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, i n the Shepparton irrigation area. Using TOPOG_Dynamic, an ecohydrological m odel, we simulate leaf and stem growth, water use and salt accumulation of the plantation, and assess likely future trends. The model results are comp ared to measurements of leaf area index, transpiration, and water table dyn amics over a 4-year period, and to water table dynamics, stem growth, and s alt accumulation over the life of the plantation. The model generally perfo rmed well but tended to underestimate stem growth, and bias root growth too heavily towards the surface. Shallow root activity affects the depth of th e majority of salt accumulation within the soil column. Simulations are use d to examine the effects of water table depth at planting, groundwater sali nity 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 fo und to be highly dependent on the salinity of that water table. If the grou ndwater 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 drawd own, the water table rises again. The sensitivity of the plantation to salt concentration appears to have the most significant influence on growth, es pecially if groundwater has a salinity above 2000 mg l(-1). Simulations of harvesting a plantation and returning the site to pasture suggested that th ere may be a severe degradation of future pasture production on that site, as the salt which had accumulated beneath the plantation rises into the roo t zone of the pasture when irrigation recommences. It is likely that a subs tantially 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 shal lower water table would need to be considered before irrigation could be re commended. The simulations presented here suggest that finding the best pla ntation rotation involves a balance between pasture and plantation producti vity. (C) 1999 Elsevier Science B.V. All rights reserved.