Cumulative Infiltration Research Articles

An Analysis of Vertical Infiltration Responses in Unsaturated Soil Columns from Permafrost Regions

Rainfall infiltration affects permafrost-related slope stability by changing the pore water pressure in soil. In this study, the infiltration responses under rainfall conditions were elucidated. The instantaneous profile method and filter paper method were used to obtain the soil–water characteristic curve (SWCC) and hydraulic conductivity function (HCF). During the rainfall infiltration test, the vertical patters of volumetric moisture contents, total hydraulic head or suction and wetting front were recorded. Advancing displacement and rate of the wetting front, the cumulative infiltration, the instantaneous infiltration rate, and the average infiltration rate were determined to comprehensively assess the rainfall infiltration process, along with SWCC and HCF. Additionally, the effects of dry density and runoff on the one-dimensional vertical infiltration process of soil columns were evaluated. The results showed that the variation curve of wetting front displacement versus time obeys a power function relationship. In addition, the infiltration rate–time relationship curve and the unsaturated permeability curve could be roughly divided into three stages, and the SWCC and HCF calculated by volumetric moisture content are more sensitive to changes in dry density than to changes in runoff or hydraulic head height.

Parameterizing Haverkamp Model From the Steady‐State of Numerically Generated Infiltration: Influence of Algorithms for Steady‐State Selection

ABSTRACTBEST (Beerkan Estimation of Soil Transfer parameters) methods of soil hydraulic characterisation are widely applied for estimating sorptivity, S, and saturated hydraulic conductivity, Ks. Calculating these properties requires choosing the β and γ parameters of the Haverkamp infiltration model. These parameters can be obtained from numerically simulated three‐dimensional (3D) infiltration runs reaching steady‐state. This investigation tested dependence of the estimated β and γ parameters on the algorithm for steady‐state selection using simulated 3D cumulative infiltrations for different soils and initial conditions. Two algorithms used the original simulation outputs and included using (i) a threshold defining steadiness (T‐algorithm) and (ii) the last four data points, yielding a reference value of steady‐state infiltration rate (R‐algorithm). A third algorithm, similar to the R‐algorithm, was applied to previously re‐sampled infiltration data at fixed time intervals (RR‐algorithm). The intercept, bs, of the straight line fitted to the data describing steady‐state on the cumulative infiltration plot depended on the applied algorithm more than the slope of this line. Consequently, β varied with the applied algorithm more than γ. The RR‐algorithm, yielding 0.62 ≤ β ≤ 1.99 and 0.74 ≤ γ ≤ 0.98, was preferred since it mediated between advantages and disadvantages of T‐ and R‐algorithms. The influence of the choice of proper values for β and γ on the estimates of S and Ks was evaluated using BEST. Using the default values of β (0.6) and γ (0.75) yielded accurate estimates of S but not of Ks. Soil dependent β and γ values should be used in this case. A check of the reliability of the estimates of bs can be made by a sequential analysis of the cumulative infiltration data. Future developments include considering sources differing in size and establishing if the suggested β and γ values apply in general to the available BEST algorithms.

Infiltration Characteristics and Hydrodynamic Parameters in Response to Topographic Factors in Bare Soil Surfaces, Laboratory Experiments Based on Cropland Fields of Purple Soil in Southwest China

