Scientific registration n : 2159 Symposium n : 20 Presentation : poster. REJMAN Jerzy, USOWICZ Boguslaw, KARPETA Krzysztof

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1 Scientific registration n : 2159 Symposium n : 20 Presentation : poster The effect of surface roughness and sealing on runoff and wash on a loess soil of south-east Poland Effets du microrelief et de la fermeture de la surface sur le ruissellement et l'érosion d'un sol développé sur loess dans le Sud-Est de la Pologne REJMAN Jerzy, USOWICZ Boguslaw, KARPETA Krzysztof Institute of Agrophysics, Polish Academy of Sciences, Dolwiadczalna Str. 4, Lublin, P.O.Box 201, Poland. 1. Introduction Soil reaction on rainfall appears initially in leveling of surface roughness and development of seal. Sealing causes a decrease of infiltration and an increase of runoff, and thus affects soil erosion. The process of sealing is very rapid on soils developed from loess that are characterized by very unstable aggregates. Especially susceptible for sealing and erosion processes are soil conditions after seedbed preparation. However, the proper evaluation of the micro-relief and sealing effect on erosion process, needs establishing the real collecting area of runoff and soil loss. Inside the USLE, the most widespread technology of field erosion studies, it is assumed that soil loss and runoff is collected from the whole area of the standard plot (Wischmeier and Smith, 1978). Use of the threshold value of 12.3 mm for erosive rainfalls to calculate rainfall and runoff erosivity factor R also assure that soil loss is collected from the whole plot area. For regions characterized by smaller rainfall intensity than in the USA, the collecting area of soil loss and runoff still needs to be tested. So, the purpose of the studies was to quantify the effect of surface roughness changes and influence of tillage implements on runoff and sediment yield on basis of the real distance from which soil loss and runoff is collected on loess soil of south-east Poland. 2. Methods 2.1 Microrelief measurements Studies were provided on a soil developed from loess (Orthic Luvisol), classified as a colluvial at Czes³awice (20 km from Lublin). The soil was characterized by 20% content of sand (including fine sand 19.1%), 70% of silt, 10% of clay, and 1.69% OM. Measurements of surface elevations were performed with a contact meter on two bare plots, the first one with random (non-oriented) and the second one with oriented microrelief. The first surface was achieved by field cultivator operations in crossed direction, the second by harrowing. A plot with random micro-relief was situated on a leveled area and a plot with oriented micro-relief on a 10% slope. 1

2 Measurements of surface elevations were taken in a grid 1cm x 1cm over the area 924 cm 2 (28 x 33 cm) after each period of rainfalls. There were 6 measurement periods, from to On the basis of elevations measurements, the following surface indexes were calculated: random roughness RR (Allmaras et al., 1966), microrelief index MI and modified microrelief index MIF (Römkens and Wang, 1986), and SSA (Helming et.al, 1993). The rainfall data were taken from a meteorological station, situated 500 m from the experimental site. On the basis of recording raingauge data, kinetic energy of rainfalls was computed using the equation E=29.0 ( exp(-0.05 I)) after Brown and Foster, Field erosion measurements Field studies of erosion were conducted on a soil developed from loess (Orthic Luvisol), classified as a slightly eroded at Bogucin (10 km from Lublin). The soil was characterized by 20% sand (including 19% of fine sand), 69% silt, 11% clay, and 1.8% OM. The frame of the experiment consists of plots of different length, two plots of 20 m, one of 10 m, and one of 5 m. The width of each plot was 3 m. To define runoff and soil loss collecting area, plastic borders were driven into soil to depth of 10cm. The plots were established in April 1997 on a uniform slope of 12%. At the lower part of the plots, runoff collectors were placed. At the distance of 2 m from the lower part of the plots, TDR probes were placed to monitor soil moisture content changes. Soil loss and runoff measurements started from 15 May and lasted to 31 August There were 9 measurement periods during that time. The plots were kept in continuous fallow with 3 tillage implements (with a hand rake) destroying seal at the soil surface (May 10, June 30, August 11). Rainfall characteristics were determined on the basis of data collected from a recording raingauge situated near the plots. 3. Results 3.1. Microrelief studies Under total rainfall amount of mm and kinetic energy of MJ/ha, the roughness indexes ranged from 1.70 to 1.29 (RR), mm -1 (MI), (MIF), (SSA) on random surface, and from 1.49 to 0.99 (RR), mm -1 ((MI), (MIF), (SSA) on oriented surface. The biggest change in micro-relief was observed in the first and last measurement period. The former change was connected with soil setting, and the latter with heavy rainfall event (47% of total amount) and runoff. Relationships between indexes of random (r) and oriented (o) surface can be described by the equations: MIr = MIo (R 2 =0.96), MIFr = MIFr (R 2 =0.82), SSAr = SSAo (R 2 =0.79), and Rrr = RRo (R 2 =0.70). To reflect changes in microrelief a spatial analysis was applied. Parameters of semivariograms were calculated according to Variowin 2.1 (Panatier, 1994). The spherical model of the semivariogram was predominant. It is in contradiction to the data of Lehrsch et al who found the hole model predominant. In the case of the experimental data from Czes³awice, the main model was modified by a secondary spherical or power model, and the zone of influence was independent on time and ranged from 6.1 to 8.6 cm. 2

