Evaluation of a combined penetrometer for simultaneous measurement of penetration resistance and soil water content

A combined penetrometer is an appropriate tool to measure the soil cone resistance and the water‐content profile. As a relatively new technique, a combined capacitance‐penetrometer for the simultaneous measurement of cone index and soil water content was developed at the Department of Agricultural Engineering of Bonn University in 2002. The objective of this study was the evaluation of the effectiveness and applicability of the innovated penetrometer with a focus on three aspects: (1) A capacitance sensor with two electrode configurations was calibrated for silt loam, sandy loam, and sand. The calibration results show that both electrode configurations have sufficient water‐content sensitivity, but soil‐specific calibrations seem necessary. (2) Under laboratory conditions, the dynamic resolution and response of the capacitance‐penetrometer were validated, and its radius of influence was determined. (3) The field measurement results demonstrate that this measurement technique can be used to improve the interpretation quality of soil cone index data.     

On-the-go measurement of soil water content and mechanical resistance by a combined horizontal penetrometer

A combined horizontal penetrometer was designed for the on-the-go and simultaneous measurement of soil water content and mechanical resistance. The maximum sampling rate for both sensors was 10 Hz and the maximum operating depth was 20 cm. For the water-content sensor, its measurement principle depends on the electric field of the fringe-capacitance. In order to evaluate the applicability of this combined penetrometer, four experiments in the field were carried out. These experiments included: (1) soil water content profiles test; (2) soil compaction measurement test; (3) effect of the operating velocity on the water content and resistant force measurement; (4) effect of operating depth on the force measurement. The experimental results show that the combined horizontal penetrometer is a practical tool since it can provide more useful information of soil physical properties.     

Comparison of empirical and partly deterministic methods of time domain reflectometry calibration, based on a study of two tropical soils

The use of time domain reflectometry for the determination of volcanic soils' moisture content in tropical areas has shown the necessity to set up an accurate calibration. We chose to test the most representative calibration models on two tropical volcanic soils coming from two different climatic areas of the Réunion Island. The empirical model of Topp et al. [Topp, G.C., Davis, J.L., Annan, A.P., 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resources Research, 16 (3) 574–582.] was found to offer the best accuracy/simplicity trade-off for simulations with a cambisol, with an accuracy of ±2.46 dielectric constant points (or about ±2.7% for a water content close to 45%). In contrast, it gave poorer results for analysis of the andosol containing the greatest quantity of amorphous minerals (±5.74 dielectric constant points). The partly deterministic models were found to be better for this latter application, including that of Ansoult et al. [Ansoult, M., De Backer, L.W., Declercq, M., 1985. Statistical relationship between apparent dielectric constant and water content in porous media. Soil Sci. Soc. Am. J., 49, 47–50.] which gave the best accuracy (±3.84 dielectric constant points). For both soil materials studied, there was only a slight gain of accuracy by introducing a specific polynomial calibration determined in the laboratory: ±1.72 and ±3.46 dielectric points for the cambisol and the andosol, respectively. Choices of models to be used should thus be based on the type of soil to be analysed and accuracy requirements.     

The effect of initial soil water content and rainfall intensity on near-surface soil hydrologic conductivity: A laboratory investigation

This paper investigates the influence of rainfall intensity and initial soil water content on changes in the near-surface soil hydraulic conductivity. While numerous papers have examined surface sealing, this paper outlines two important innovations: the design and application of time-domain reflectometry probes with the ability to measure and record soil water content at very short time and length scales; the design and application of stainless steel tensiometers to measure soil water potential at the same, very short, time and length scales. These permit the Richards Equation to be applied and the near-surface hydraulic conductivity computed. An exponential curve was fitted to the temporal changes in hydraulic conductivity, as both a function of water potential or soil water content, with a high degree of fit. As rainfall intensity increased, the degree of scatter about the best-fit line increased, probably due to the ability of high intensity rainfall to disrupt the near-surface soil structure. While utilising deterministic models, such as the Richards Equation, may be appropriate at the column scale there are practical difficulties due to the very small length scale variability in each of the input variables. The experimental results detailed here indicate that even with data at very short time and length scales it is still not possible to utilize the Richards Equation, or at least, it may be difficult to transfer the laboratory results to the field due to the effect of the specific laboratory experimental conditions on the results.     

