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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. [ABSTRACT FROM AUTHOR]

ABSTRACT Potential evapotranspiration represents the maximum evaporative capacity for a region, which underscores the importance of analysing its changes and attribution to provide information for water resource management. This study focused on the northern Loess Plateau of China, investigated the changes in potential evapotranspiration ( ET0) using data from 34 stations during 1960-2013 and conducted its temporal and spatial attribution through the differentiation equation method. The results show that the annual mean ET0 is 1004.9 mm with a decreasing trend at the rate of 0.33 mm year−1. The lowest values were found in the northeast and southwest regions, while the greatest ET0 was in the northwest region. An 'evaporation paradox' existed as a whole, but not obviously. The ET0 was most sensitive to actual vapour pressure ( ea), followed by solar radiation ( Rs), mean temperature ( Tmean) and wind speed ( U2). The inter-annual variation in the sensitivity of ET0 in relation to climatic factors showed that ET0 became more sensitive to U2 and Tmean but less sensitive to Rs and ea from 1960 to 2013. Temporally, the increasing Tmean resulted in the increase of ET0; while its effect was weakened by the other three factors, especially by decreasing U2. The contributions of climatic factors to the ET0 spatial variation trends were also analysed along two transects, but the dominant factors were different for each transect. Furthermore, the possible impacts of human activities on ET0 changes were also discussed. [ABSTRACT FROM AUTHOR]

Vegetation restoration efforts (planting trees and grass) have been effective in controlling soil erosion on the Loess Plateau (NW China). Shifts in land cover result in modifications of soil properties. Yet, whether the hydraulic properties have also been improved by vegetation restoration is still not clear. The objective of this paper was to understand how vegetation restoration alters soil structure and related soil hydraulic properties such as permeability and soil water storage capacity. Three adjacent sites with similar soil texture, soil type, and topography, but different land cover (black locust forest, grassland, and cropland) were selected in a typical small catchment in the middle reaches of the Yellow River (Loess Plateau). Seasonal variation of soil hydraulic properties in topsoil and subsoil were examined. Our study revealed that land-use type had a significant impact on field-saturated, near-saturated hydraulic conductivity, and soil water characteristics. Specifically, conversion from cropland to grass or forests promotes infiltration capacity as a result of increased saturated hydraulic conductivity, air capacity, and macroporosity. Moreover, conversion from cropland to forest tends to promote the creation of mesopores, which increase soil water-storage capacity. Tillage of cropland created temporarily well-structured topsoil but compacted subsoil as indicated by low subsoil saturated hydraulic conductivity, air capacity, and plant-available water capacity. No impact of land cover conversion on unsaturated hydraulic conductivities at suction > 300 cm was found indicating that changes in land cover do not affect functional meso- and microporosity. Our work demonstrates that changes in soil hydraulic properties resulting from soil conservation efforts need to be considered when soil conservation measures shall be implemented in water-limited regions. For ensuring the sustainability of such measures, the impact of soil conversion on water resources and hydrological processes needs to be further investigated. [ABSTRACT FROM AUTHOR]

Soil water has become a major limiting factor in agriculture and forestry development on the Loess Plateau of China. In the past 20–30 years, large areas of apple orchards have been built in this region, which have resulted in excessive consumption of deep soil water and soil desiccation. To evaluate the effects of orchard development on deep soil water content (SWC), a meta-analysis of 162 sampling sites on the loess tableland from 44 peer-reviewed publications was conducted in this study. The results showed that the deep SWC in orchards depended on stand age, planting density and annual precipitation. In regions with 550–600 mm precipitation, the orchard with lower planting density showed no soil desiccation in young and early fruiting stages, while deep soil (>2 m) desiccation occurred in full fruiting and old orchards. The effect of planting density on deep SWC varied with stand age. There were significant differences in SWC among different planting densities in early fruiting orchards (p < 0.05), in which soil desiccation occurred in orchards with higher planting density. However, with the continuous consumption of soil water by apple trees, deep soil desiccation occurred in old orchards regardless of planting density. Further, affected by the spatial variation of annual precipitation, deep SWC in orchards significantly decreased with annual precipitation from 650 to 500 mm among the 44 study sites (p < 0.05). Our results suggest that the planting density should be reasonably regulated on the level of annual precipitation, and apple trees need to be pruned appropriately with a goal of moderate productivity, so as to achieve the sustainable use of regional water resources, food security and economic development. [ABSTRACT FROM AUTHOR]

