2 in the absence of advection, which is the most complementary case. When k < 2, warm advection plays a role, and the value of k gradually decreases as the advection influence increases. Brutsaert (2015) considered the effect of minimal advection, and used the potential ET (Priestley and Taylor,1972) as ET’s constraint to determine the first-order BC in completely wet environments for the polynomial model of CR, which is a case that fits quite well with a large number of observed data. When k = 0, the CR is no longer valid, and the ET is always equal to ETpa, which reflects the ET of a small wet surface. When 0≤x≤xmin, y is equal or approximately equal to 0. xmin and Priestley-Taylor coefficient α can be determined by the values of x at y close to 0 and to 1 in GCR-EXP model, respectively. For instance, the value of x at y=0.001 can be taken as the value of xmin. k reflects advection effects and the corresponding degrees of CR. Moreover, the GCR models, which satisfy the four BCs including dy/dxx=1=k, can be also expressed as a power-exponential function form and other ones besides the proposed exponential one (Supported by Project 41971049 of NSFC).

The water table fluctuation (WTF) method is popular for groundwater recharge (GR) estimation, but its accuracy is challenged when applied in areas with thick vadose zones because of the signal lag and attenuation with depth and uncertainties from barometric pressure effect and lateral flow. Improvement of the WTF method used the linear regression method and Darcy's law, and has been assessed to give satisfactory results. In particular, the improved method presented lower GR (20-34%) relative to the conventional method. GR decreased from the centre to the edge of the tableland. The regional average GR was 63-81 mm year-1, equivalent to 11-14% of annual average rainfall. Lag times between recharge and rainfall ranged from 1 to 9 months. Rainfall and vegetation dominated the spatiotemporal variability of GR. Our study provides reference and technical support for GR estimation with the WTF method in regions with a thick vadose zone.

Understanding the relationship between water and nitrate in soils is critical to agricultural production and aquifer vulnerability assessments, especially in regions with intensive agricultural activities and thick unsaturated zones. Deep soils usually record long-term variations in coupling water and nitrate processes, influenced by multiple environmental factors. As such, this study aims to investigate water and nitrate-nitrogen (NO3--N) contents and their relationships in soil profiles of 13-23 m deep under farmland and neighboring apple orchards with different ages. Soil properties were considered to explore the individual effect of a single factor or combined effects of multiple factors by wavelet analysis and partial least squares path models. Compared with farmland, apple orchards had large water deficit and NO3--N accumulation due to root water uptake and synthetic fertilizer inputs, with high spatiotemporal asynchrony in deep unsaturated zones. Specifically, water deficit and NO3--N accumulation were respectively observed in relatively stable (5-15 m) and active layers (0-5 m, spatial asynchrony), with higher water deficit rates under young apple trees (e.g., 12 and 22 years old) but larger NO3--N accumulation rates under old apple trees (e.g., 23-32 years old, temporal asynchrony), respectively. Under farmland and orchard, soil texture-organic carbon regulated soil water movement, while soil water-organic carbon affected nitrate transport and transformation processes, which further altered soil pH and electric conductivity conditions in deep vadose zones. The growth of apple trees leads to asynchronous water depletion and fertilizer accumulation in soils, and the > 20 years apple trees are limited by insufficient water despite the excessive fertilizer. The dried soil layers not only reduce potential groundwater recharge, but also prolong the residence time of nitrate bombs in thick unsaturated zones. This study provides essential information for sustainable management of vegetation and water resources, and benefits the complicated hydrological and biogeochemical cycles in thick vadose zones.

With the vigorous promotion of apple tree planting on the Chinese Loess Plateau (CLP), a mosaic pattern of apple orchards and traditional farmland has emerged in the region. However, the interaction between adjacent farmland and apple orchards in terms of soil water and nutrient migration remains unclear. This study aimed to investigate the spatial distribution of the soil water content (SWC) and soil nutrients, as well as their interactions, in 0-10 m soil layers of apple orchards of different stand ages and adjacent farmland under the orchard-farmland land use pattern. The results revealed a significant decrease in average-depth SWC in the orchards in the 0-10 m soil profile (R-2 = 0.90) with increasing orchard age, accompanied by a significant increase in soil water deficit (SWD, R-2 = 0.94). The annual growth rate of SWD was found to be as high as 5.94 mm center dot m(-1)center dot y(-1). Deep soil and surrounding farmland were identified as important sources of soil water for apple tree growth, particularly in older orchards; a 25-year-old orchard absorbed approximately 10.8 % of soil water towards farmland with a horizontal distance of 7 m. The distribution of soil organic carbon (SOC) and total nitrogen (TN) in the 0-10 m soil profile exhibited a similar pattern, decreasing and then stabilizing with increasing depth. However, the peak depth and accumulation of soil nitrate (NO3--N) significantly increased with orchard age. The migration and accumulation of NO3--N in the soil provided a scientific basis for the transport of soil water, with NO3--N accumulation (405.32 mg kg(-1)) observed in the deep soil (3-6 m) of farmland located 5 m away from a 30-year-old orchard, confirming the exchange of soil water and nutrients between the orchard and the farmland. The stand age of the orchards was identified as the main environmental factor influencing soil water transport under the orchard-farmland land use pattern. Overall, our findings provide valuable insights into the interaction between apple orchards and farmland under the mosaic pattern, which can contribute to a more rational layout of apple orchards and farmland in the future, promoting the efficient utilization of soil water and nutrients on the CLP.