A comparison between seasonal changes in soil water storage and penetration resistance under conventional and conservation tillage systems in Aragón

Low and extremely variable precipitations limit dryland crop production in the semi-arid areas of Aragón (NE Spain). These areas are also affected by high annual rates of topsoil losses by both wind and water erosion. A long-term experiment to determine the feasibility of conservation tillage in the main winter barley production areas of Aragón was initiated in 1989 at four locations, three on loam to silt loam soils (Xerollic Calciorthid) and one on a silty clay loam (Fluventic Ustochrept), receiving between 300 and 600 mm of average annual rainfall. In this study, we compared, under both continuous cropping and cereal-fallow rotation, the effects of conventional tillage (mouldboard plough) and two conservation tillage systems, reduced tillage (chisel plough) and no-tillage, on soil water content and penetration resistance during the first two growing seasons. Whereas reduced and conventionally tilled treatments generally had similar soil water content during the experimental period, the effects of no-tillage were inconsistent. No-tilled plots had from 26% less to 17% more stored soil water (0–80 cm) than conventional tilled plots at the beginning of the growing season. In contrast to the conventional and reduced tillage treatments, penetration resistances were between 2 and 4 MPa after sowing in most of the plough layer (0–40 cm) under no-tillage at all sites. Fallow efficiencies in moisture storage in the cereal-fallow rotation, when compared with the continuous cropping system, ranged from −8.7 to 12%. The highest efficiencies were recorded when the rainfall in the months close to primary tillage exceeded 100 mm. Since this event is very unlikely, long fallowing (9–10 months) appears to be an inefficient practice for water conservation under both conventional and conservation management. Our results suggest that, up to now, only reduced tillage could replace conventional tillage without adverse effects on soil water content and penetration resistance in the dryland cereal-growing areas of Aragón.     

Effect of data resolution on soil hydraulic conductivity prediction

The aim of the paper is to compare results of the instantaneous profile method (IPM) for measurement and calculation of unsaturated hydraulic conductivity k(ψ) of soils obtained with different measurement data resolution. The application of IPM allows to realize a great number of k(ψ) measurements for the purpose of mapping soil properties on large areas. Application of shorter samples i.e. less sensors makes the method even more quick and cheap. The calculation of unsaturated soil water conductivity by the IPM method bases on measurements of time and space variability of water content and water pressure within the soil sample in a cylinder. The spatial resolution of data depends on the number of probes applied in the core. The question arises how the number of compartments within one core influences the calculation of soil hydraulic conductivity. Application of three sensors instead of five reduced the accuracy of calculation but allowes to use 5 cm long standard cores during unsaturated flow experiment.<?show $6#>     

Laboratory evaluation of a model for soil crumbling for prediction of the optimum soil water content for tillage

A model for soil crumbling, called the capillary crumbling model (CCM) was introduced by Aluko and Koolen [Aluko, O.B., Koolen, A.J., 2000. The essential mechanics of capillary crumbling of structured agricultural soils. Soil Till. Res. 55, 117–126]. According to the CCM, the optimum soil water content for tillage (θ_OPT) may be defined as the water content at which the capillary bonding strength between aggregates is minimum. The objective of this study was to evaluate the CCM for the arable layer of 10 agricultural soils (sandy loam to clay textures) from semi-arid regions in western Iran. The results were compared with conventional soil workability limits such as 0.85 of the soil plastic limit (0.85θ_PL), Proctor critical water content (θ_Proctor), 0.6 or 0.7 of water content at matric suction of 50 hPa (0.6–0.7θ_50 hPa), and the Kretschmer optimum water content (θ_Kretschmer = θ_PL − 0.15(θ_LL − θ_PL)) where θ_LL is the soil liquid limit. Repacked soil cores were prepared from intact soil aggregates (0.50–4.75 mm) to 0.9 of the critical bulk density (to represent the soil conditions before tillage). Tensile strength and matric suction of the cores were determined at different soil water contents obtained by slow drying. The CCM provided evidence for the physics and mechanics of crumbling in the studied soils. It revealed that effective stresses are the dominant inter-aggregates forces, at least for the wet range of soil water content. A fall in strength of inter-aggregate bonds (i.e. tensile strength) was recorded due to water emptying from structural pores in a narrow range of matric suction (h_OPT) which was consistent with the model. With increasing soil organic matter and clay contents the fall became more distinct, indicating increased structural stability. The θ_OPT values determined by the CCM were found in the h_OPT range 551–612 hPa corresponding to 0.91–0.79θ_PL, which was in agreement with published values for the soil workability limit. Negative correlations between h_OPT and clay and organic matter contents clearly confirmed the increasing effect of soil structure on the enlargement of inter-aggregate pores. High correlations were observed between θ_OPT and 0.85θ_PL, θ_Proctor or 0.7θ_50 hPa. The results showed that the CCM might be recommended as a physically based method for the determination of θ_OPT. Considering the 1:1 relationships between θ_OPT and 0.85θ_PL or θ_Proctor, and easy determination of θ_PL and θ_Proctor, use of these indices is recommended in situations where the CCM is not applicable.     

