Relationship between surface roughness decay and surface seal development and its effects on interril erodibilty and pesticide transport

1. Objectives This study investigated processes affecting soil aggregate breakdown, soil surface roughness decay and surface seal development. Furthermore relationships between roughness decay and seal development and their effects on the partitioning of water, sediments and pesticides were examined. 2. Materials and Methods Experiments were carried out in the lab using rainfall simulations and in the field under natural rainfall conditions. The used soil was classified as loam. Three initial soil surface roughness conditions were investigated. In the field they were achieved by rotary tiller, chisel plough and moldboard plough. In the lab the different roughness was obtained by using soil sieved through 20 mm- (smooth), 63 mm- (medium), or 200 mm-sieve (rough). In the laboratory air dried top soil was placed in an erosion pan and exposed to simulated rainfall with constant (40 and 60 mm h-1 ), increasing (20-40-60 mm h-1 ) and decreasing (60-40-20 mm h-1 ) rainfall intensity sequences. Soil loss, splash erosion, splash water, runoff and percolation were measured in five minute intervals throughout each experiment. Soil moisture content and surface shear strength were measured at selected times during the runs. Each experiment was replicated at least once. Measurements in the field were done during growing seasons 1998-2000. For each roughness variation 1m-plots with at least one replication were installed. Runoff and erosion were measured after each erosive rainfall event. Temporal changes of soil microtopography were examined with a reliefmeter (1998) and an automatic laser scanner device (1999, 2000). 3. Results 33 rainfall simulations were performed in the lab. Various soil surface roughness indices were calculated and compared. All indices decreased with accumulated rainfall amount between 2 and 80 % from initial value. The relationship between roughness decay and accumulated rainfall amount could be expressed by regression curves in the form of second order polynoms. Surface runoff was influenced significantly by initial roughness condition and rainfall intensity sequence. Smoother surface condition and higher initial rainfall intensities led to earlier start of runoff and to a more rapid increase of runoff rates. Soil loss depended significantly on rainfall intensity sequences but not on roughness condition. Splash erosion amounts were appr. four times higher than amounts of soil loss, hence sediment was not a limiting factor in the erosion process. Splash erosion did not vary significantly with roughness, whereas influence of intensity was obvious. Fast wetting of soil surface induced by higher intensities at beginning enforced slaking and led to significantly higher splash rates. For field measurements seedbed condition showed highest roughness decay, moldboard ploughed plots lowest. Roughness decay can be described as function of kinetic rainfall energy by exponential regression curves. Field measurements showed significant higher 3-year-average runoff values for rotary tilled plots than for chisel and moldboard plough conditions. Runoff coefficient was highest for rotary tilled plots (2,51 %), while moldboard (0,87 %) and chisel plough (0,82 %) produced similar values. Average soil loss from field plots did not differ significantly between tillage practices which corresponds to lab experiments. Average measured values of the variations were 9,22 t ha -1 for rotary tiller, 7,49 t ha -1 for chisel plough and 6,37 t ha -1 for moldboard plough.