Topography is an important factor that impacts the hydrological processes on sloping farmlands. Yet, few studies have reported the combined influences of slope gradient and slope position on infiltration characteristics and hydrodynamic parameters on sloping croplands in purple soil regions, an important area for agricultural productivity in Southwest China. Here, laboratory-simulated rainfall experiments were conducted in a steel trough (5 m long, 2 m wide, and 0.45 m deep), and rainfall lasted for 1 h at a rate of 90 mm h−1 to examine the variations in the infiltration rates and hydrodynamic parameters under varying slope gradients (i.e., 3°, 6°, 10°, 15°, 21°, and 27°) and slope positions (i.e., upper, middle, and lower), and explore the relationships between the infiltration rate and the soil detachment rate. The results showed that the infiltration rate decreased gradually with duration rainfall and ultimately approached a steady state in the six slope treatments. Cumulative infiltration ranged from 15.54 to 39.32 mm during rainfall, and gradually reduced with the increase of slope gradient. The Horton’s model outperforms other models for predicting the infiltration rate with an R2 value of 0.86. Factors such as Darcy–Weisbach friction, flow shear force, Manning friction coefficient, unit energy, and runoff depth varied in the following order: upper slope > middle slope > lower slope, whilst the Reynolds number and Froude number gradually increased along the slope transect from the upper to lower slope positions. A significant linear function was fitted between the soil detachment rate and the infiltration rate at the gentle slopes (3°, 6°, 10°), whereas an exponential relationship was observed at the steep slopes (15°, 21°, and 27°). Observation also suggested that 15° was the critical slope gradient of sediment detachment, infiltration characteristics, and hydrodynamic parameters. Our results provide theoretical insight for developing models that predict the impacts of topographic factors on hydrological characteristic and soil erosion in hilly agricultural landscapes of purple soil fields.

Multifactorial Analysis of the Effect of Applied Gamma-Polyglutamic Acid on Soil Infiltration Characteristics

To investigate the mechanism and influence of applying gamma-polyglutamic acid (γ-PGA) on soil water infiltration, laboratory experiments and numerical simulations were conducted using Hydrus-1D. These studies assessed the impact of various application rates of γ-PGA on soil water characteristic parameters. Orthogonal simulation experiments on soil bulk density, γ-PGA application rates, and burial depths were performed utilizing predefined soil water characteristic values (twelve groups: nine groups of numerical simulation experiments and three groups of laboratory verification tests), and the soil infiltration characteristics were analyzed. Concurrently, an empirical model was developed to elucidate the relationships between the empirical model parameters and influencing factors, as well as to examine the sensitivity of these factors to changes in soil infiltration rate. The relationship between cumulative infiltration and the distance of wetting front movement, based on the water balance equation, was refined. The results indicated that γ-PGA significantly affected soil water characteristic parameters, where the saturated water content and the reciprocal of soil intake suction increased with rising γ-PGA applications (p < 0.01), while the saturated hydraulic conductivity and the parameter n decreased (p < 0.01), with no notable changes in the retained water content (p > 0.05). The trend in cumulative infiltration influenced by various factors could be modeled by a capacitive charging model function, which yielded a superior fit. A negative correlation existed between the sensitivity index and all the influencing factors (p < 0.05), with the order of influence being soil bulk density, γ-PGA application rate, and γ-PGA burial depth, respectively. Utilizing the modified water balance equation, the ratio of cumulative infiltration to wetting front migration distance corresponded more closely with a power function. These findings provide a theoretical foundation for further studies on the effects of γ-PGA on crop growth characteristics in fields and the optimization of γ-PGA technical element combinations.

Soil water repellency under Eucalyptus stands of different age and its relationship with soil hydrological function and soil organic matter

Eucalyptus stands can modify soil hydrological functioning, by several mechanisms including the effect of the growing root system, changes in soil organic matter (SOM) and the occurrence of soil water repellency (SWR). This effect is expected to evolve with time, as tree root system develop and SOM is formed from tree litter. SWR can prevent water from entering the soil, reducing infiltration, while the effect of roots and SOM can improve structure, favoring infiltration. The aims of this study were: i- to determine the influence of Eucalyptus stands of different ages on soil physical and hydraulic properties and SWR phenomena; ii- to evaluate the effects of SOM content and composition on SWR; iii- to determine the relationship between SWR and soil physical properties; iv- to compare different methodologies for SWR evaluation. The soil studied was a in a silty loam Typic Argiudoll with Eucalyptus stands of 3, 7, 11 and 32 years. Samples were taken to determine saturated and near saturation hydraulic conductivity, water retention curve, total organic carbon and different fractions of SOM, and SWR. Different parameters were used to estimate SWR, including the repellency index (RI), water repellency cessation time (WRCT), modified repellency index (RIm), and a qualitative analysis of the shapes of cumulative infiltration curves. As main results, a reduction in total porosity (TP) in the first years of Eucalyptus plantation was found, followed by a steep increased in TP, mainly due to an increase in microporosity. Hydraulic conductivity decreased with stand age, which was attributed to a decrease in macro and mesoporosity and porosity connectivity. SOM and most SOM fractions were not affected by stand age. RI was a consistent method to estimate SWR in Eucalyptus stands, while WRCT and RIm determination was more difficult when non classical shapes of infiltration curves were obtained.

