V. Bagarello

Calibrating storage tanks for soil erosion measurement from plots

Many plots for soil loss measurements are equipped with a sequence of tanks for measuring runoff volume and sediment concentration. The stored water volume is easily determined by a water depth measurement while the sediment concentration is often measured by collecting samples of the mixed suspension. In this paper, using the Π-theorem of dimensional analysis, the functional relationship describing the mixing of the suspension in the tank is expressed in a dimensionless form. The recognized dimensionless groups allow the establishment, for given soil and water depth in the tank, of the relationship (calibration curve) between the actual and the measured concentration. The calibration curve, using measured concentration values obtained from sampling taps located at different heights on the vertical of a tank wall, is shown to be linear. For a given soil, the slope of the calibration curve is related to the water depth. We also show that the same dimensionless groups allow the deduction of a scale-up procedure and the possibility of using a small tank, similar to the field one, for investigating additional effects neglected in the theoretical analysis. The effects of the following factors on the calibration curve are examined: (1) sampling direction along the vertical, i.e. starting the sampling from the upper tap to the lower one or vice versa; (2) mathematical shape of the concentration profile; (3) representativeness of the calibration cufve; (4) sampling volume; (5) water depth into the tank (i.e. filling level); (6) suspension mixing time; (7) grain size distribution of the eroded soil particles for given soil type; (8) time between the end of the erosive event and the starting of the sampling procedure (delay time). Finally, an attempt is made to define a simpler sampling procedure based on a single measurement with a dipped sampler. Copyright

An assessment of the global impact of 21st century land use change on soil erosion

Human activity and related land use change are the primary cause of accelerated soil erosion, which has substantial implications for nutrient and carbon cycling, land productivity and in turn, worldwide socio-economic conditions. Here we present an unprecedentedly high resolution (250 × 250 m) global potential soil erosion model, using a combination of remote sensing, GIS modelling and census data. We challenge the previous annual soil erosion reference values as our estimate, of 35.9 Pg yr−1 of soil eroded in 2012, is at least two times lower. Moreover, we estimate the spatial and temporal effects of land use change between 2001 and 2012 and the potential offset of the global application of conservation practices. Our findings indicate a potential overall increase in global soil erosion driven by cropland expansion. The greatest increases are predicted to occur in Sub-Saharan Africa, South America and Southeast Asia. The least developed economies have been found to experience the highest estimates of soil erosion rates.Human activity and related land use change are the primary cause of soil erosion. Here, the authors show the impacts of 21st century global land use change on soil erosion based on an unprecedentedly high resolution global model that provides insights into the mitigating effects of conservation agriculture.

Measuring hydraulic conductivity in a cracking clay soil using the guelph permeameter

Knowing the variation of field-saturated hydraulic conductivity (Kfs) with space, time and antecedent soil moisture conditions can be important for optimum management of cracking clay soils. The Guelph Permeameter (GP) method is a potentially valuable technique for measuring Kfs and its variation in cracking soils, but concerns exist regarding (1) impacts of smearing and compaction in the GP well on the Kfs measurement, (2) accurate detection of when steady flow from the GP is attained, and (3) time required for the GP to reach steady flow (equilibration time). These concerns were investigated for a series of GP measurements in a cracking clay soil (Vertic Xerochrept) in Sicily by testing a plucking implement for removal of smearing and compaction, and by comparing three approaches for detecting steady flow and equilibration time. The mean Kfs values (11.4-22.1 mm h–1) obtained from the GP measurements were 1-3 orders of magnitude larger than what might be expected for a clayey texture, indicating that ponded infiltration from the GP wells occurred primarily through highly permeable soil shrinkage cracks, rather than through the low permeability clayey matrix. Wells that were treated with the plucking implement produced a non-significant (P < 0.05) factor of 1.94 increase in mean Kfs relative to untreated wells, suggesting that this method for removing smearing and compaction functioned primarily by reopening partially smeared-over shrinkage cracks or other macropores. Relatively short mean equilibration times were obtained for the GP measurements (11-42 min), suggesting that the GP method can measure the soil’s antecedent Kfs before wetting from the well causes enough soil swelling to produce a declining permeability. The traditional “four equal readings” (FR) procedure for estimating GP equilibration time substantially underestimated the more recent “visual estimation” (VE) and the new “cumulative drop” (CD) procedures. This underestimation by the FR procedure resulted in overestimates of Kfs relative to the VE and CD procedures for Kfs values less than about 4 mm h–1. It was concluded that the GP method can be a viable technique for measuring Kfs in cracking clay soil.