3 Changes of surface elevations for the first and the last measurement period estimated by kriging are shown on Fig. 1. The estimated kriging standard deviation of the spatial distribution of the surface elevation were not higher than 3.77 mm, and 2.75 mm for random micro-relief, and than 2.3 mm, and 1.29 mm for oriented micro-relief, on the first and the last period of measurements, respectively. A B Figure 1. The estimated maps of soil surface elevation for the first and last measurement period for random (A), and oriented (B) micro-relief. The surface variograms presented on Fig. 2 show structure of two micro-reliefs. The isotropy of the random micro-relief was maintained during the whole period of measurements, whereas anisotropy of the oriented micro-relief showed a new direction of changes of roughness structure. The direction of changes on oriented plot (localized on slope) can be identified with an appearance of runoff preferred direction. Usually, the relationship between micro-relief parameters and rainfall is described as a ratio of parameter after rainfall to its initial value before rainfall and this ratio is related to exponential function of rainfall amount or kinetic energy (Zobeck and Onstad, 1987). On the basis of the experimental data from Czes³awice a simple model of another type was suggested that described changes of micro-relief on the basis of kinetic energy and initial status of micro-relief. As a micro-relief parameter SSA index (ratio of total surface area to map area, Helming et al., 1993) was used. It was assumed that micro-relief difference A B 3

4 Figure 2. Surface semivariograms of relative elevations, random (A) and oriented (B) micro-relief. ( SSA C = SSA i+1 -SSA i ) is dependent on the kinetic energy of rainfall (EK) and on the initial status of micro-relief (SSA i ). The function should reflect the quick change of initial rough micro-relief, so the exponential form was applied. Calculated difference of microrelief ( SSA C ) could then be described by the following equation : SSA C = a EK exp[k(ssa i -1)] where: a, k are experimentally derived parameters. Parameter (k) was constant for both micro-relief (k= 10) and could be connected with soil type, whereas parameter (a) was equal 1000 for oriented micro-relief situated on slope, and equal for random micro-relief situated on leveled area. Coefficients of correlation (r) between calculated and experimental difference of micro-relief are for random site, and for oriented site. Table 1. Ratio of total surface area to map area, measured difference SSA exp, calculated difference SSA C, and kinetic energy and initial SSA i. Measur. period Kinetic energy, MJ/ha Random micro-relief 4 Oriented micro-relief SSA exp SSA i SSA c SSA exp SSA i SSA c Soil erosion studies Soil loss and runoff Rainfall characteristics and erosion are presented in Table 2. Comparing soil loss and runoff data from plots of different size it was found that erosion per unit area decreases with plot size. For all measurement periods, the highest soil loss and runoff per unit area was found on the smallest plot of 15 m 2 (5 x 3m) despite the existence of rainfalls higher than 12.3 mm (in 6 for 9 measurement periods). Regressing erosion against rainfall characteristics, the best correlation was found for erosion and rainfall amounts of intensity higher than 5 mm/h (Fig. 3). The value most