Analytical solution for the saturation line in dry-stack tailings dams under cumulative rainfall infiltration for many years

Analytical solution for the saturation line in dry-stack tailings dams under cumulative rainfall infiltration for many years

Numerical simulation of rainfall-induced deformations of embankments considering the coupled hydro-mechanical behavior of unsaturated soils

This paper presents numerical simulations of the deformation response of unsaturated embankments subjected to rainfall infiltration using a coupled hydro-mechanical constitutive model for unsaturated soils. The constitutive model accounts for the influence of degree of saturation on the stress–strain behavior and the influence of void ratio on the water retention behavior. The constitutive model was implemented in the finite difference program FLAC, calibrated using triaxial test data, and validated using measurements of wetting-induced deformations of embankment models from centrifuge tests. The validation demonstrates that the constitutive model can capture the coupled hydro-mechanical behavior of unsaturated soils under wetting conditions. Simulations of the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration indicate that the differential settlement across the top surface of the embankment between the centerline and shoulder increases significantly during rainfall infiltration, which could result in severe damage to overlying transportation infrastructure. As the rainfall infiltration increases, shear strains accumulate and form a potential failure surface at a shallow depth of approximately 2 m from the slope surface. Insights into the hydro-mechanical response of unsaturated embankments subjected to rainfall infiltration gained will be useful for considering climate change effects in design and construction of compacted embankments.

Spent coffee waste-derived biochar improves physical properties, water retention, and maize (Zea mays L.) growth in sandy soil

Adding organic soil amendments can improve the physical and hydrological properties of soil, subsequently enhancing fertility for better crop production. In this study, spent Arabica and Columbian coffee wastes and their respective biochars were evaluated as soil amendments to improve the physical and hydrological properties of loamy sand soil and enhance maize (Zea mays L.) crop growth. Spent Arabica coffee (AC) and Columbian coffee (CC) wastes were collected and transformed into biochar through pyrolysis process at 550 °C with a residence time of 3 h and pyrolysis rate of 5 °C per minute. The AC and CC derived biochar were termed as ABC and CBC, respectively. The produced soil amendments were applied to soil at 0% (control), 1%, 3%, and 5% in a column setup. The moisture characteristics, including water infiltration, evaporation, and water retention, were investigated. Thereafter, the prepared amendments were applied to loamy sand soils at 0% (control), 1%, 3%, and 5% (w/w) application rates. Maize growth was then observed for a period of 30 days under greenhouse conditions. Results of the column trials showed that ABC and CBC applied at 5% reduced the cumulative water evaporation by 57%–66% and cumulative infiltration by 124%–181% compared to control. Likewise, 5% application of ABC and CBC resulted in 101 to 130% higher water retention in loamy sand soil. Results of the greenhouse experiment showed that 5% application of ABC and CBC amendments resulted in root biomass of 2.12 and 2.38 g, respectively, as compared to 0.51 g in control treatment. Similar treatments resulted in shoot biomass of 9.70 and 9.93 g respectively, as compared to 7.37 g in control. Likewise, 5% application of CBC and ABC increase plant height from 15.71 to 30.94 cm in ABC and 33.23 cm in CBC. Overall, 5% application of coffee waste-derived biochars significantly reduced water evaporation and infiltration, while increasing soil water retention and maize plant height, root biomass, and shoot biomass. Therefore, spent coffee waste-derived biochar could effectively be employed to improve physical and hydrological properties of loamy sand soils for better crop productivity.

Response of soil moisture to rainfall following deep soil drying in China's hilly loess lands

Deep soil drying is a physical soil phenomenon that has become increasingly characteristic to artificial afforestation on China's Loess Plateau. Current research is largely short of conclusive reports on soil moisture recovery following deep soil drying in afforested lands. In this study, a 10-m deep underground column was constructed at Pengyang Experimental Station in Ningxia. The CS650-CR1000 automatic soil moisture monitoring system and BLJW-4 small meteorological observation stations were used to respectively monitor soil moisture and meteorological conditions in the study area for the period 2014–2019. The local rainfall was classified and the characteristics of soil infiltration analyzed at both monthly and annual scales. The results showed that: i) Deep soil moisture recovery in the semi-arid Loess Plateau region depended mainly on 25–49.9 mm and >50 mm types of rainfall; together accounting for 35.44 % of the precipitation. ii) Deep soil moisture replenishment occurred mainly for the period from April to October. While this accounted for 30.13 % of the precipitation, evaporation loss accounted for 69.87 % of it. With increasing monthly rainfall (Pm), the variation in monthly infiltration depth (Zm) was quadratic in shape — where Zm = −0.0094 Pm2 + 3.7702 Pm (R2 = 0.9577). iii) At the annual scale, deep soil moisture replenishment was mainly driven by year-on-year infiltration water accumulation. This is because a single year precipitation infiltration was not enough to replenish deep soil moisture. The cumulative infiltration depth for 2014–2019 was 180, 260, 400, 700, 1000 > 1000 cm. It suggested that soil water infiltration and deep dry soil recovery occurred at different times under rainfed conditions in the semi-arid loess hills in China. This is key for in-depth studies of the hydrological process in dry soil regions.

Multifactor analysis of the infiltration characteristics of film hole irrigation under muddy water conditions

Film hole irrigation under muddy water conditions is a new and effective water-saving irrigation technology. In order to determine the influence of multiple factors on the infiltration process of film hole irrigation under muddy water conditions, 12 sets of indoor infiltration tests were conducted to investigate the effects of four key influencing factors (muddy water sand content, hole diameter, soil bulk density, and infiltration time) on the infiltration characteristics of irrigation with film holes under muddy water conditions, in this study. Based on the experimental data, accurate and effective soil water infiltration and vertical and horizontal wetting front transport models were constructed. Based on the modeling results, the standard regression coefficients of each influencing factor against the fitted parameters were calculated and the effects of the factors on the fitted parameters were analyzed. Error analysis showed that both models could effectively simulate the soil water infiltration process under experimental conditions. Infiltration time was the dominant factor influencing the cumulative infiltration per unit of film hole area, followed by hole diameter, muddy water sand content and lastly, soil bulk density. Infiltration time was also the main influencing factor of the vertical wetting front transport distance, followed by soil bulk density, muddy water sand content, and lastly, hole diameter. Similarly, infiltration time exerted the greatest effect on the horizontal transport distance, followed by hole diameter, soil bulk density and muddy water sand content. The model validation revealed that both the calculated and measured values were distributed around the 1:1 line, reflecting the accuracy of the models. The results of this study can provide theoretical support for the design of film hole irrigation systems under muddy water conditions.

Use of rice straw nano-biochar to slow down water infiltration and reduce nitrogen leaching in a clayey soil