Subsoil compaction by heavy sugarbeet harvesters in southern Sweden: III. Risk assessment using a soil water model

Due to its persistence, subsoil compaction should be avoided, which can be done by setting stress limits depending on the strength of the soil. Such limits must take into account soil moisture status at the time of traffic. The objective of the work presented here was to measure soil water changes during the growing period, use the data to calibrate a soil water model and simulate the soil susceptibility to compaction using meteorological data for a 25-year period. Measurements of soil water content were made in sugarbeet (Beta vulgaris L.) from sowing until harvest in 1997 on two sites classified as Eutric Cambisols in southern Sweden. Sampling was carried out at 2-week intervals in 0.1 m layers down to 1 m depth, together with measurements of root growth and crop development. Precompression stress of the soil at 0.3, 0.5 and 0.7 m depth was determined from uniaxial compression tests at water tensions of 6, 30, 60 and 150 kPa and adjusted as a logarithmic function of the soil water tension. Soil water content was simulated by the SOIL model for the years 1963–1988. Risk calculations were made for a wheel load of 8 t and a ground pressure of 220 kPa, corresponding to a fully loaded six-row sugarbeet harvester. Subsoil compaction was expected to occur when the major principal stress was higher than the precompression stress. The subsoil water content was very low in late summer, but increased during the autumn. At the end of August, there was practically no plant available water down to 1 m depth. There was in general good agreement between measured and simulated values of soil water content for the subsoil, but not for the topsoil. In the 25-year simulations, the compaction risk at 50 cm depth was estimated to increase from around 25% to nearly 100% between September and late November, which is the period when the sugarbeet are harvested. The types of simulation presented here may be a very useful tool for practical agriculture as well as for society, in giving recommendations as to how subsoil compaction should be avoided.     

Study of soil respiration and fruit quality of table grape (Vitis vinifera L.) in response to different soil water content in a greenhouse

ABSTRACT

To understand the response of grape (Hutai No.8) quality and soil respiration (R_s) to different soil relative water contents (SRWCs), this study was designed with three soil moisture levels (A: 80–95%, B: 60–75%, and C: 40–55% of SRWC) for grape cultivation. Meanwhile, environmental factors, including air temperature (T_a), air relative humidity, and light intensity, were also recorded. The results showed the following: (1) Through the comprehensive analysis of fruit quality by the method of subordinate function, we concluded that the optimum soil moisture treatment was 60–75% SRWC, and the soluble sugars, proanthocyanidin, and resveratrol were most abundant. In addition, vitamin C (Vc) content was the largest under C treatment. (2) Photosynthetic characteristic under high soil moisture was better than those under low soil moisture condition during grape coloring periods, and it was largest under A treatment in 2015. R_s rate was in accordance with the trend of grape photosynthesis. High soil moisture could accelerate the photosynthetic rate of grape leaves and increase R_s. (3) Correlation analysis showed that higher soil moisture and air humidity and lower soil temperature (T_s) and T_a could promote the accumulation of more nutrients in grape berries; it also could increase photosynthetic rate and R_s during grape coloring periods. In conclusion, 60–75% SRWC was the optimum soil moisture condition, which could improve the nutrient contents and accumulate more bioactive substances. Of course, keeping a lower T_s and T_a, as well as higher air humidity, was also necessary.