Estimating the USLE Soil Erodibility Factor in Sicily, South Italy

The Universal Soil Loss Equation (USLE) is used by professionals and technicians to predict soil loss by water erosion and to establish soil conservation measures. One of the key elements of the USLE is the K factor, which is a measure of the soil erodibility. Given the difficulty in collecting sufficient data to adequately measure K, early in the USLEs history the soil erodibility nomograph method was developed to allow estimation of K based on standard soil properties. Since the nomograph approach was developed based on a small number of soils in the United States, it is necessary for other contexts to check the nomographs ability to predict the soils true erodibility. Considering that soil organic matter data are difficult to obtain, an estimation procedure of the soil erodibility factor, K, based only on soil textural data is desirable. In this investigation, the soil erodibility factor was first experimentally determined for the clay soil at the Sparacia (Sicily) experimental station. A relatively low value (0.039 t ha h ha-1MJ-1mm-1) was determined, and summer erodibility was found to be more than twice the value of winter erodibility. This measured K value was 1.85 times the nomograph K, which for many practical applications is not a large difference. Finally, using 1813 data points, a procedure for estimating K using only soil textural data was developed for Sicily. The errors of the predictions did not exceed a factor of two and three for 94.4% and 99.2% of the data points, respectively, suggesting a satisfactory ability of the developed procedure to yield an estimate of K with a reduced input dataset.

INFLUENCE OF THE PRESSURE HEAD SEQUENCE ON THE SOIL HYDRAULIC CONDUCTIVITY DETERMINED WITH TENSION INFILTROMETER

An increasing and a decreasing sequence of pressure head, h0, values were applied with the tension infiltrometer (TI) to determine the corresponding hydraulic conductivity, K0. The pressure head sequence is expected to influence the K0 results given the hysteretic nature of the hydraulic conductivity relationship. The objective of this study was to evaluate the influence of the selected pressure head sequence on the hydraulic conductivity of a sandy loam soil measured by a multipotential TI experiment. Twenty experiments were carried out by applying h0 values varying between -150 and +5 mm (site A). The h0 values ranged from -150 to -10 mm in another 20 spots (site B). Both wetting and drying values of K0 corresponding to h0 = -150, -75, and -30 mm were calculated for each experiment using the measured steady-state flow rates. At both sites, higher K0 results were obtained with the descending h0 sequence than with the ascending one. The deviations between the two sequences were more noticeable in site A (deviations by a factor ranging from 2.1 to 3.3, depending on h0) than in site B (deviations by a factor ranging from 1.0 to 2.2), and the values decreased as h0 increased. For most of the considered type of site/pressure head combinations, the differences between the K0 results were statistically significant (P = 0.05). In all cases, the coefficients of variation of the K0 data obtained with the two sequences differed at most by a factor of 1.2, suggesting that the applied h0 sequence did not affect appreciably the relative variability of the K0 results. It was concluded that the dependence of the K0 estimates on both the pressure head sequence (ascending or descending) and the highest value of h0 used within a descending sequence experiment may be neglected for a rough hydraulic characterization of the selected area. However, both factors should be maintained constant in order to obtain truly comparable K0 data from different experiments.

MANUAL SAMPLING AND TANK SIZE EFFECTS ON THE CALIBRATION CURVE OF PLOT SEDIMENT STORAGE TANKS

In many experimental soil erosion plots, runoff is collected and carried by a conveyance system to a sequence of storage tanks. If the soil loss is measured by collecting, after mixing, samples of the stored suspension, then a calibration curve between the actual mean concentration (C) and the measured concentration (Cm) in the storage tank occurs. The aim of this article was to evaluate experimentally the factors affecting the relationship between C and Cm. For a sandy loam soil, the replicated measurements of Cm (20 samples) for two values of the actual concentration (C = 5 and 25 g/L) showed that the variability of the measurements of Cm is low and confirmed the reliability of a calibration curve obtained by a single series of runs. Results from experiments carried out with a clay soil to compare the calibration curves obtained by four field workers suggested that the maximum uncertainty in the soil loss measurement due to the choice of the calibration curve should not exceed 100% of the true value. Moreover, the slope of the calibration curve was independent of both the water level in the tank and the field worker. Finally, a comparison among the calibration curves of a prototype tank and some model tanks was carried out for both a sandy loam and a clay soil in order to establish a scaled-up relationship among tanks of different size. Soil-specific and theoretically based scaled-up relationships were deduced.