Biochar exhibits numerous advantages in enhancing the soil environment despite a few limitations due to its lower surface energy. Nanomodified biochar combines the advantages of biochar and nanoscale materials. However, its effects on water infiltration and N leaching in a clayey soil remain unclear. Therefore, this study prepared rice straw nano-biochar by a ball milling method, and investigated its physicochemical properties and effects of bulk biochar and nano-biochar at various addition rates (0 %, 0.5 %, 1 %, 2 %, 3 %, and 5 %) on wetting peak migration, cumulative infiltration, water absorption and retention, and N leaching. The results showed that, compared with bulk biochar, nano-biochar presented a more abundant pore structure with an increase in specific surface area of approximately 1.5 times, accompanied by a 20 % increase in acid functional groups. Compared with those for clayey soil without biochar addition, the wetting front migration time was increased by 10.2 %–123.9 % and 17.0 %–257.9 %, and the cumulative infiltration volume at 60 min was decreased by 26.0 %–48.4 % and 14.1 %–62.4 % for bulk biochar and nano-biochar, respectively. The parameter S of Philip model and the parameter a of Kostiakov model for nano-biochar were lower than those for bulk biochar, whereas the parameter b of Kostiakov model was greater, indicating that nano-biochar decreased initial soil infiltration rate and increased attenuation degree of the infiltration rate. Nano-biochar increased water absorption by 8.03 % and subsequently enhanced water retention capacity relative to bulk biochar. In addition, bulk biochar and nano-biochar reduced NH4+-N leaching by 3.0 %–13.1 % and 5.7 %–39.2 %, respectively, and NO3−-N leaching by 2.7 %–3.6 % and 9.0 %–43.3 %, respectively, by decreasing N concentration and leachate volume relative to those with no biochar addition. This study provides new knowledge for nano-biochar application in a clayey soil.

Approximate analytical solution of a soil water movement equation under different ponding radii on the basis of numerical simulation

AbstractThis study addresses the problem of 2D soil water movement under ponding radii of 1, 2, and 3 cm. The soil water movement characteristics (shape parameters of the water content profile, ratio of horizontal wetting front to vertical wetting front, relationship between infiltration time and horizontal wetting front, and relationship between infiltration time and cumulative infiltration) under the above three kinds of water ponding radius were analyzed. On the basis of the assumption that the soil wetting body is a semi‐ellipse and the analytical solution of the 1D soil water movement equation at any angle, the approximate analytical solution of the 2D soil water movement equation under ponding conditions is optimized. The function relationships between infiltration time, wetting front, and cumulative infiltration are established. We applied the numerical data simulated by HYDRUS‐3D to validate the parameters in proposed analytical solutions and evaluated the relationships between the wetting front and hydraulic parameters. The results indicate that as the water ponding radius increases, the wetting body and 2D water content distribution becomes larger. When the water ponding radius was 2 cm, the numerical and analytical solution of 1D soil water distribution showed the best comparison results, and the model error was the smallest. The ratio of wetting fronts was linearly increased with the increase of air‐entry suction with R2 = 0.9969.

Effects of gravel on the water infiltration process and hydraulic parameters of stony soil in the eastern foothills of Helan Mountain, China

The investigation into the impact of gravel on water infiltration process and hydraulic parameters in stony soil could offer a theoretical basis to enhance water availability in rocky mountain area. A one-dimensional vertical infiltration experiment was used in this study. Six groups of gravel content of 0% (CK), 10% (W1), 20% (W2), 30% (W3), 40% (W4) and 50% (W5) were established to explore the changes in the wetting front, cumulative infiltration volume and infiltration rate. Then the accuracy of four infiltration models in simulating soil water infiltration processes was evaluated. Finally, Hydrus-1D was used to perform numerical inversion of the soil water content after infiltration. The findings revealed that: (1) When the infiltration time reached 300 min, the wetting front of the W1, W2, W3, W4 and W5 treatments was 11.00%, 17.00%, 32.25%, 38.75% and 54.50% lower than CK, the cumulative infiltration volume was 29.80%, 38.97%, 45.62%, 54.74% and 73.17% lower than CK, and the stable infiltration rate was 50.98%, 52.94%, 66.67%, 68.63% and 86.27% lower than CK. (2) The soil–water infiltration processes were accurately described by the Horton model, the coefficient of determination (R2) > 0.935. (3) The simulation results of Hydrus-1D showed that with the increase of gravel content, the values of the retention water content (θr), saturated water content (θs), shape coefficient (n) and saturated hydraulic conductivity (Ks) were decreased, the values of the reciprocal of air-entry (α) were increased. The value of R2 was more than 0.894, the root mean square error (RMSE) and mean absolute error (MAE) were less than 2%, which demonstrated that the Hydrus-1D model exhibited superior capability in simulating the changes of water content in stony soil in rocky mountain area. The findings of this study demonstrated that gravel could decrease the water infiltration process and affect the water availability. It could provide data support for the water movement process of stony soil and rational utilization of limited water resources in mountainous area.

