This chapter describes a method to estimate moisture availability in the 1.1 million km² Murray–Darling Basin (MDB) in southeast Australia. Remotely sensed data from the advanced very high resolution radiometer (AVHRR) are combined with meteorological data to estimate the normalised difference temperature index (NDTI). The NDTI provides a measure of moisture availability, the ratio of actual to potential evapotranspiration. Eighty-five per cent of variation in the modelled NDTI could be explained by surface temperature minus air temperature, per cent vegetation cover and net radiation. These three covariates can be used across the network of meteorological stations to calculate NDTI images, which map changes in moisture availability across the MDB. The method uses a ‘calculate then interpolate’ (CI) approach: the per-pixel variation present in the AVHRR data is used as the backbone for the spatial interpolation. Using the spatially dense AVHRR-based covariates in a CI approach avoids errors that occur between measurement points when interpolating variables for regional hydrological modelling, most significantly the spatial pattern of rainfall. The NDTI provides a link into regional water balance modelling that does not require spatial interpolation of daily rainfall. Assessing spatial and temporal interactions between the NDTI and the normalised difference vegetation index (NDVI) provides useful information about regional hydroecological processes, including agricultural management, within the context of Australia’s highly variable climate and sparse network of meteorological stations.