Soil water under forests (SWUF): a model of water flow and soil water content under a range of forest types K.I. Paul a,* , P.J. Polglase a , A.M. O’Connell a , J.C. Carlyle a , P.J. Smethurst a,b , P.K. Khanna a , D. Worledge a,b a CSIRO Forestry and Forest Products, PO Box E4008, Kingston, ACT 2604, Australia b Cooperative Research Centre for Temperate Hardwood Forestry, GPO Box 252-12, Hobart, Tasmania 7001, Australia Received 6 August 2002; received in revised form 3 October 2002; accepted 12 January 2003 Abstract A new model (soil water under forest, SWUF) is suitable for predicting the daily water content within both surface soil layers and the sub-soil under a range of forest types, and is suitable for use in models of mineralisation of soil organic matter as well as models of forest production. This empirical cascading bucket-type model was largely derived by combining algorithms from well-tested models for prediction of soil water under agriculture. However, it extends these to predict the water content of the litter layer, and the influence of the litter layer, weeds and understorey, and site mounding, on SWUFs. Measurements of soil water content under native forest, and pine and eucalypt plantations, were available for 59 sites across southern Australia. The model was parameterised to about half (27) of these datasets, while the remainder (32) were used for validation, for which the model explained 86% of the variation in observed water content. Sensitivity analysis indicated that important input data required were the observed upper limit of water content, bulk density, and climatic data, particularly solar radiation. The estimated area of ground that was covered by litter and canopy were also important inputs. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Runoff; Drainage; Unsaturated flow; Evaporation; Tree water use; Litter 1. Introduction For many native forests and plantations, one of the main factors limiting productivity and decomposition or mineralisation of soil organic matter is availability of soil water. In some intensively managed forests, there has been concern regarding deep drainage of nutrients into waterways. Models of forest produc- tion, drainage and turnover of organic matter and nutrients all require accurate predictions of soil water content. A number of models have been developed to esti- mate soil water availability for forest growth. They generally predict water content within the root zone of stands (Corymbia maculata syn. Eucalyptus maculata and E. populnea (Hatton et al., 1993), E. regnans (Vertessy et al., 1996), E. globulus (Hingston et al., 1998), Pinus radiata (Myers and Talsma, 1992; Landsberg and Waring, 1997; Kirschbaum, 1999), E. grandis (Soares and Almeida, 2001)). These models are unlikely to accurately account for the influence of water availability on decomposition or mineralisation. This is because the microbial activity responsible for Forest Ecology and Management 182 (2003) 195–211 * Corresponding author. Tel.: þ61-2-6281-8269; fax: þ61-2-6281-8312. E-mail address: keryn.paul@csiro.au (K.I. Paul). 0378-1127/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-1127(03)00048-3