Long-term effects of management practices on water-driven soil erosion in an intense agricultural sub-watershed: monitoring and modelling

The overarching objective of this research was to provide an improved understanding of the role of land use and associated management practices on long-term water-driven soil erosion in small agricultural watersheds by coupling the established, physically based, distributed parameter Water Erosion Prediction Project (WEPP) model with long-term hydrologic, land use and soil data. A key step towards achieving this objective was the development of a detailed methodology for model calibration using physical ranges of key governing parameters such as effective hydraulic conductivity, critical hydraulic shear stress and rill/inter-rill erodibilities. The physical ranges for these governing parameters were obtained based on in situ observations within the South Amana Sub-Watershed (SASW) (~26 km²) of the Clear Creek, IA watershed where detailed documentation of the different land uses was available for a period of nearly 100 years. A quasi validation of the calibrated model was conducted through long-term field estimates of water and sediment discharge at the outlet of SASW and also by comparing the results with data reported in the literature for other Iowa watersheds exhibiting similar biogeochemical properties. Once WEPP was verified, 'thought experiments' were conducted to test our hypothesis that land use and associated management practices may be the major control of long-term erosion in small agricultural watersheds such as SASW. Those experiments were performed using the dominant 2-year crop rotations in the SASW, namely, fall till corn-no till bean (FTC-NTB), no till bean-spring till corn (NTB-STC) and no till corn-fall till bean (NTC-FTB), which comprised approximately 90% of the total acreage in SASW. Results of this study showed that for all crop rotations, a strong correspondence existed between soil erosion rates and high-magnitude precipitation events during the period of mid-April and late July, as expected. The magnitude of this correspondence, however, was strongly affected by the crop rotation characteristics, such as canopy/residue cover provided by the crop, and the type and associated timing of tillage. Tillage type (i.e. primary and secondary tillages) affected the roughness of the soil surface and resulted in increases of the rill/inter-rill erodibilities up to 35% and 300%, respectively. Particularly, the NTC-FTB crop rotation, being the most intense land use in terms of tillage operations, caused the highest average annual erosion rate within the SASW, yielding quadrupled erosion rates comparatively to NTB-STC. The impacts of tillage operation were further exacerbated by the timing of the operations in relation to precipitation events. Timing of operations affected the 'life-time' of residue cover and as a result, the degree of protection that residue cover offers against the water action on the soil surface. In the case of NTC-FTB crop rotation, dense corn residue stayed on the ground for only 40 days, whereas for the other two rotations, corn residue provided a protective layer for nearly 7 months, lessening thus the degree of soil erosion. The cumulative effects of tillage type and timing in conjunction with canopy/residue cover led to the conclusion that land management practices can significantly amplify or deamplify the impact of precipitation on long-term soil erosion in small agricultural watersheds.