5 probably is close to final infiltration rate for the soil as it suggests our initial experiments with rainfall simulator. Rainfalls with intensities above 5 mm/h were 42% of total rainfall amounts. Usually higher value of correlation coefficient between soil loss and erosion index EI 30, in this case was smaller (R 2 =0.56). Better correlation between soil loss and rainfall amount could be the effect of high soil moisture content at topsoil (above 23% m 3 m -3 ) that was maintained at a similar level during the all measurement periods. Table 2. Characteristics of rainfall above 0.1 mm, soil loss and runoff per unit area on the smallest plot of 15m 2, and average effective distance, Bogucin. Measurement Rainfall characteristics Plot 15m 2 Effective distance, m period amount, EI 30 index, soil loss runoff for soil loss for runoff mm MJ mm ha -1 h -1 kg m -2 dm 3 m -2 avg std avg std * ** total * predicted data ** not all runoff collected on larger plots The value of soil erodibility calculated as a ratio of total soil loss to EI 30 index ( Mg h MJ -1 mm -1 ) was similar to the average value of soil erodibility determined for slightly eroded loess soil in 3-year field studies conducted in another site by Rejman et al. (1996) Effective distance for soil loss transport and runoff It can be assumed that soil loss and runoff volume collected on the smallest plot should be similar to soil loss and runoff volume collected from the same area of larger plots. Other words, it means that for the same area, despite differences in plot size, unit soil loss and runoff is similar. It is very probable that the surplus of soil loss and runoff registered on larger plots comes from further area. Taking into account unit loss, the real collecting area of soil loss and runoff can be calculated. The collecting area divided by the plot width enables to establish the effective distance from which soil loss and runoff was collected (Table 2). In spite of erosive rainfalls with amounts higher than 12.3 mm, the effective distance for soil transport does not exceed 15 m, i.e. it was shorter than length of standard erosion plot. In each case, the distance from which soil loss was collected was longer than the distance from which runoff was collected. Application of tillage implements disturbing seal surface resulted in decrease of the effective distance for runoff, and especially for soil transport, but seems to have no effect on the runoff and soil loss per unit area (Fig. 3). The significant decrease in soil transport distance was observed only for the period of measurements just after the tillage 5

6 implements. Excluding the measurement periods just after tillage implements, variations of the soil loss distance can be explained in 75% by the rainfall amount of intensities higher than 5 mm/h. A B 0,15 Soil loss, kg m -2 0,12 0,09 0,06 0,03 after tillage implement y = 0,0053x - 0,0277 R 2 = 0,87 0, Rainfall, mm C D 15 Effective distance for runoff, m after tillage implement predicted data y = 0,2423x + 2,358 R 2 = 0, Rainfall, mm Figure 3. The relationships between runoff (A), soil loss (B), effective distance for runoff (C) and soil transport (D), and rainfall amount with intensities higher than 5 mm/h. Regression function on Fig. 3D is determined without data after tillage implement. 4. Summary Field erosion studies on a loess soil conducted on plots of different size show that both soil loss and runoff are collected from a certain distance not the whole length of standard erosion plot. Despite the existence of rainfalls above 12.3 mm, the soil transport distance was in the range from 4 to 15 m. The effective distance for runoff was shorter and was on average 75% of soil transport distance. Application of tillage implements destroying surface seal decreased the distance on which soil and runoff is transported, however it seems to have no influence on soil loss and runoff inside the distance i.e. soil loss and runoff per unit area. The effect of tillage was significant only for the first periods of rainfall after the tillage implement. It corresponds with exponential change of microrelief, that was one of the assumption of the proposed model to predict microrelief on the basis of kinetic energy and initial status of microrelief. A semivariogram analysis was used to describe spatial dependence of 6

7 elevation heights. The dependence was best identified by a spherical model modified by secondary spherical or power models, with zones of influence ranged from 6 to 8 cm. 5. References Allmaras, R.R., Burwell R.E., Larson W.E., Holt R. F., Total porosity and random roughness of the interrow zone as influenced by tillage. USDA Conserv. Res. Rep. 7, 22 p. Brown L.C., Foster G.R Storm erosivity using idealised intensity distributions. Transactions of the ASAE, 30: Helming K., Roth Ch. H., Wolf R., Diestel H., Characterization of rainfallmicrorelief interactions using parameters derived from digital elevation models (DEMs). Soil Technology, 6: Lehrsch G. A., Whisler F. D., Römkens, M.J.M., Spatial variation of parameters describing soil surface roughness. Soil Sci. Soc. Am. J., 52: Rejman J., Turski R., Paluszek J., Spatial and temporal changes in erodibility of loess soil. Soil Technology, in press. Römkens, M.J.M., Wang, J.Y, Effect of tillage on surface roughness. Transactions of the ASAE, 29: Wischmeier W.H., Smith D.D Predicting rainfall erosion losses. Agr. Handbook U.S. Dept. Agr., Washington D.C Zobeck T.M., Onstad C.A., Tillage and rainfall effects on random roughness: A review. Soil & Tillage Res., 9: Acknowledgments This study was supported by Polish Committee of Scientific Research under the grant 5PO6B Key words: surface roughness, sealing, runoff, soil loss, loess soil, geostatistics Mots clés: microrelief, croûte, ruissellement, perte en sol, loess, géostatistique 7