Influence of well preparation on field-saturated hydraulic conductivity measured with the Guelph Permeameter

Abstract An investigation on the influence of the well preparation technique on field-saturated hydraulic conductivity K fs measured with the Guelph Permeameter (GP) method was performed in a sandy loam soil. In particular, the influence of adopting the following procedures to prepare the well was investigated: use of a wire screen insert to prevent sinking of the water outlet tip of the GP into the base of the well during a measurement; use of a metallic, sharpened rod to remove the smear layer created during the digging process from the well walls. The influence of the well radius and the antecedent soil water content on the K fs estimates was also evaluated. Five different types of wells were prepared. Higher and less variable K fs values were obtained from the wells treated with the wire screen insert and the metallic rod than from the non-treated ones. These results were consistent with the hypothesized occurrence of sinking and smearing phenomena in the non-treated wells and with the effectiveness of the used procedures to prevent or reduce the influence of these phenomena on the K fs measurement. The estimates of field-saturated hydraulic conductivity increased with the radius of the well. Differences between the sampled soil volumes and a more pronounced influence of sinking phenomena in the small-size wells likely contributed to produce this result. In relatively dry soil conditions, the antecedent soil water content did not affect the K fs measurements, independently of the well preparation technique.

EFFECT OF CONTACT MATERIAL ON TENSION INFILTROMETER MEASUREMENTS

A layer of contact material is often used to establish and maintain hydraulic contact between a tension infiltrometer membrane and the soil surface. The aims of this study were to determine the change in hydraulic properties of two types of contact materials after repeated use of the materials and to evaluate the effect of contact material on steady–state infiltration rates. Re–using a natural sand contact material resulted in increasing values of satiated hydraulic conductivity, Kst (Kst = 246 to 311 mm h –1 ), due to a progressive loss of fine–textured particles. However, Spheriglass No. 2227 glass spheres, previously proposed as a suitable contact material, maintained stable hydraulic properties after repeated use both in the laboratory and in the field (Kst = 264 to 267 mm h –1 , water entry pressure head hw = –400 to –360 mm, air entry pressure head ha = –640 to –650 mm). The steady–state tension infiltrometer infiltration rate, i0, at a pressure head h0 = –50 mm was reduced by about 30% by removing the contact material from the smoothed and leveled surface of a sandy loam soil. It was concluded that Spheriglass No. 2227 glass spheres are adequate for both use and reuse as tension infiltrometer contact material, and that contact material is required to maintain good hydraulic connection with the soil surface even when the surface has been smoothed and leveled.

Establishing a Soil Loss Threshold for Limiting Rilling

AbstractIn this paper a frequency analysis of event soil loss measurements collected in the period 1999–2012 at the microplots and plots of the Sparacia Experimental Area in Sicily, southern Italy, was developed. The analysis was carried out using the annual maximum soil loss measurements normalized by the mean soil loss measured at a given temporal and spatial scale. The empirical frequency distribution of the normalized variable was well fitted by two Gumbel’s theoretical probability distributions discriminated by a value of the normalized variable equal to 2. This last value discriminates between the relatively low and frequent values of the normalized variable and the high and rare ones. The annual maximum soil loss was demonstrated to be representative of the total annual soil erosion at the Sparacia Experimental Area. Then, a threshold soil loss value at the annual temporal scale was calculated by multiplying the frequency factor, equal to 2, by the mean annual maximum soil loss values for each give...

Effects of Laboratory Procedures on the Integrity of a Sandy-Loam Soil Sample for Bi-Directional Measurement of Saturated Hydraulic Conductivity

Practical and validated methods to measure anisotropy of saturated, Ks, or field-saturated, Kfs, soil hydraulic conductivity on a single soil sample are still lacking. The objective of this investigation was to test factors affecting anisotropy measured in a sandy-loam soil by the constant-head laboratory permeameter and the modified cube method. The ratio, RK, between the conductivity results obtained with a long (6 h) and a short (0.5 h) duration run varied from a minimum of 0.29 to a maximum of 0.88 with the considered experimental procedure, differing by both the employed constant-head device (Mariotte bottle, siphon) and the initial soil water content (unsaturated, saturated). Maximum time stability was detected with a siphon and an initially saturated soil sample, suggesting that soil alteration at the surface may influence the measured conductivity, and more reliable anisotropy measurements have to be expected for Ks than Kfs. A short-duration run carried out with a siphon and a calcium solution was also suggested for laboratory determination of Ks anisotropy since more representative results were obtained for a single cube of soil (i.e., horizontal, Kh, to vertical, Kv, conductivity ratio varying from 1.78 to 1.82 with the measurement sequence) as compared with the ones associated with using tap water (ratio varying from 1.24 to 2.09). The Kh/Kv ratio varied between 1.80 and 2.81 on two dates (April and July 2009) differing by the soil water content at the time of sampling (close to field capacity; lower than the permanent wilting point, respectively), and a significant temporal variability was only detected for Kv. Therefore, temporal variability of Ks anisotropy cannot be predicted by measuring one-dimensional Ks.