3.2 Process connectivity framework and observed erosion
In terms of process connectivity overland flow enhancement factors, on-the-ground measures of soil infiltration capacity and soil erodibility (aggregate stability) across the different agricultural plots was beyond the scope of this drone survey-based study. We hypothesised, however, that at a higher level, correlation of plot cultivation direction with high density of rill erosion could be illustrative of cultivation controls on potential overland flow generation in the context of process connectivity concepts that encompass impacts on overland flow generation and slope length.
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Figure 6: Aerial photographs of key illustrative zones showing field boundaries, cropping practice and incidence of rill erosion.
Zones of different time since plot establishment, configuration and within plot practice showed different degrees of rill density and network development (Figure 6). In zone B, where the greatest density of rill erosion was observed, there were two main rill networks (labelled B1, B2, Figure 7) connecting upper slope to lower and discharging onto the main track running E-WSW which ultimately spills across the pastoral land at E. The agricultural plots in B1 are oriented across slope so boundaries traverse any potential flow lines in accord with local ‘good practice’ but cultivation lines within the plots are largely in the direction of slope (indicated by amber and red arrows, Figure 7), with the exception of the lowermost plots. On top of that, the higher plots are also developed on very steep slopes (>12°). In zone B2, again the plots themselves are largely and appropriately orientated across the slope but within plots, there is a mosaic of cultivation directions. The rill network on this slope is well developed and crosses all cultivated plots connecting them to the main track way. It is also important to note that the slope length on these plots is also longer.
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Figure 7: Comparison of first order rill density data and orientation of cultivation with respect to slope
Zones A and D contrast with the situation of zone B. Both zones A and D show more moderate observation of rill density (Figure 6). In zone A, the plots are contoured around a valley head. Plots to the northern side have crop lines contoured with plot boundaries (green arrows, Figure 7) and little evidence of rill erosion. To the southern side, crop lines show some alignment to slope (amber arrows, Figure 7) and there is moderate rill erosion in the lower plots (Figure 6). In Zone D, plot boundaries are more exactly contoured to topography than zone B (Figure 6) and there is only one incidence of cultivation lines being with the slope, the remainder being intermediate or contoured. Rills in this area do not display the same level of development and connectivity as zone B. Zone C offers a different scenario where while cropping lines are contoured, moderate rill erosion was determined (Figure 7) and apparently concentrated along plot boundaries before overtopping edge-of-field bunds (Figure 6). The extensive erosion of gentle to moderately sloped pastoral land, zone E (Figure 6), occurred at the outlet of the track fed by extensive rill erosion in zone B with some contribution from zone C.