Effects of spatial and temporal variability on simulated transpiration ratios

Sustainability studies are increasingly needed to determine management systems that protect the environment and maintain production potentials. Whether these analyses are performed by field experiments or by computer modelling, defining the appropriate spatial and temporal scales is essential. For three management types within one soil series in The Netherlands, the land quality indicator "transpiration ratio", E-ratio (ratio between actual and potential transpiration), was determined using a simulation model of water flow in the unsaturated zone. Soils were characterised by eight profiles per management type in which measurements of the retention and hydraulic conductivity characteristics were made. Variation in weather data was described by 30 years of historical data. Soil hydraulic characteristics were used as individual data and as averaged data. Differences in E-ratio obtained by using individual soil data rather than averaged data were not significant, but E-ratio values for the three management types were significantly different (0.79, 0.81 and 0.83). The results of the 30 years were used to analyse the effect of the time over which an experiment is evaluated, on the E-ratio. The differential of variance indicated that the length of an experiment should be at least seven years in order to reduce the effect of the variable weather conditions. (C) 1997 Elsevier Science B.V.