Regional mapping of root zone soil moisture using optical satellite imagery

Abstract

Root zone soil moisture is a dynamic variable subject to rapid changes in time as well as space. Accurate, detailed information on the distribution of soil moisture is difficult to obtain. Ground based methods for the measurement of temporal and spatial changes in root zone soil moisture require much time and effort and, therefore, have limited value for soil moisture monitoring at the regional scale. Existing remote sensing methods use microwaves to measure soil moisture near the soil surface (0-10 cm). In this study, the Surface Energy Balance Algorithm for Land (SEBAL) is applied to a series of LandSat TM optical images to determine the regional distribution of the evaporative fraction. From this, soil moisture conditions are derived using an empirical relationship between evaporative fraction and root zone soil moisture. Ground measurements available in the Middle Rio Grande Basin do not cover a sufficiently wide range of soil moisture values for validation of the soil moisture maps derived from optical satellite imagery. Therefore, we conducted a qualitative validation by comparing predicted soil moisture conditions along three transects perpendicular to the Rio Grande with ground observations. This analysis shows that the remote sensing approach is successful in distinguishing between moist and dry pixels in the Middle Rio Grande Basin. Further field investigation will be required to validate the product quantitatively.