A growth-irrigation scheduling model for wastewater use in forest production

Applying wastewater and sludge to land for remediation has been recommended by the Environmental Protection Agency (EPA) as a method to recycle nutrient and organic matter and conserve water resources. The level of sewage treatment can range from simple primary treatment using a lagoon to tertiary treatment using a standard wastewater treatment plant. Small communities are selecting primary treatment and land application as the most cost-effective way of treating municipal wastewater. Wastewater was used to irrigate an Eucalyptus camaldulensis plantation in Ojinaga, Chihuahua, Mexico. The overall objective of the research was to develop a daily growth-irrigation scheduling model (GISM) for Eucalyptus tree plantations based on the trees’ water needs with the source of irrigation water being wastewater from a lagoon sewage treatment system. A second objective was to check this model against measured growth data to determine the limitations of using a simple irrigation-scheduling model to manage the irrigation system to maximize tree growth and wood production. The GISM calculated the evapotranspiration (Et) from the volume balance soil water model and a tree biomass sub-model, based on a water use efficiency (WUE) (biomass/Et) that partitions biomass determined from Et into the component parts of leaves and stems plus branches. The water balance portion of the model computes the Et for grass growing between the trees until a closed canopy system is reached.Weather data and a soil water stress function were used to calculate Et based on calculated reference Et and crop coefficients (Kcs) for both the trees and grass scaled to nonstressed Et. The GISM model accurately modeled height and diameter growth, although, it slightly overestimated the height growth of Eucalyptus for the high irrigation treatment in the second and the third years. The GISM model was successful in predicting height and diameter growth within a 95% confidence level of the measured height and diameter of the trees under all irrigation treatments. Based on the modeled and measured data analysis, the GISM model can be a useful tool to predict tree growth and schedule irrigations for Eucalyptus tree plantations, understand the trees response to environmental and water stress, and to provide better analyses for future research efforts. The climate-driving variables (temperature and rainfall) needed by the model are readily available for any location in the world from the National Climatic Data Center (NCDC)