Droughts have become widespread in the Northern Hemisphere including in China where they have affected farmland resources on the Loess Plateau Given this background we proposed a new index the Normalized Day-Night Surface Temperature Index (NTDI) to estimate moisture availability (m(a)) defined as the ratio of actual to reference evapotranspiration The NTDI is defined as the ratio of the difference between the maximum daytime surface temperature and the minimum nighttime surface temperature to the difference between the maximum and minimum surface temperatures estimated from meteorological data by applying energy balance equations To calculate the index we used data of 20 clear-sky meteorological observations made during the 2005 growing season at a natural grassland station in the Liudaogou River basin on the Loess Plateau The NTDI showed a significant inverse exponential correlation with m(a) (R-2 = 0 97 p < 0 001) whereas the numerator of the index (the maximum daytime surface temperature minus the minimum nighttime surface temperature) was only weakly correlated with m(a) (R-2 = 0 24 p = 0 03) This result indicates that normalization relative to the index denominator (maximum surface temperature - minimum surface temperature) dramatically improved the accuracy of the estimate (C) 2010 Elsevier Ltd All rights reserved