Using the transpiration regime to estimate biomass production

The design and implementation of a new or improved irrigation or drainage system should be preceded by a study of the soil water regime (SWR) to define the need for such a system and to optimize its design. This requires estimates of appropriate parameters characterizing the long-term (multiyear) dynamics of the SWR and its relation to biomass production. Parameters that could be used for this purpose are the water content or pressure head at one or several points in the soil root zone or the average water content of the root zone, as affected by precipitation and evapotranspiration or other variables. Because these variables change from year to year, it is necessary to characterize somehow statistically the SWR during a relatively long period. Because the water content is not easily related directly to biomass production, we used the calculated seasonal transpiration rate as an indirect characteristic of the canopy water regime. This article presents a method for estimating plant production as a function of the seasonal transpiration rate as calculated retrospectively with the HYDRUS-ET software package. New in our approach is the use of an empirical relationship between seasonal transpiration and yield. The cumulative frequency distribution of seasonal transpiration was chosen as the basic characteristic of the SWR as a source of water for plants. The approach allows one to estimate cumulative frequency curves of actual and potential yields. The difference between these two curves is the cumulative frequency distribution of yield to be optimized by the irrigation system. The method permits a better cost-benefit analysis by comparing expected yield increases with the investment and operational expenses of the newly designed irrigation system, or of newly invoked water management practices. The approach is illustrated using a 31-year time series of data for a corn crop (Zea mays L.) grown on loess soil in Slovakia.