S. Hansen et al Modelling nitrate leaching at different scales.
519
3. only one model simulated all main processes like water dynamics, plant growth and nitrogen dynamics with the same quality, the others are able to reproduce the measured dynamics only in part. The second conclusion strongly emphasizes the importance of proper training of model users as the user is responsible for the model parameterization. The model singled out in the third conclusion was the Daisy model as presented by Svendsen et al. (1995). Examples of the performance of Daisy at model validation exercise are shown in Fig. 8 through 10 illustrating time courses in simulated and observed soil water content, soil water potential, soil temperature, shoot dry matter and leaf area index, and soil mineral nitrogen content and shoot nitrogen content. It should be noted that all the data are from the same site, the so called Intensive Loam Site, and the same year, viz. 1989, where winter wheat was grown. Furthermore, it should be noted that parameters, which could be obtained from the data set, were used directly (e.g. soil organic matter content, soil water retention data, etc.), while the parameters which were not measured at the sites were obtained from Hansen et al. (1990). Hence, regarding the parameterization of the nitrification and denitrification processes, default parameters were used, and regarding the mineralization model the only calibration parameter used was the distribution of soil organic matter between SOM1 and SOM2 (see the model description and discussion earlier this chapter). The simulation of the soil water dynamics is illustrated in Figs. 8 and 9 in terms of volumetric soil water content and soil water pressure potential (tension), respectively. Discrepancies between simulated and observed values are most clearly identified in the more sensitive soil water pressure potential. Discrepancies especially develop in the upper soil layers in May. According to Fig. 11, the winter wheat attains a closed canopy at that time, and hence the depletion of soil water in the upper soil is mainly due to water uptake by plants. At 40 cm depth, the simulation is very satisfactory and in general the simulation of soil water dynamics is satisfactory. Fig. 10 compares simulated and measured values of soil temperature. Soil temperature is of importance because it influences biological processes in the soil. The agreement between simulated and measured values are very satisfactory especially taking into account the extent to which the influence of temperature on biological processes is known. Simulated and measured values of leaf area index (LAI) and accumulated shoot dry matter in the shoot part of the plant is shown in Fig. 11. The version of the Daisy crop model used in these simulations does not simulate the proper LAI. Instead the model simulated a crop area index (CAI, also including stem area etc.). However, in the first part of the growth period leaves are the dominant contributor to the crop area index, so in this period the measured LAI and simulated CAI can be compared. The figure shows a satisfactory agreement between measured and simulated values. Fig. 12 shows comparisons between simulated and measured values of accumulated nitrogen in the shoot and accumulated mineral nitrogen in the rooting zone, respectively. A satisfactory agreement is found for the accumulated nitrogen in the shoot, but a less satisfactory agreement is found for the accumulated mineral nitrogen in the rooting zone. Especially during late autumn the model underestimates the soil mineral nitrogen content. The measured values were unexpectedly high during this period as well as in 1990 (not shown). McVoy et al. (1995) comments that there is a possibility of unreported organic manure having been applied, but this is unsubstantiated.