Fine scale 20‐year timeseries of plantation forest evapotranspiration for the lower limestone coast

Abstract

AbstractCommercial plantation forestry is a vital industry worldwide, providing natural resources for humans and playing an important role in carbon sequestration. Historically, impacts of plantation development to natural resources such as surface and groundwater was not considered, however demand for water and drying climates has focussed attention on sustainable water use and extraction. The Lower Limestone Coast region of south‐east South Australia has an extensive softwood plantation estate. The region is characterised by minimal surface water and substantial, fresh karstic groundwater resources, often <6 m below ground. Rapid expansion over shallow groundwater of the plantation hardwood estate from 2000, prompted concern about impacts of deep‐rooted plantations on water resources and notably contracting wetlands. To quantify plantation estate water use, an extensive evapotranspiration (ET) field study (1999–2008) identified plantation areas with <6 m to groundwater as groundwater users, leading to water policy whereby plantations are considered licenced water users. To enable plantation expansion in the region, fine resolution, broadscale ET data is required to ensure optimum use of water licences in the forest industry and account for plantation groundwater extraction. As continual field studies are not feasible, this 20‐year, monthly 30 m ET dataset for the region was developed to improve estimates of plantation water use, enabling improved, dynamic estimates of groundwater extraction. The dataset was developed using 21 long‐term field sites where ET was derived based on field measured transpiration, soil moisture, canopy interaction and rainfall data. Region specific field ET was combined with machine learning and additional remote sensing layers to estimate monthly ET (R2 = 0.86) across the plantation estate, improving on a national remote sensing ET product. Currently, groundwater extraction is accounted for via modelling which excludes dynamic changes in plantation groundwater extraction, limiting understanding of the hydrological processes influencing available water resources and potentially impeding economic growth.