Abbreviations: SRWC: soil relative water content; A, 90-95% SRWC; B, 70-75% SRWC; C, 40-55% SRWC; Rs: soil respiration; Ta: air temperature; Ts: soil temperature; OPC: proanthocyanidin; TSS: total soluble solids.     

Errors in the estimation of soil water properties and their propagation through a hydrological model

Abstract. The Agricultural Catchments Research Unit model (ACRU) includes a decision support system (DSS) for estimating the water content of soil at field capacity (θ _fc ) and wilting point (θ _wp ) when these characteristics are not directly measurable. Three methods of estimation are proposed: (a) based on silt and clay content and bulk density, (b) based on clay content only, and (c) based on soil series. These three pedotransfer functions are compared with respect to both the estimation of θ _fc and θ _wp and the propagation of errors when the actual evapotranspiration of a wheat crop (E) is predicted over the growing season by the ACRU model.
The standard error of estimation was between 0.066 and 0.082 m³/m³ for θ _fc , between 0.056 and 0.069 m³/m³ for θ _wp and between 29.9 and 34.8 mm of water for E. The method based on silt and clay contents and bulk density predicted θ _fc and θ _wp for non-swelling soils most precisely. The method based on soil series was better than other methods for swelling soils. It also performed better for estimating available water capacity and consequently for predicting E from a conceptual soil water model. The propagated error of estimating θ _fc and θ _wp using the DSS reached 15–18% of the simulated E. The error in the prediction of E can reach 26–30% when spatial variation in soil properties is also estimated.     

Vulnerability of Bavarian silty loam soil to compaction under heavy wheel traffic: impacts of tillage method and soil water content

Soil compaction caused by traffic of heavy vehicles and machinery has become a problem of world-wide concern. The aims of this study were to evaluate and compare the changes in bulk density, soil strength, porosity, saturated hydraulic conductivity and air permeability during sugar beet (Beta vulgaris L.) harvesting on a typical Bavarian soil (Regosol) as well as to assess the most appropriate variable factors that fit with the effective controlling of subsequent compaction. The field experiments, measurements and laboratory testing were carried out in Freising, Germany. Two tillage systems (conventional plough tillage and reduced chisel tillage) were used in the experiments. The soil water contents were adjusted to 0.17 g g⁻¹ (w₁), 0.27 g g⁻¹ (w₂) and 0.35 g g⁻¹ (w₃).Taking the increase in bulk density, the decrease in air permeability and reduction of wide coarse pore size porosity (−6 kPa) into account, it seems that CT (ploughing to a depth of 0.25 m followed by two passes of rotary harrow to a depth 0.05 m) of plots were compacted to a depth of at least 0.25 m and at most 0.40 m in high soil water (w₃) conditions. The trends were similar for “CT w₁” (low soil water content) plots. However, it seems that “CT w₁” plots were less affected than “CT w₃” plots with regard to bulk density increases under partial load. In contrast, diminishments of wide coarse pores (−6 kPa) and narrow (tight) coarse pores (−30 kPa) were significantly higher in “CT w₁” plots down to 0.4 m. Among CT plots, the best physical properties were obtained at medium soil water (w₂) content. No significant increase in bulk density and no significant decrease in coarse pore size porosity and total porosity below 0.2 m were observed at medium soil water content. The soil water content seemed to be the most decisive factor.It is likely that, CS (chiselling to a depth of 0.13 m followed by two passes of rotary harrow to a depth 0.05 m) plots were less affected by traffic treatments than CT plots. Considering the proportion of coarse pore size porosity (structural porosity) and total porosity, no compaction effects below 0.3 m were found. Medium soil water content (w₂) provides better soil conditions after traffic with regard to wide coarse pore size porosity (−6 kPa), air permeability (at 6 and 30 kPa water suction), total porosity and bulk density. Proportion of wide coarse pores, air permeability and bulk density seems to be suitable parameters to detect soil compaction under the conditions tested.     

