ESTIMATING SPRING WHEAT YIELD VARIABILITY WITH EPIC

The Environmental Policy Integrated Climate (EPIC) model has been used on the semiarid temperate Canadian Prairies to estimate crop yield, s oil erosion loss, and water and nitrate dynamics. While its estimates of long-term average yields are accurate for most purposes, additional model development is desirable to fully reflect year-to-year variabil ity. We tested the precision of EPIC (version 5300) in estimating mean yields and in replicating yearly yield variability as influenced by t he potential evapotranspiration (PET) method, using field data from a 27-yr crop rotation experiment at Swift Current, Saskatchewan. Rotatio ns tested ranged from continuous wheat (Triticum aestivum L.) to fallo w-wheat-wheat. Mean estimated yields were compared with measured yield s (MY) and detrended yields (DY). Estimated yields and MYs were furthe r compared by regression, ratio of variances due to lack-of-fit and to experimental errors (R), and model efficiency (EF). Mean yields estim ated using the Penman-Monteith and the Priestly-Taylor PET methods res ulted in significant underestimations, associated with high annual PET values, and were not analysed further. The Hargreaves (H) and Baier-R obertson (BR) PET methods resulted in mean yields not different than M Y or DY for most cases, especially the BR method. EPIC with the H meth od accounted for 18 to 66% of the variability in annual yield estimati on, whereas the BR method accounted for 29 to 60%. These were slightly , but not significantly, lower than results obtained with regionally d erived statistical crop models. Overall EPIC with the BR PET method pr ovided yield estimates accurate and precise enough for long term studi es. The relatively high R and low EF values obtained, though, suggest further improvements in EPIC are necessary to better replicate yearly yield variability. Analysis of yield residuals indicated that EPIC may not be simulating accurately enough the water balance and its effects throughout the off-season and in the early part of the growing season .