3. Study Area and Data
Te-Chi reservoir, located in Central Taiwan, with a drainage basin of 592 km2, has multiple functions including water supply for domestic use, irrigation, flood control, and a hydropower plant. The reservoir pool is approximately 14km long and covers an area of 454 hectares. Based on local relationships between water quality parameters, the following modified TSI model was developed for Te-Chi reservoir:



TSI cutoff values are 0:52 for oligotrophic, 53:60 for mesotrophic, 61:65 for meso-eutrophic, 66:77 for eutrophic, and 78:100 for hypereutrophic. These categories reflect the reservoir's nutrient and clarity levels.

4. TSI Estimation using LANDSAT TM Data
Landsat TM data from four dates (8/23/1993, 10/17/1994, 1/10/1995, and 7/22/1996) were analyzed. Corresponding to each TM image, water quality samples were collected at five cross sections within the reservoir pool. Digital numbers of TM bands 1, 2, 3, and 4 are related to water quality parameters:



Combining use of Eqs.(4)-(7) and Eqs.(8)-(10) yields pixel-based images of the reservoir pool. The overall reservoir trophic state index is expressed as the areal average of



where (x) represents the pixel-based TSI value and W the area extent of the reservoir pool.

5. Random Field Simulation
In order to develop a probabilistic approach of reservoir trophic state evaluation, a geostatistical random field simulation model HYDRO_GEN (Bellin and Rubin, 1996) is utilized to generate six hundred realizations using parameters mean, variance, and variogram obtained from the TM-derived image. A brief summary of theoretical background of the HYDRO_GEN is given below.