Thermal and visible remote sensing for estimation of evapotranspiration of rainfed agrosystems and its impact on groundwater in SE Australia Rakhshan Roohi*, John A. Webb Department of Ecology, Environment and Evolution, School of Life Sciences, College of Science, Health and Engineering, La Trobe University, VIC 3086, Australia ABSTRACT Rainfed agrosystems are important components of the world’s food production system and account for 65 - 95% of total agriculture. In contrast to irrigated production systems, relatively little attention has been paid to understanding the hydrological interactions between the components of rainfed agrosystems and their impact on water resources, especially groundwater. A new model, the Surface Energy Balance Algorithm for Rainfed Agriculture (SEBARA), has been developed to estimate the spatial pattern of evapotranspiration in these agrosystems using satellite images (thermal, infrared and visible spectra). The model was calibrated for two competing land uses (Eucalyptus globules tree plantations and pastures) in adjacent catchments in western Victoria, southeastern Australia. Using measurements from a flux tower in the pasture catchment and adjusted sapflow measurements in the plantation catchment, an estimation accuracy of 95% was achieved. The tree plantations had higher available net radiation, lower soil heat flux and higher latent heat flux, resulting in 15- 20% higher evapotranspirative demand than the pasture, depending upon the age and canopy of plantations. The evapotranspiration rate of plantations declines where groundwater depth is >12m or where shallow groundwater is saline. The shallow root system of the pasture means that it relies solely on soil moisture to meet its water requirements and thus has lower evapotranspiration, which varies according to the pasture species. Keywords: Evapotranspiration, groundwater, eucalyptus, pasture, SEBARA, SEBAL, Landsat, Victoria Australia *r.roohi@latrobe.edu.au ; phone 6 450113663; latrobe.edu.au 1. INTRODUCTION In arid and semi-arid areas with limited water resources like much of Australia, sustainable management of rainfed agricultural systems requires a substantial understanding of available water resources and the interaction of the agricultural systems with climate, vegetation and soil. Groundwater at variable depths and limited rainfall mean that vegetation can have a major impact on the finite water resources. The increasing demand for water globally is expected to increase stress on water supply, especially in the context of changing rainfall and potential evapotranspiration (ET) 1 so any change in the vegetation cover can alter the delicate balance of the hydrological cycle. The contrasting evapotranspirative water use of plantations and grasslands through has been documented by a diverse array of approaches such as plot studies, paired catchments, and hydrological modeling 2,3 . Changes in vegetation, particularly from grassland to forests, alter the ecosystem water balance and soluble salt fluxes 4 . Hydrological studies from across the world demonstrate that compared to pastures, forests often have an aerodynamically rough surface well coupled to the atmosphere as well as deep root systems that access deeper water sources, resulting in higher evapotranspiration rates 4-13 , lower moisture content in the soil and vadose zone 14-16 , and reduced groundwater recharge 17,18 . In Australia, the area under plantation forestry has increased by ∼6000 km 2 in the past few decades. As a proportion of the total area of agricultural land, the change is small but it can be very important in individual catchments, where new plantation forestry represents a significant change in landuse 18 , with potentially negative consequences for water Thermosense: Thermal Infrared Applications XXXVIII, edited by Joseph N. Zalameda, Paolo Bison, Proc. of SPIE Vol. 9861, 98610A · © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2223725 Proc. of SPIE Vol. 9861 98610A-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 05/24/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx