To measure water levels from remote sites using a narrowband channel, this paper propose a difference image based JPEG communication scheme and water level measurement scheme using sparsely sampled images in time domain. In the slave system located in the field, the image is converted to difference image, and are compressed using JPEG, then larger changes are sampled and transmitted. To measure the water level from the images received in the master system which may contain noises caused by various sources, the averaging filter and Gaussian filter are used to reduce the noise and the Y-axis profile of an edge image is used to read the water level. Considering the wild condition of the field, a simplified camera calibration scheme is also introduced. The implemented slave system was installed in a river and its performance has been tested with the data collected for a year.