Diabetes is a global public health problem that is easy to ignore and difficult to detect early. Here we introduce a photonic crystal hydrogels microneedle (PCHMN) patch with a core -shell structure for painless, in -situ and visual monitoring of glucose concentration. The microneedles (MN) have a high specific surface area (tip height: 1600 mu m, base width: 500 mu m), high mechanical toughness (tensile strength:10.49 MPa, breaking elongation: 11%), and high load (0.09 N per needle) to achieve painless penetration of the skin stratum corneum. Based on the deprotonation of the carboxyl group of the polymer after hydrolysis, the microneedle patch is responsive to pH and achieves a response to glucose due to the specific binding of the phenylboronic acid group to the glucose molecule in the polymer chain segment, and performs well in the detection application in mice. Because these response processes can lead to changes in the swelling degree and volume of microneedles, and then produce changes in structural color and reflection spectrum, so as to realize the monitoring of the above detection substances. These characteristics indicate that the microneedle patch prepared by us can be customized to design various wearable sensing devices for painless and in situ monitoring of various biomarkers in tissue fluid by simply changing the types of response groups.

Studies on the spatiotemporal dynamics in ecosystem carbon and water exchanges are essential in predicting the effects of climate change on regional carbon and energy budgets. Using the eddy covariance technique, carbon and water fluxes were observed in a typical winter wheat ecosystem (WWE) and an agroforest ecosystem (AFE) in the southern Loess Plateau from 2004 to 2010. The seasonal and inter-annual variability in gross primary productivity (GPP), net ecosystem exchange (NEE), evapotranspiration (ET), and water use efficiency (WUE) were examined and the main influencing factors were identified using the Pearson correlation. The results indicate that the seasonal GPP and NEE showed a bimodal distribution in WWE, while this was unimodal in AFE. The sinusoidal function did well in the characterization of seasonal ET dynamics for both ecosystems, with the determination coefficients being 0.85 and 0.94, respectively. In WWE and AFE, the annual mean GPP were 724.33 and 723.08 g C m−2 a−1, respectively, and the corresponding ET were 392.22 and 410.02 mm a−1. However, the difference in NEE between the two ecosystems was obvious, NEE were −446.28 and −549.08 g C m−2 a−1, respectively, showing a stronger carbon sink in AFE. There were strong coupling relationships between the GPP and ET of both ecosystems; the overall slopes were 1.71 and 1.69, respectively. The seasonal trend of WUE was bimodal in WWE, with peak values of 3.94 and 3.65 g C kg−1 H2O, occurring in November and April, respectively. However, the monthly WUE in AFE had one single peak of 4.07 g C kg−1 H2O in January. Photosynthetically active radiation (PAR) and soil temperature (Ts) were most positively correlated with GPP, net radiation (Rn) and Ts were the major factors influencing ET, while vapor pressure deficit (VPD) and soil water content (SWC) were the major influencing factors for WUE. These results provide observational support for regional carbon neutrality simulations.

The Chinese Loess Plateau has undergone extensive revegetation to restore degraded land and enhance carbon sequestration. However, soil organic carbon (SOC) sequestrated in the soil profiles of deep-rooted plants has not been fully studied. Here, we investigated the SOC within a 0–23 m profile in farmlands and apple orchards converted from farmlands with different ages (A5, <5 years; A10, ~10 years; A15, ~15 years; A20, >20 years) and the controlling factors on three loess tablelands (Changwu, Qingyang, and Luochuan). The results show that SOC stocks among farmlands and orchards showed no significant difference (p = 0.88); however, SOC stocks showed a trend with tree ages, i.e., a decrease for A5 and A10 but an increase for A15 and A20. For the vertical variability, the SOC stock was the highest within 0–1 m, regardless of the standing age; however, the SOC stock in this layer only accounted for 8.8% of the total SOC stock (97.93 ± 9.18 kg m−2). Climate accounted for 82% of the variations and controlled the changes in SOC in the 0–1 m range, while soil texture dominated the SOC in the soil below 1 m, accounting for 57% of the SOC variations. The variations in SOC in the thick, unsaturated zones provide implications for future land use management and the sustainability of apple orchards in arid regions.

