Improved process descriptions in the PESERA model

Table of Contents

1. Authors
2. Abstract
3. Introduction
4. General extensions to PESERA
   1. Grazing
   2. Fire
   3. Wind Erosion
5. Particular extensions to PESERA
   1. Mulching and/or maintaining ground cover vegetation within tree crops
   2. Retention of crop residues as litter layer at harvesting of arable and other crops
   3. Irrigation and water harvesting for croplands
   4. Invasion and clearance of unpalatable species
   5. Terracing and strip cropping
   6. Nitrogen budgeting and rotations
6. Other extensions to PESERA
   1. Mass movements
   2. Data collection calibration of PESERA against reservoir data
7. Conclusions
8. References

Terracing and strip cropping

It is possible to represent patterns of terracing and strip cropping with a sub-grid model, explicitly representing the morphology and management patterns at a finer resolution within a single (1 km) grid cell. Here we illustrate this approach for strip cropping and terracing across a uniform 100 m slope with 15 m elevation. In Figure 2.4, the area has been separated into equal strips with different land covers, represented here by different runoff thresholds of 30mm and 90mm respectively. Curves show the calculated sediment transport (in red) and the denudation (averaged from the top of the slope to the point in question) for every point on the slope, for an average year of storms, with a mean rain per rain-day of 10mm, falling on 50 days in the year. It can be seen that the denudation varies between limits of +2.4mm to -3.6mm. However the average (0.51mm) is very similar to that estimated for a uniformly covered slope with the average runoff threshold for the two types of strip (60mm), which gives an average denudation of 0.48mm. We are therefore modelling such strip-cropped areas with a runoff threshold that is the areally weighted average of the land cover types within the cell.

Figure 2.4. Sub-grid model for strip cropping on a 15% uniform slope.

Similarly terracing has been simulated at the sub-grid scale, with both 'soft' terraces in which the riser is not protected in any way and 'hard' terraces in which the riser is protected with stones or vegetation to increase its infiltration and reduce its erodibility. Figure 2.5 shows example output from such a model. It can be seen that the terrace risers produce local peaks in erosion, but that the overall effect is almost identical to the erosion from a uniform slope, at the gradient of the terrace step, but with the weighted average runoff threshold (across read and riser). This then provides the simple modelling rule that is used at the coarser grid scale of PESERA. It can also be seen that local erosion is concentrated on the tops of each riser, which should be reinforced and perhaps protected by diverting any pooled runoff away from the edge.

The effects on the effective modelled relief are more ambiguous for terracing. The lower gradients improve water retention in the lower part of the terrace treads, and this is accentuated by the re-deposition of any material eroded from above. However, in semi-arid climates, most rainfall is evaporated so that this effect is thought to be quite slight. However experiments suggest that the effective relief used in the grid cell should be reduced in the same proportion as the ratio of terrace tread gradient to overall average gradient.

Figure 2.5. Sub-grid model of 'hard' terraces, with 6% treads on an average 15% slope.