Digital mapping of a soil drainage index for irrigated enterprise suitability in Tasmania, Australia D. B. Kidd A,B,C , B. P. Malone B , A. B. McBratney B , B. Minasny B , and M. A. Webb A A Department of Primary Industries Parks Water and Environment Tasmania, 171 Westbury Road, Prospect, Tas. 7250, Australia. B Faculty of Agriculture and Environment, University of Sydney, 1 Central Avenue, Australian Technology Park, Eveleigh, NSW 2015, Australia. C Corresponding author. Email: darren.kidd@dpipwe.tas.gov.au Abstract. An operational Digital Soil Assessment was developed to inform land suitability modelling in newly commissioned irrigation schemes in Tasmania, Australia. The Land Suitability model uses various soil parameters, along with other climate and terrain surfaces, to identify suitable areas for various agricultural enterprises for a combined 70 000-ha pilot project area in the Meander and Midlands Regions of Tasmania. An integral consideration for irrigable suitability is soil drainage. Quantitative measurement and mapping can be resource-intensive in time and associated costs, whereas more ‘traditional’ mapping approaches can be generalised, lacking the detail required for statistically validated products. The project was not sufficiently resourced to undertake replicated field-drainage measurements and relied on expert field drainage estimates at ~930 sites (260 of these for independent validation) to spatially predict soil drainage for both areas using various terrain-based and remotely sensed covariates, using three approaches: (a) decision tree spatial modelling of discrete drainage classes; (b) regression-tree spatial modelling of a continuous drainage index; (c) regression kriging (random-forests with residual-kriging) spatial modelling of a continuous drainage index. Method b was chosen as the best approach in terms of interpretation, and model training and validation, with a concordance coefficient of 0.86 and 0.57, respectively. A classified soil drainage map produced from the ‘index’ showed good agreement, with a linearly weighted kappa coefficient of 0.72 for training, and 0.37 for validation. The index mapping was incorporated into the overall land suitability model and proved an important consideration for the suitability of most enterprises. Additional keywords: decision trees, digital soil mapping, land suitability, regression trees, random forests, soil drainage, spatial modelling. Received 25 March 2013, accepted 24 September 2013, published online฀6฀March฀2014 Introduction Tasmania has a population of ~500 000 people, with a cool temperate climate, and rainfall ranging from >500 mm year –1 in the central Midlands to <1800 mm year –1 on the West Coast. It has some of the most productive soils in Australia, ranging from the fertile and well-draining Red Ferrosols (Isbell 2002) (Nitisols or Acrisols; IUSS Working Group WRB 2007) in the north-west, to the more poorly drained duplex (prominent change in texture between the A and B horizons) Sodosols (Isbell 2002) (Solonetz or Lixisols; IUSS Working Group WRB 2007) in the drier parts of the state (Cotching and Kidd 2010). Government-commissioned irrigation schemes have been introduced primarily to intensify and diversify agricultural and horticultural production, capitalising on the state’s favourable climate and soils to ensure food security and economic prosperity. The Tasmanian Department of Primary Industries, Parks, Water and Environment (DPIPWE), along with the University of Sydney Faculty of Agriculture and Environment, the Tasmanian Institute of Agriculture (TIA), and the Department of Economic Development, Tourism and the Arts (DEDTA), have developed the ‘Wealth from Water’ project, which aims to classify land within these schemes for suitability of 20 agricultural enterprises (http://www.dpiw.tas.gov.au/ wealthfromwater). The land suitability assessment provides comprehensive soil, climate, and enterprise data, complemented by market and business information (Kidd et al. 2012). Completed in late 2012, two irrigation areas were chosen to pilot the suitability and Digital Soil Mapping (DSM) process, namely the Meander Valley of Tasmania and the Tunbridge district of the Midlands Water Scheme, totalling 70 000 ha. Both areas are representative of a range of typical Tasmanian cropping soils and climatic conditions (Fig. 1). An integral component of any suitability assessment is appropriately detailed soils information. Existing soil mapping for the project areas was not of the scale, format or Journal compilation Ó CSIRO 2014 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2014, 52, 107–119 http://dx.doi.org/10.1071/SR13100