Effects of Muddy Water Infiltration on the Hydraulic Conductivity of Soils

Despite the high sand content of Yellow River water in arid Northwest China, locals in the region opt to use muddy water to meet the demand for agricultural irrigation. Muddy water irrigation is a complex process and is still poorly understood. In this study, six sets of saturated soil column infiltration tests were designed, considering soil texture (silt loam, sandy loam, and sand) and muddy water sand content (3%, 6%, 9%, and 12%) as the influencing factors, with two sets of validation tests. Change in hydraulic conductivity (Kh), the average change rate of hydraulic conductivity (ΔK), and cumulative infiltration volume (I) were experimentally studied in the context of muddy water infiltration to respectively establish the separate functional models and developed to fit their relationship with time. The study results indicated that the hydraulic conductivity (Kh) decreased with increasing muddy water infiltration time. For silt loam and sandy loam, Kh stabilized at 0.0030 and 0.0109 cm/min, respectively, after 70 min of infiltration. In contrast, Kh in the saturated sandy soil column significantly declined throughout the muddy water infiltration, showing a 90.84% reduction after 90 min compared to the saturated hydraulic conductivity of the sandy soil. As the sand content of the muddy water increased from 3% to 12%, Kh decreased by 83.99%, 90.90%, 91.92%, and 92.21% for 3%, 6%, 9%, and 12% sand content, respectively, in the saturated sandy soil columns at the end of the infiltration period. The I values were 21.20, 9.29, 7.90, and 6.25 cm for 3%, 6%, 9%, and 12% sand content, respectively. The ΔK values were 0.0037, 0.0041, 0.0043, and 0.0044 cm/min2 for the respective sand contents, at an infiltration time of 80 min. The validation test demonstrated that the segmented function model accurately emulated the changes in hydraulic conductivity of sandy soil textures throughout the infiltration period. Results from this study provide a significant basis for understanding the mechanisms to hinder muddy water infiltration and to efficiently utilize muddy water for irrigation.

The infiltration variability under oil palm plantation related to moss and understory vegetation covers in terrestrial and riparian sites in Jambi, Indonesia

Abstract The capacity and contribution of each understory vegetation group to infiltrate transmitted precipitation and moisten the soil are to be revealed. Recently, common findings on infiltration rate under oil palm are well reported, however, it does not include the role of each vegetation group to support water availability to the oil palm plantation. The specific objective of this study is to identify the role of each understory vegetation group consisting of moss and weeds on cumulative infiltration rate and infiltration capacity in the terrestrial area and riparian zone of oil palm plantations consisting of smallholder and corporate farmers in dry and rainy seasons. The total covering rate of both vegetation types amounted to 61.7%. In the active path of terrestrial area weed growth dominated the surface cover of oil palm plantations in both seasons, where the cover of weeds reached 67.8% and moss covers at the level of averagely 11.3%. The total covering rate by the vegetation was 79.1%, and 20.9% of the surface area was without any vegetative protection. In the riparian area, there was no evidence of the difference of infiltration rate measured in Ks both in the circle area and active path area, which reached very low at the value of 1.74-4.02 cm h−1 in the dry season and 3.29-3.88 cm h−1 in the rainy season. However, in the terrestrial area at the active path area, the Ks value reached an average of 12.13 cm h−1 in the dry season and 5.86 cm h−1 in the rainy season. The Ks-value in the oil palm circle in the terrestrial area showed no difference value, which was a very low category value ranging from 3.37-3.80 cm h−1. The cumulative value of infiltrated water during the first four hours in the riparian zone in the active path ranged from 10.35 to 18.52 cm. This value is lower than cumulative infiltrated water in the terrestrial area, ranging from 25.83-110.55 cm in the first 4 hours of infiltration time.

