Trophic State Index estimation from remote sensing of lake Chapala, México
The Trophic State Index (TSI) was estimated for Lake Chapala by remote sensing techniques as an indicator of water quality. Our results show that multispectral satellite images can be successfully used to monitor surface water parameters related to trophic state (i.e., chlorophyll-a and Secchi disc depth). Lake Chapala is the largest water body in Mexico and supplies Guadalajara City with over 60% of its freshwater requirements. The Lerma River is the main tributary of Lake Chapala and exhibits significant water quality deterioration associated with nutrient enrichment coming from drainage from urban, agriculture and livestock areas. This study is based on the modification of the optical properties of the water caused by increased values of turbidity and Chl-a. We estimated turbidity on the basis of empirical relationships between reflectance data in band SPOT S2 and Secchi disc depth, and Chl-a concentration values from the relationship between band ratio SPOT S3/S2 and Chl-a concentration in the water samples. The results show that application of defoliants to eradicate aquatic weeds did not ameliorate the trophic state of the lake. The Trophic State Index determinations are based on multispectral image reflectance values, which revealed Chapala to be in the range from oligotrophic to mesotrophic. The use of remote sensing data allows a more efficient, continuous and low-cost Trophic State Index monitoring of Lake Chapala to assist in the implementation of regulations toward water quality improvement.