Studying the groundwater recharge mechanism in regions with thick unsaturated zone can greatly improve our understanding of hydrological processes since these regions have complex groundwater processes. This study attempted to discuss the groundwater recharge in a region covered by loess over 130 m deep in China's Loess Plateau. The water stable isotope, tritium and chloride in precipitation, groundwater and soil water were determined and used as inputs of mass balance methods. The tracer technique is found to be applicable and effective this region with thick unsaturated zone. The groundwater originates from rapid precipitation infiltration through some fast flow paths. The total recharge is likely to be 107 ± 55 mm yr − 1 accounting for 19 ± 10% of average annual precipitation, while the recharge from preferential flow accounts for 87 ± 4% of the total recharge. The identified recharge mechanism has important implication to groundwater management and recharge modeling for regions covered by thick loess. [ABSTRACT FROM AUTHOR]

The Loess Plateau of China is subject to severe water shortages, and the runoff reduction observed in most watersheds exacerbates the problem. Quantifying the contributions of environmental changes to runoff reduction is thus very important for water resources management. Taking the Jing River catchment as the study area, the changes in runoff for the period of 1961–2010 at three gauge stations—the catchment outlet (Zhang Jia Shan, ZJS) and the two others from the upper reach (Yang Jia Ping, YJP, and Yu Luo Ping, YLP) were analyzed in this study. Using the Budyko framework, the spatiotemporal variations of the contributions of precipitation ( P ), potential evapotranspiration ( ET 0 ), and land surface conditions (represented by the parameter n in the Choudhury–Yang equation) to runoff changes were evaluated. A significant downward trend in runoff was detected for the ZJS and YJP stations. The sensitivity of the runoff changes to the different environmental factors considered was different. The sensitivity coefficient was the greatest for P , intermediate for ET 0 , and smallest for the land surface condition ( n ). However, the sensitivity coefficients are becoming greater over time. The decrease in P was the dominant factor in the runoff reduction at the three stations, but its effect was largely offset by the increase in n at YLP. The contribution of land surface alteration to runoff reduction has been increasing in recent years, which indicates that the improvement of vegetation coverage and the construction of terraces and check dams have strengthened their impacts on runoff. Therefore, careful attention should be paid to the hydrological effects of soil conservation measures on runoff. [ABSTRACT FROM AUTHOR]

[Display omitted] • Water-nitrate coupling is explored under farmland and apple orchards • Large water deficit and nitrate accumulation are observed under apple orchards • Water deficit and nitrate accumulation are spatiotemporally asynchronous • Soil properties significantly affect water-nitrate coupling in deep soils 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 (NO 3 –−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 NO 3 –−N accumulation due to root water uptake and synthetic fertilizer inputs, with high spatiotemporal asynchrony in deep unsaturated zones. Specifically, water deficit and NO 3 –−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 NO 3 –−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. [ABSTRACT FROM AUTHOR]

• Deep soil and adjacent farmland were both important water sources for apple trees. • SWD increased at a rate of 5.94 mm m−1 yr−1 in apple orchard before 30 years of age. • The migration and accumulation of NO 3 –-N confirmed the soil water exchange. • The orchard-farmland pattern can promote efficient water and nutrient use in the CLP. 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 (R2 = 0.90) with increasing orchard age, accompanied by a significant increase in soil water deficit (SWD, R2 = 0.94). The annual growth rate of SWD was found to be as high as 5.94 mm·m−1·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 (NO 3 –-N) significantly increased with orchard age. The migration and accumulation of NO 3 –-N in the soil provided a scientific basis for the transport of soil water, with NO 3 –-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. [ABSTRACT FROM AUTHOR]

• α e-0 and α e-c are integrated parameters involving α in the P-T equation and other factors. • Basic GNAA with α e-0 function could accurately estimate E at a catchment scale. • α e-c in the extended GNAA is closer to the original α value in P-T equation. The generalized nonlinear advection aridity model (GNAA) for evaporation (E) estimation can be expressed in a basic form with a single parameter α e-0 , or an extended form using two parameters, α e-c and c. The implications of these model parameters in the model and the accurate estimation of E are receiving increasing attention. Our study shows that α e-0 and α e-c are affected by precipitation (P), the aridity index (E pa /P) and the climate seasonality and asynchrony index (SAI), etc, with which α e-0 has stronger correlations. This demonstrated that annual α e-0 and α e-c could cover the α parameter in the Priestley–Taylor formula, as well as other factors, particularly the aridity index. For the basic GNAA form, annual α e-0 is smaller than 1.00 in most catchments, but the GNAA model with the developed empirical formula between α e-0 and E pa /P can accurately estimate E at an annual scale. For the extended GNAA form, most annual α e-c were larger than 1.00, with a mean value of 1.08 for the Loess Plateau; α e-c tended to restore the original α values in the Priestley-Taylor equation. Functional differences exist between the basic and extended GNAA forms in estimating E and explaining the complementary relationship. Our results bridge some gaps in understanding the GNAA model from previous studies, and provide useful information to extend the application of the GNAA model. [ABSTRACT FROM AUTHOR]

