An irrigation-scheduling model for application of saline water to tree plantations

Abstract WATSKED is a simple spreadsheet model that uses a water-budgeting approach to schedule irrigation of eucalypt plantations in Australia. The model was developed as part of the Wagga Wagga Effluent Plantation Project (WEPP) in New South Wales, Australia. Pan coefficients (the ratio of water-use to pan evaporation) for 33 reference sites within 10 biogeoclimatic zones were calculated using the Penman–Monteith model calibrated to water-balance measurements in Eucalyptus grandis (flooded gum). The model estimates plantation water-use from these pan coefficients and measured pan evaporation. Simple equations are used to calculate run-off, canopy interception of water during irrigation and rainfall, soil water distribution, and drainage. The model divides the soil into two layers and predicts daily soil water deficit, daily average levels of root-zone soil salinity, and the timing and amount of irrigation and leaching fraction required to control salinity. Input data include pan evaporation, rainfall and applied irrigation, and estimates of basic soil physical characteristics. For prediction of salinity, input data also include estimates of initial average soil salinity, salinity of the irrigation water, threshold and critical soil salinity, and a user-nominated maximum allowable level of soil salinity. The model was tested and performed well at two sites, one under freshwater sprinkler irrigation, and the other under freshwater flood irrigation. Predicted volumetric soil moisture contents at fortnightly intervals were in close agreement with measured values. The model is yet to be tested for scheduling irrigation with saline water.