Channel 10 has the elevation for each pixel expressed in metres.
Channel 7 has a mask where a pixel value of 27 means the pixel is in the reservoir area, any other value means it is not. (This mask was created from the elevation channel by altering the elevation data to show the line of the dam at its maximum final built height, thresholding at the surface elevation of the reservoir (program THR), burning the resulting bitmap into a channel (MAP), growing polygons for contiguous areas (IPG), then identifying the polygon number which represents the reservoir (DCP)).
The following model calculates the volume of the reservoir at a user-entered water level.
! Reservoir Capacity Model
!
input 'Enter water level in metres: ' #level;
!
if (%7 = 23) then
#volume = (#level - %10) * @metrex * @metrey;
if (#volume < 0) then #volume = 0; endif;
endif;
!
title ' Capacity of Reservoir in Cubic Metres';
report #volume by sum;