Landfill cap models under simulated climate change precipitation: assessing long-term infiltration using the HELP model

The Hydrologic Evaluation of Landfill Performance (HELP) model (v 3.04) was used to assess the hydrological performance of two large-scale (~ 80 × 80 × 90 cm) laboratory-based landfill cap models. Desiccation cracking under simulated climate change precipitation (CCP) events was modelled to predict the performance of landfill caps over a 50-year period in Cumbria England, UK. Cumulative infiltration values under different climate conditions were highly sensitive to macro-pore development in the clay liner which is represented by the changing hydraulic conductivity in HELP simulations. Simulation results of cap design 1 (a clay barrier layer only) show that cumulative annual infiltration under normal precipitation (NP) and CCP events, with the initial hydraulic conductivity, ranged between 1 and 2% of the annual precipitation; however, cumulative annual infiltration was 10 to 68% when maximum (degraded) hydraulic conductivity values were used in simulations. A geosynthetic clay liner (GCL) included in cap design 2 significantly reduce cumulative infiltration.

A simple, accurate, and explicit form of the Green–Ampt model to estimate infiltration, sorptivity, and hydraulic conductivity

AbstractFinding an explicit solution to the widely used Green–Ampt (G–A) one‐dimensional infiltration model has been subject of efforts for more than half a century. We derived an explicit semiempirical approach that combines accuracy with simplicity, a concept that has been generally neglected in previous studies. The equation is , with F (L), Ks (L/T), S (L/T0.5) and t (T) being cumulative infiltration, saturated hydraulic conductivity, sorptivity, and time, respectively. Relative errors (ɛ) by the application of this equation generally do not exceed ±0.3% in most applied infiltration problems faced by water resources engineering today. We show both numerically and mathematically that │ɛ│> 0.3% could only occur if Kst/F > 0.904, a criterion that could apply to sand and loamy sand soils (i.e., coarse texture) and if they experience infiltration rates for over 6 h and 19 h, respectively. Hence, we also derived a simple linear adjustment in the model as Fadj ≅ 0.9796 F + 0.335 S2/Ks to address these longer infiltration rates, and to assure that ɛ remains within the expected ±0.3% range of uncertainty. A linearized regression technique was also developed to accurately estimate S and Ks when the G–A model is used. We numerically demonstrated that our fitting method could be used even when the G–A approach is less valid (diffusive soils), provided that the actual value of the capillary length (λ) is initially known. An added benefit of our approach is that by setting λ equal to 1/3 and 2/3, it can significantly limit the range of initializing, unknown, a priori values of S and Ks, as these two parameters are estimated through the inverse solution of implicit infiltration models. Due to the model's simplicity and accuracy, our solution should find application among hydrologists, natural resource scientists, and engineers who wish to easily derive accurate estimations from the G–A infiltration approach and/or estimate sorptivity and hydraulic conductivity without encountering divergence problems.

Soil moisture dynamics and associated rainfall-runoff processes under different land uses and land covers in a humid mountainous watershed

Identifying soil moisture dynamics is critical for understanding watershed hydrological processes. Soil moisture responses to rainfall vary with land use and land covers (LULCs) and thus influence rainfall-runoff processes, however, the knowledge about how different LULCs affect such processes was less revealed, especially in humid areas. In this study, we investigated the characteristics of soil moisture content (SMC) and rainfall-related soil moisture responses under four typical LULCs, i.e., waxberry forest, farmland, sloping farmland, and bamboo forest, based on in-situ observations in a humid mountainous watershed. We then used the HYDRUS-1D model to analyze the patterns of the rainfall-infiltration processes and quantified corresponding runoff generation to reveal the partitioning between infiltration and runoff at the event scale. We found significant differences of the average SMC at multiple depths for four LULCs. The deep soil (80 cm) had the largest SMC for the waxberry forest, farmland, and sloping farmland, while the surface soil (10 cm) was the wettest soil layer for the bamboo forest. During the representative rainstorms, although soil moisture responses exhibited depth gradients, the earlier responses at some deep layers indicated the occurrence of preferential flow. Generally, the cumulative infiltration increased while the infiltration rate decreased with the rainfall grade. Farmland and bamboo forest showed the most and the least infiltration, respectively. These results suggested that both the rainfall properties and LULCs had a strong impact on the soil moisture responses. Meanwhile, the estimated average runoff coefficients of four typical LULCs increased with rainfall grade, and the sloping farmland generated the most runoff during the representative rainfall events. Our findings provide new insights into soil moisture dynamics and infiltration regimes under different LULCs in humid areas, which can contribute to hydrological modeling at the field-scale and regional water resource management.

