Stochastic use of the LEACHN model to forecast nitrate leaching in different maize cropping systems

This paper proposes a stochastic application of a deterministic model (LEAC HN) with the aim of forecasting the probability of exceeding given nitrate leaching levels for different cropping systems and soil hydrological charac teristics. The understanding of the level of probability associated to the prediction of leaching is an important criteria for the judgement of croppi ng systems. After calibration of organic matter mineralization and nitrific ation rates in both a sandy-loam and a loamy soil of the Western Po river v alley (Northern Italy), LEACHN was used as a stochastic tool to evaluate th e meteorological variability and the spatial variability of hydrological pa rameters of a soil. Meteorological variability was generated using series o f measured air temperature, rainfall and global radiation for a period of a t least 15 years, and these were then expanded to 100 years using the clima te simulator CLIMGEN. soil variability was simulated using the scale factor approach. The scale factor mean and standard deviation were obtained in ei ght locations within a 1000-m(2) area. The stochastic scale factors were ap plied to parameters a and b in Campbell's water retention function and to h ydraulic conductivity. The following combinations of crops were simulated i n the two soils: (a) continuous maize for silage (MM); (b) continuous maize for grain (MG); (c) a combination of late harvested Italian ryegrass and s hort cycle silage maize (LRM); and (d) early harvested Italian ryegrass and late maturing silage maize (ERM). The crops were fertilized with 200 or 30 0 kg N ha(-1) year(-1) (or 450 kg for MM) and submitted to three water regi mes: no irrigation, irrigation on the basis of water balance and convention al irrigation, which resulted in the highest volume. The simulated leaching was higher when fertilization and irrigation inputs were higher. It was fu rther reduced by the introduction of cover-crop and was higher in the sandy soil. All these factors interacted, creating different levels of nitrate l oss risk, that ranged from a minimum leaching of 4 kg N ha(-1) year(-1), wi th a 10% breakthrough probability of 16 kg N ha(-1) year(-1) (low fertilize d and irrigated ERM in the sandy-loam soil) to a maximum average leaching o f 146 kg N ha(-1) year(-1) with a 10% breakthrough probability of 235 kg N ha(-1) year(-1) thigh fertilized, conventionally irrigated MM in the sandy soil). The breakthrough probability curves associated to nitrate leaching a re skewed, showing that lower than average values are more frequent than hi gher ones. The standard deviations of yearly leaching were closely correlat ed to the means and were frequently greater than the means, particularly in the sandy soil. The stochastic simulation results offered the possibility of ranking cropping systems into classes of probability of exceeding a give n value of leaching and the possibility of deriving suggestions for improve d crop management. (C) 2000 Elsevier Science B.V. All rights reserved.