Spatial and seasonal variations in soil respiration in a temperate deciduous forest with fluctuating water table

Our objectives were to determine both spatial and temporal variations in soil respiration of a mixed deciduous forest, with soils exhibiting contrasting levels of hydromorphy. Soil respiration (R_S) showed a clear seasonal trend that reflected those of soil temperature (T_S) and soil water content (W_S), especially during summer drought. Using a bivariate model (RMSE=1.03), both optimal soil water content for soil respiration (W_SO) and soil respiration at both 10 °C and optimal soil water content (R_S10) varied among plots, ranging, respectively, from 0.25 to 0.40 and from 2.30 to 3.60 μmol m⁻² s⁻¹. Spatial variation in W_SO was related to bulk density and to topsoil N content, while spatial variation in R_S10 was related to basal area and the difference in pH measured in water or KCl suspensions. These results offer promising perspectives for spatializing ecosystem carbon budget at the regional scale.     

Quantitative assessment of effectiveness of soil conservation measures using a combination of Cs radioactive tracer and conventional techniques

Quantitative assessment of soil redistribution in landscapes remains a challenging task. In this study we used radioactive soil redistribution tracer ¹³⁷Cs together with soil morphological characteristics and empirically-based modeling for quantitative assessment of long-term soil conservation effectiveness. Three pairs of arable slopes were selected, all located within the territory of the Novosil experimental station (the Orel Region, central European Russia). One slope in each pair undergone creation of artificial terraces with forest shelter belts located parallel to topography contour lines and spaced at approximately 100 m from each other.Preliminary results have shown that slopes with soil-protective measures are characterized by a 11–80% reduction of average soil redistribution rates, as shown by soil profile morphology and ¹³⁷Cs methods. Discrepancy in values obtained can be attributed to differences in temporal resolution of methods as well as possible influence of individual extreme events on results yielded by the ¹³⁷Cs method. On the other hand, more significant decrease in average soil degradation rates on slopes with soil conservation (62–75% for each pair of slopes) was predicted by the model.The ¹³⁷Cs method overestimates gross and net soil redistribution rates, as a result of the influence of extreme erosion prior to tillage mixing of a fresh fallout isotope, not accounted for by calibration models used. Another shortcoming of the estimations obtained is that sediment redeposition directly within forest belts was not taken into account. Therefore, net erosion rates obtained for slopes with forest belts should be regarded as overestimation. Nevertheless, it can be generally concluded that the multi-technical approach has allowed acquiring much more detailed information on temporal and spatial variability of soil redistribution rates than single method-based studies.     

Compatible measurements of volumetric soil water content using a neutron probe and Diviner 2000 after field calibration

Field calibrations for a neutron probe and a capacitance sensor (Diviner 2000) for measuring the soil water content of a shrinking–swelling clay soil were substantially different from commonly used default values. Using our field calibrations, the two instruments estimated similar changes in the cumulative water content of a soil profile (0–1 m depth) over one growing season.     

Integral energy of conventional available water, least limiting water range and integral water capacity for better characterization of water availability and soil physical quality