The accurate quantification of actual evapotranspiration (ET) is crucial for water resources management in agricultural production and ecological construction. Estimating ET using both rescaled and non-rescaled complementary relationship (CR) models has become a hotspot in the research on terrestrial ET. This study explores the relationship between these two CR models and improves the method for calculating xmin, which is the value of the independent variable x when the dependent variable y is equal to zero in rescaled CR models. The rescaled and non-rescaled CR models can be functionally interconvertible, i.e., the non-rescaled CR model enables the rescaled simulation of ET and the rescaled CR model can also conduct a non-rescaled simulation. The parameter b or c in the non-rescaled CR models plays a role similar to xmin in the rescaled models and vice versa. Based on the data from 15 catchments in the Loess Plateau of China, we validate this relationship between two CR models. Meanwhile, we evaluate the formulation for xmin proposed by Crago et al. (2016) (xmin-d) and the results show that the range of variation for xmin-d values is smaller than that for the xmin values obtained by inverse method from the models (xmin-i) in the interannual process. The mean xmin-i value of RCR-C2016 is larger than that of RCR-S2017, while the mean xmin-d value is in between these two values. The empirical function for xmin is developed using the aridity index (AI) and normalized difference vegetation index (NDVI) as independent variables in the interannual fluctuations. The empirical function for xmin is expressed only using the AI in the spatial variations at a mean annual scale. Cross-validation results show that the rescaled CR models with the empirical functions of xmin can more accurately estimate ET and simulate its interannual and spatial changes.

[Objective] To investigate the spatial distribution and cooperative utilization characteristics of soil water under the orchard-farmland mosaic pattern in the southern Loess Tableland. [Methods] The 10-year-old, 21-year-old, and 25-year-old apple orchards (AO10, AO21, and AO25) and their adjacent farmland were selected in Changwu tableland were selected to quantitatively calculate the contribution of soil water storage to water consumption of orchard. By measuring the soil water content at relevant sites in the orchard-farmland interface zone after the rainy season in 2021. [Results] The precipitation in 2021 is 756 mm, which was a typical wet year. The rainfall infiltration depth of farmland and AO21, AO25 orchards reached 8.4, 7.0, and 5.0 m at the end of November, respectively. AO10 orchard-farmland boundary zone is 4 m deep, the soil water content in the lower part is larger than that in the upper part, and the average soil water content was 25.5% in the 4—10 m soil layer. In AO21 orchard, the average soil water content of the 0—7 m soil layer was 22.1%, and that of the 7—10 m soil layer was 15.0%. the average soil water content of the 0—5 m soil layer of AO25 orchard was 20.9%, and that of 5—10 m was 13.6%. The soil dry layer of AO21 and AO25 orchard was below 7.0 and 5.0 m, respectively. In the horizontal direction, the distance of soil water used by AO21 and AO25 orchards from adjacent farmland reached 5 and 8 m, respectively, and the water supply from fields to orchards at the agro-fruit interface was 0.08 and 0.25 m3/m2 when the soil profile was divided by the upper boundary of the dry layer. Below is the actual water supply, which is 0.45 and 0.81 m3/m2, respectively. [Conclusion] The mosaic layout of apple orchards and farmland in the Loess tableland region is a reasonable utilization structure, and factors such as the age of apple trees and the width of adjacent farmland should be considered in land planning and management. The research results will contribute to promote the sustainable utilization and spatial optimization of regional soil water resources.

The water and organic carbon stored within unsaturated zones are crucial in terrestrial ecosystems; however, the coupling effects of deep soil water and organic carbon are still poorly documented. To explore the water-carbon coupling effects within soils under tree plantations, we investigated the water and organic carbon content to 20 m deep under cultivated farmland and orchards with apple trees of varying ages (5, 10, 15, 20, and 24 years) in the loess-covered region of China. A conditional process model was used to explore the impacts of vegetation and edaphic factors on the water-carbon coupling processes. Compared with farmland, young apple orchards (0-10 years) showed lower water deficit, but the soil water storage reduced by 21%- 32% under old apple orchards (15-24 years). The organic carbon content exhibited negligible variation among apple trees of different ages. However, the water-carbon coupling effects varied with apple tree ages. A positive correlation existed between water storage and organic carbon density under young apple orchards (p < 0.05), but a negative correlation was found between water deficit and organic carbon sequestration under old apple orchards (p < 0.01). The conditional process model suggested that the water-carbon coupling relationship under young apple orchards was primarily controlled by clay content due to lower root biomass, while that under old apple orchards was concurrently shaped by silt content and coarse roots because of the developed root system of apple trees. This study offers novel insights into the sustainable management of agriculture and tree plantations in regions with deep-rooted vegetation and thick unsaturated zones.