Modeling spatial variation in productivity due to tillage and water erosion

The advent of precision farming practices has heightened interest in managi ng field variability to optimize profitability. The large variation in yiel ds across many producer fields demonstrated by yield-monitor-equipped combi nes has generated concern about management-induced causes of spatial variat ion in soil productivity. Soil translocation from erosion processes may res ult in variation in soil properties across field landscape positions that p roduce long-term changes in soil productivity. The objective of this study was to examine the relationships between soil redistribution caused by till age and water erosion and the resulting spatial variability of soil product ivity in a soil catena in eastern South Dakota. An empirical model develope d to estimate tillage erosion was used to evaluate changes expected in the soil profile over a 50-year period on a typical toposequence found in easte rn South Dakota and western Minnesota. Changes in the soil profile due to w ater erosion over a 50-year period were evaluated using the WEPP hillslope model. The tillage erosion model and the WEPP hillslope model were run conc urrently for a 50-year period to evaluate the combined effect of the two pr ocesses. The resulting changes in soil properties of the root zone were eva luated for changes in productivity using a productivity index model. Tillag e erosion resulted in soil loss in the shoulder position, while soil loss f rom water erosion occurred primarily in the mid to lower backslope position . The decline in soil productivity was greater when both processes were com bined compared to either process acting alone. Water erosion contributed to nearly all the decline in soil productivity in the backslope position when both tillage and water erosion processes were combined. The net effect of soil translocation from the combined effects of tillage and water erosion i s an increase in spatial variability of crop yields and a likely decline in overall soil productivity. (C) 1999 Elsevier Science B.V. All rights reser ved.