Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 10, 1995
Publication Date: N/A
Interpretive Summary: Although about 19% of the world's arable land is salt-affected, the problem is of most economic importance in highly productive irrigated areas. In these areas, salts accumulate in the root zone of plants as they extract water from the soil whenever drainage is inadequate, or insufficient water is applied to leach salts from the root zone. This paper demonstrates how traditional techniques (soil and plant sampling) and newer techniques (aerial multispectral videography, or satellite observation; and image analysis) can be used jointly to help quantify and map effects of soil salinity on growth and yield of irrigated crops such as sugarcane. We relied on the growth responses of the plants as observed in the video and satellite data to sort the sugarcane into growth reduction classes using image analysis. Then, because we knew the salinity and growth responses for the sample sites we were able to extrapolate the relations for the 56 samples sites to the whole field (51,000 spectral samples in the video and 1600 in the satellite image). We mapped the field into the damage or salinity classes and determined the fraction of the field in each class. From those we could tell how much salinity was costing the producer, or alternatively, how much it was worth to society to reclaim the land.
The effects of soil salinity on growth and yield of sugarcane are used to illustrate how soil and plant samples (ground truth), digital videographic or SPOT HRV spectral observations, and image analysis by unsupervised classification, as summarized in Fig. 2, can be used jointly to detect and map variations in weighted electrical conductivity (WEC, dS m-1) of the root zone and YIELD (metric tons of millable stalks ha-1). The combined data for the 1992 and 1993 growing seasons of the study showed that each dS m-1 increase in WEC reduced stalk population by 0,6 stalks m-2, stalk weight by 0.14 kg, and stalk yields by 13.7 metric tons ha-1. Sugarcane growth and yield were not affected by root zone salinities less than about 2 dS m-1, but no millable stalks were produced at salinities in excess of 10 dS m-1. The 25 pixels ha-1 of SPOT is a good scale for documenting salt stress patterns and taking site-specific ameliorative actions. The combination of satellite or aerial spectral observations, ground truth, and image classification procedures demonstrated in this study is readily applicable to other vegetation stresses. The normalized difference vegetation index (NDVI), which measures the amount of photosynthetically active tissue in plant canopies, was also used in spectral components analysis (SCA) to interrelate soil salinity, spectral responses, and sugarcane stalk yields. Although independent of the combined remote spectral, ground sampling, and image analysis procedures, the SCA relationships complement and strengthen them.