Modelling the effects of soil moisture and solute conditions on long-term tree growth and water use: a case study from the Shepparton irrigation area, Australia

This paper discusses the growth and hydrologic impact of a small (2 ha) 21-year old plantation growing over a shallow saline water table at Kyabram, in the Shepparton irrigation area. Using TOPOG_Dynamic, an ecohydrological model, we simulate leaf and stem growth, water use and salt accumulation of the plantation, and assess likely future trends. The model results are compared to measurements of leaf area index, transpiration, and water table dynamics over a 4-year period, and to water table dynamics, stem growth, and salt accumulation over the life of the plantation. The model generally performed well but tended to underestimate stem growth, and bias root growth too heavily towards the surface. Shallow root activity affects the depth of the majority of salt accumulation within the soil column. Simulations are used to examine the effects of water table depth at planting, groundwater salinity and tree salt sensitivity on growth, salt accumulation and water table dynamics. The effect of depth to water table at the time of planting is found to be highly dependent on the salinity of that water table. If the groundwater salinity is 700 mg l(-1) the water table is drawn down, but if the water has a salinity of 6000 m l(-1), then after an initial period of drawdown, the water table rises again. The sensitivity of the plantation to salt concentration appears to have the most significant influence on growth, especially if groundwater has a salinity above 2000 mg l(-1). Simulations of harvesting a plantation and returning the site to pasture suggested that there may be a severe degradation of future pasture production on that site, as the salt which had accumulated beneath the plantation rises into the root zone of the pasture when irrigation recommences. It is likely that a substantially increased leaching fraction would be required to negate this salt rise. When the plantation is irrigated, wood production increases, but the increased water requirements, and lesser environmental benefit from a shallower water table would need to be considered before irrigation could be recommended. The simulations presented here suggest that finding the best plantation rotation involves a balance between pasture and plantation productivity. (C) 1999 Elsevier Science B.V. All rights reserved.