• Parameter c of GNAA model was calibrated to 6.94 at annual scale on the Loess Plateau. • The upper and lower limits of SGCF were constrained within (0, 1) at annual scale. • GNAA and SGCF could estimate annual actual evaporation well in study area. Generalized complementary functions, which describe the relationship between the ratio of actual evaporation over the Penman potential evaporation (E/E Pen) and the proportion of the radiation term in E Pen (E rad /E Pen) have not been widely used at annual time scales. In this study, the generalized nonlinear advection-aridity function (GNAA) and sigmoid generalized complementary function (SGCF) were evaluated for annual evaporation estimation with calibrated parameters in the Loess Plateau of China. For all 15 catchments, the lowest values of annual E rad /E Pen were found to be much larger than zero, and the annual E/E Pen increased approximately linearly with annual E rad /E Pen. This complementary evaporation relationship at an annual timescale differs from those at daily timescales, and requires different parameterizations of the GNAA and SGCF. For the GNAA, parameter c , which is often set to zero for daily time scales, need to be well calibrated with available data. For the SGCF, the upper and lower limits of E rad /E Pen must be constrained. After calibration, both the SGCF and GNAA performed well at estimating annual evaporation. In addition, the calibrated Priestley-Taylor coefficient from the SGCF was found to be closer to the widely accepted value (1.26) than that determined from the GNAA. [ABSTRACT FROM AUTHOR]

• Interactions between factors impacting ET varies at different time scales. • Interannual change of vegetation coverage is independent from climate seasonality. • Climate seasonality and topography impact vegetation condition on the long-term scale. • Vegetation and fraction of precipitation falling as snow are selected to fit mean annual ω. Vegetation, climate and topography have been empirically formulated into the controlling parameter of the Budyko model (ω) to estimate evapotranspiration (ET). However, these variables, if simultaneously employed, may induce multicollinearity problems because of their potential interactions. Further, these interactions may vary with different time scales and subsequently result in the inaccurate estimation of ω. As such, we investigated the interactions of vegetation, climate and topography and their corresponding effects on ET modelling at different time scales by employing vegetation coverage (M), an improved climate seasonality and asynchrony index (SAI), the fraction of precipitation falling as snow (f s) and relative basin relief (BR/ B R ¯), in 30 catchments in China's Loess Plateau. We found that, on annual scale, M and SAI were significantly related to ω, while being independent from each other; in consequence, both of them should be parameterized into the Budyko model on the annual scale for better ET modelling. However, the links between M and SAI became stronger with increased time scales, the parameterization of ω should thus be reformatted for longer periods. When extended to a 30-year period, ω was closely related to the above variables, but M was highly intercorrelated with SAI and BR/ B R ¯ , and f s was significantly related to BR/ B R ¯. The independent M and f s were finally selected to fit ω, which allowed mean annual ET to be accurately modelled on long-term scale. Identification of the dominant factors applicable at different time scales can simplify the empirical parameterization of the Budyko formula and thereby facilitate more accurate estimation of ET. [ABSTRACT FROM AUTHOR]

• GNAA becomes a rescaled complementary model when α e-B takes a functional form. • GNAA-based evaporation increased at a rate of 0.99 mm/yr during 1982–2017 across China. • Water yield in the loess plateau may not decrease because P increase faster than e after revegetation. The generalized nonlinear advection aridity (GNAA) model has been widely applied for evaporation estimations and drought assessments at regional and global scales. Here, the GNAA model was combined with a semi-empirical function for the parameter α e-B to estimate actual evaporation (E) in nine Level-I water resources regions and one typical revegetation area (i.e., the Loess Plateau) in China. The results showed that the semi-empirical function for α e-B with aridity index rescaled the original independent variable (x B) in GNAA; thus, the model satisfied the boundary condition of " y = 0, at x B-αf =0". Moreover, the data points of E/E pa (E pa : apparent potential evaporation) and E po /E pa (E po : potential evaporation) were captured well by the GNAA curve with the semi-empirical function of α e-B. Further, the GNAA model showed high accuracy in simulating evaporation (E CR-B) when compared with the water balance equation, with the mean absolute error, root mean square error, and Nash-Sutcliffe efficiency values being 52 mm, 66 mm, and 0.84 respectively, over 49 basins. The multi-year mean E CR-B was 443.1 ± 14.7 mm/yr and 431.6 ± 21.2 mm/yr across China and Loess Plateau, respectively. Moreover, the annual E CR-B showed a significant upward trend during 1982–2017, and E CR-B increased rapidly before 1998, but decreased slightly after 1998 when considering 1998 as the turning point across China. Since the implementation of the large scale "Grain for Green" project on the Loess Plateau, nomalized difference vegetation index (NDVI) and precipitation increased significantly, while E CR-B showed no significant increases in both China and Loess Plateau. For the Loess Plateau, annual precipitation increased faster than annual evaporation, implying that water yield may increase. Overall, the findings would assist in clarifying the characteristics of evaporation across China and contribute to better management and sustainable utilization of regional water resources. [ABSTRACT FROM AUTHOR]