Changes and influence factors of soil matrix infiltration in Chinese fir plantations with different stand ages in northern Guangxi

Soil matrix infiltration is an important pathway for plantations to obtain water, which affects ecological benefits and water conservation function of plantations. The changes of soil matrix infiltration and its influencing factors in different growth stages of Chinese fir plantations remain unclear. We measured soil matrix infiltration process using a tension infiltrometer in Chinese fir plantations (5, 8, 11, and 15 years old) of Beijiang River Forest Farm in Rongshui, Guangxi, and analyzed soil basic physicochemical properties to identify the dominant factors influencing soil matrix infiltration. The results showed that initial infiltration rate, stable infiltration rate, and cumulative infiltration increased with stand ages. The ranges of different stand ages were 141-180 mm·h-1, 109-150 mm·h-1, and 188-251 mm, respectively. The initial infiltration rate, stable infiltration rate, and cumulative infiltration were significantly positively correlated with soil capillary porosity, soil organic matter, soil water stable macroaggregate, sand content, and clay content, while negatively correlated with soil bulk density and silt content. Early thinning had a positive effect on soil matrix infiltration, but thinning measures after 11 years did not enhance soil matrix infiltration further. Philip model was optimal for describing soil matrix infiltration process in this region. In conclusion, soil matrix infiltration capacity of Chinese fir plantations gradually increased from young to middle-aged stands, but matrix infiltration capacity tended to stabilize after 11 years old. Silt content and water stable macroaggregate were the dominant factors influencing matrix infiltration.

Infiltration and Leaching Characteristics of Soils with Different Salinity under Fertilizer Irrigation

Salt and nutrient transport and transformations during water infiltration directly influence saline soil improvement and the efficient use of water and fertilizer resources. The effects of soil initial salinity (18.3 g/kg, 25.5 g/kg, 42.2 g/kg, 79.94 g/kg, and 165 g/kg, respectively, labeled S1 to S5) on the infiltration and leaching characteristics of water, salt, and nitrogen were analyzed via a one-dimensional vertical fertilizer infiltration experiment. Meanwhile, the estimation models of cumulative infiltration and wetting front, including the effect of soil initial salinity, were established. The results showed that, with the increase in soil initial salinity, the cumulative infiltration within the same time decreased, and the migration time of wet front to 45 cm was longer. The time required for S5 to reach the preset cumulative infiltration was more than six times that of S1, and, for the wet front migration to 45 cm, the time requirement for S5 was about four times that of S1. In the established Kostiakov model and wetting front model, the coefficients all decreased with the increase in soil initial salinity, and the test index R2 values both reached 0.999. In the Kostiakov model, coefficient K had a linear relationship with the natural logarithm of initial soil salt content, while index a had a direct linear relationship with initial soil salt content. The cumulative leachate volume decreased with the increase in soil initial salinity, and the corresponding data of S3 and S5 were reduced by 37% and 57.3%, respectively, compared with S1. The electrical conductivity values of S1, S3, and S5 were 15.4, 209.8, and 205.6 ms/cm, respectively, being affected by the initial content in soil, soil moisture transport rate, and exogenous potassium nitrate (KNO3) addition. The NO3−-N concentrations in the leachates of S1, S3, and S5 at the end of leaching were 55.26, 16.17, and 3.2 mg/L, respectively. Based on the results of this study, for soil with high initial salinity, the conventional irrigation amount (2250 m3/ha) of the general soil in the study area could not meet the requirements of leaching salt. These results can provide a reference for the formulation of irrigation and fertilization strategies for soils with different salinity and contribute to the sustainable development of saline soil agriculture and the ecological environment.