Different approaches have been proposed for quantification of soil water availability for plants but mostly they do not fully describe how water is released from the soil to be absorbed by the plant roots. A new concept of integral energy (E_I) was suggested by Minasny and McBratney (Minasny, B., McBratney, A.B. 2003. Integral energy as a measure of soil-water availability. Plant and Soil 249, 253-262) to quantify the energy required for plants to take up a unit mass of soil water over a defined water content range. This study was conducted to explore the E_I concept in association with other new approaches for soil water availability including the least limiting water range (LLWR) and the integral water capacity (IWC) besides conventional plant available water (PAW). We also examined the relationship between E_I and Dexter's index of soil physical quality (S-value). Twelve agricultural soils were selected from different regions in Hamadan province, western Iran. Soil water retention and penetration resistance, Q, were measured on undisturbed samples taken from the 5-10 cm layer. The PAW, LLWR and IWC were calculated with two matric suctions (h) of 100 and 330 hPa for field capacity (FC), and then the E_I values were calculated for PAW, LLWR and IWC. There were significant differences (P < 0.01) between the E_I values calculated for PAW₁₀₀, PAW₃₃₀, LLWR₁₀₀, LLWR₃₃₀ and IWC. The highest (319.0 J kg⁻¹) and the lowest (160.7 J kg⁻¹) means of E_I were found for the E_I(IWC) and E_I(PAW330), respectively. The E_I values calculated for PAW₁₀₀, LLWR₁₀₀ and LLWR₃₃₀ were 225.6, 177.9 and 254.1 J kg⁻¹, respectively. The mean value of E_I(PAW330) was almost twice as large as the mean of E_I(IWC) showing that IWC is mostly located at lower h values when compared with PAW₃₃₀. Significant relationships were obtained between E_I(IWC) and h at Q = 1.5 MPa, and E_I(LLWR100) or E_I(LLWR330) and h at Q = 2 MPa indicating strong dependency of E_I on soil strength in the dry range. We did not find significant relationships between E_I(PAW100) or E_I(PAW330) and bulk density (ρ_b) or relative ρ_b (ρ_b-rel). However, E_I(LLWR100) or E_I(LLWR330) was negatively and significantly affected by ρ_b and ρ_b-rel. Both E_I(PAW100) and E_I(PAW330) increased with increasing clay content showing that a plant must use more energy to absorb a unit mass of PAW from a clay soil than from a sandy soil. High negative correlations were found between E_I(PAW100) or E_I(PAW330) and the shape parameter (n) of the van Genuchten function showing that soils with steep water retention curves (coarse-textured or well-structured) will have lower E_I(PAW). Negative and significant relations between E_I(PAW100) or E_I(PAW330) and S were obtained showing the possibility of using S to predict the energy that must be used by plants to take up a unit mass of water in the PAW range. Our findings show that E_I can be used as an index of soil physical quality in addition to the PAW, LLWR, IWC and S approaches.     

Time stability of soil water storage measured by neutron probe and the effects of calibration procedures in a small watershed

The knowledge of soil water storage (SWS) of soil profiles is crucial for the adoption of vegetation restoration practices. With the aim of identifying representative sites to obtain the mean SWS of a watershed, a time stability analysis of neutron probe evaluations of SWS was performed by the means of relative differences and Spearman rank correlation coefficients. At the same time, the effects of different neutron probe calibration procedures were explored on time stability analysis, mean SWS estimation, and preservation of the spatial variability of SWS. The selected watershed, with deep gullies and undulating slopes which cover an area of 20 ha, is characterized by an Ust-Sandiic Entisol and an Aeolian sandy soil. The dominant vegetation species are bunge needlegrass (Stipa bungeana Trin.) and korshinsk peashrub (Caragana Korshinskii kom.). From June 11, 2007 to July 23, 2008, SWS of the top1 m soil layer was evaluated for 20 dates, based on neutron probe data of 12 sampling sites. Three calibration procedures were employed: type I, most complete, with each site having its own linear calibration equation (TrE); type II, with TrE equations extended over the whole field; and type III, with one single linear calibration curve for the whole field (UnE) and also correcting its intercept based on site specific relative difference analysis (RdE) and on linear fitting of data (RcE), both maintaining the same slope. A strong time stability of SWS estimated by TrE equations was identified. Soil particle size and soil organic matter content were recognized as the influencing factors for spatial variability of SWS. Land use influenced neither the spatial variability nor the time stability of SWS. Time stability analysis identified one site to represent the mean SWS of the whole watershed with mean absolute percentage errors of less than 10%, therefore, this site can be used as a predictor for the mean SWS of the watershed. Some equations of type II were found to be unsatisfactory to yield reliable mean SWS values or in preserving the associated soil spatial variability. Hence, it is recommended to be cautious in extending calibration equations to other sites since they might not consider the field variability. For the equations with corrected intercept (type III) , which consider the spatial variability of calibration in a different way in relation to TrE, it was found that they can yield satisfactory means and standard deviation of SWS, except for the RdE equations, which largely leveled off the SWS values in the watershed. Correlation analysis showed that the neutron probe calibration was linked to soil bulk density and to organic matter content. Therefore, spatial variability of soil properties should be taken into account during the process of neutron probe calibration. This study provides useful information on the mean SWS observation with a time stable site and on distinct neutron probe calibration procedures, and it should be extended to soil water management studies with neutron probes, e.g., the process of vegetation restoration in wider area and soil types of the Loess Plateau in China.     