• Root water uptake depth increased in "S" shape with tree age and stabilized at 20 m after 20 years in an apple orchard. • Soil water deficit increased in depth of 3–21 m at a rate of 5.8 mm m−1 yr−1 from 10 to 24 years of tree age. • Apple trees could draw water horizontally up to 7 m from deeper soil layers of adjacent cropland. • Orchard-cropland mosaic structure is able to optimize the use of soil water resources in the Loess Plateau. It is of great significance to explore soil water distribution and consumptions of the agroforestry systems in water-deficient areas. In recent decades, continuous development of apple orchards in the southern Chinese Loess Plateau has formed the orchard-cropland mosaic vegetation pattern. In this study, soil water content (SWC) in two series were measured, (1) 0–21 m soil profiles of five apple orchards with different stand ages (5, 10, 15, 20 and 24 years) and one cropland; (2) 0–6 m soil profiles at different sites within the orchard-cropland border zone (with apple orchard aged 10 and 24 years old, denoted as AC10 and AC24, respectively). Results showed that (1) the water consumption depth of apple orchards increased in an "S"-shaped pattern with stand age and reached 20 m after 20 years, while the average SWC within 3–21 m layer decreased and the soil water deficits increased at a rate of 5.8 mm m−1 yr−1; and (2) the average SWC in 3–10 m layer of 15, 20 and 24-year-old orchards accounted for 69.3%, 64.2% and 57.9% of the field capacity, respectively, these proportions came to 71.2%, 67.4% and 61.1% in the 3–21 m layer, respectively; and (3) AC24 had greater influence on SWC of adjacent cropland than AC10. The soil water storage of 2–6 m layer in AC24 ranged from 713 mm to 879 mm within 7 m away from the orchard, which were significantly lower than those of 7–15 m away from the orchard (933–1042 mm). Overall, the apple tree root system can absorb soil water from a depth of 20 m in the vertical direction and up to 7 m from the adjacent cropland in the horizontal direction. Therefore, a reasonable orchard-cropland mosaic pattern should be arranged in regional agricultural planning, so as to promote the optimized utilization of regional soil water resources, and to coordinate the development of fruit industry and food security, in which the width of a cropland can't be < 14 m. [ABSTRACT FROM AUTHOR]

Highlights • Nitrous oxide emissions were higher in the apple orchard than in the wheat field. • Summer rainfall and winter snowfall were the principal controllers of N 2 O fluxes. • A moisture threshold associated with rainfall triggered higher N 2 O emissions. • Nitrous oxide emission due to land use change was altered by precipitation in arid region. Abstract The conversions of cropland to forest, or other tree-based systems, are considered to be important processes affecting regional and global greenhouse gas budgets, especially for nitrous oxide (N 2 O). From April 2007 to March 2009, in the rain-fed semi-arid climate of the Loess Plateau, China, soil N 2 O emissions were measured using static chambers from a winter wheat field and an apple orchard, which had been established in part of the wheat field 23 years earlier.Annual average N 2 O emissions from the apple orchard (2.40 kg N 2 O ha−1yr−1) were 12.15% higher than those in the wheat field (2.14 kg N 2 O ha−1yr−1). Seasonal rainfall, in combination with higher nitrogen fertilization, had a promoting effect on N 2 O emissions in the apple orchard compared with the wheat field. The amounts and patterns of summer rainfall and winter snowfall were the principal controllers of seasonal and annual N 2 O fluxes in these rain-fed semi-arid regions, likely through their influence on soil moisture content. Since there may be a moisture threshold associated with summer rainfall and winter snowfall triggering higher N 2 O emissions, climatic regimes should be taken into account when assessing the effects of land use on N 2 O emissions in the Loess Plateau. [ABSTRACT FROM AUTHOR]