Influence of spring preparation date and soil water content on seedbed physical conditions of a clayey soil in Sweden

Secondary tillage performed under inadequate soil water contents usually leads to a poor seedbed. Under normal Swedish weather conditions, clayey soils ploughed during autumn form a very dry top layer in spring, which acts as an evaporation barrier so that deeper layers remain wet. Thus, the conventional approach considering soil workability in relation to a single value of soil water content is difficult to apply. Hence, a field experiment was carried out to study the effect of seedbed preparation date, the associated soil water contents and traffic consequences on the physical properties of a spring seedbed. The field was autumn ploughed and the experiment started as soon as the field was trafficable after winter thawing. The seedbed preparation consisted of three harrowing operations on plots 8 m×8 m (three replications) with a spring tined harrow and a tractor mounted with dual tyres and was performed on 10 occasions from the beginning of April to the middle of May. With the exception of some short periods after rain, the soil had a clear water stratification during the experiment, with a very dry superficial layer (5–20 mm thick) contrasting to water contents over 300 g kg⁻¹ from only 40 mm depth. After the harrowing operation, the seedbed aggregate fraction less than 2 mm increased from about 40% at the beginning of April to about 60% for the last four treatments in May. Contributing factors to the rise were attributed to the lower water contents of the top layer (<40 mm) and the drying–wetting and freezing–thawing cycles that occurred in the surface layer during April. There were no significant differences in bulk density after harrowing between the treatments but an increase in penetration resistance up to a depth of 180 mm in the harrowed plots was statistically significant (P<0.001). In the non-harrowed soil, penetration resistance also increased, including in those soil layers where water contents kept nearly constant.

In conclusion, the seedbed preparation dates had only a minor effect on soil compaction, as measured by bulk density and penetration resistance, due to the slow drying beneath the dry top layer. The fraction of fine aggregates in the seedbed increased with time. Thus, the optimal time for seedbed preparation depended mainly on soil friability and not on the risk of compaction.     

水蚀区水土保持地面观测资料整编内容与方法

资料整编是水土保持监测的基础性工作.按资料的来源和属性,可将水土保持监测资料分为基础资料、观测资料、调查资料和衍生资料.水蚀区水土保持地面观测主要涉及径流小区和小流域控制站,按监测内容和对象,一般可分为影响因子、径流和泥沙观测资料整编3大类,不同资料整编内容与方法不尽相同,各有侧重.近期,应抓紧制订监测资料整编的技术标准和整编制度,规范整编行为,同时,还应加强技术培训和档案管理,提高水土保持监测资料整编的信息化和现代化水平.     

Identifying scale specific controls of soil water storage in a hummocky landscape using wavelet coherency

High spatio-temporal variability of soil water is contributed from different ecohydrological and soil processes operating in different intensities at different scales. Traditional Pearson correlation analysis only examines linear correlation at the measurement scale. In this study, the correlation between soil water storage and its controlling factors was examined at different scales and locations in a hummocky landscape using wavelet coherency. Time domain reflectometry and neutron probe were used to measure soil water storage up to 1.4 m depth along a transect of 576 m long established in a hummocky landscape at St. Denis National Wildlife Area, Saskatchewan, Canada. In spite of visual similarity of the spatial pattern of soil water storage and elevation, the value of Pearson correlation coefficient was very small. However, wavelet coherency identified strong scale- and location-specific correlations between soil water storage and elevation. The total area of significant correlations as calculated from the total number of significant coherencies at different scales and locations was higher between soil water storage and elevation than between soil water storage and any other factors, which indicated a dominant control from elevation on soil water storage in the hummocky landscape. The largest area of significant correlation was observed at large scales (> 70 m), which can be attributed to the alternating knolls and depressions. The relationship between soil water storage and elevation at different scales was persistent at different times of the year or at different seasons with a slight reduction in the magnitude of correlation. The persistent relationship indicated the dominant control from elevation with slight change in the degree of the control. The scale-location specific correlation provides a complete picture on the controls of soil water storage, which was not possible with traditional correlation analysis.