With the example you've shown what you're asking is not possible. The only way you could do what you want is if the replacement text has exactly the same number of characters as the one you replace. If it has less, you can add spaces to make it equal, but if it has more, like in your example, it's not physically possible as the rest of the file needs to be moved to make room for the extra characters. Hence you've got to rewrite everything after the replacement.
If you're willing to keep the formatting of the water and air column as it is originally, and assuming the header is always the same number of lines, then:
fid = fopen('somefile.txt', 'rt+')
for headerline = 1:10
fgetl(fid); %skip header lines
end
for row = 1:size(newdata, 1) %assuming your cell array is newdata
startline = ftell(fid); %get current position
fseek(fid, 0, 'cof'); %required because switching from reading to writing
fprintf(fid, '%s\t% +7.2f\t%+7.2f', newdata{row, 1:3}); %the format string assumes no value is above 999.99
fseek(fid, startline, 'bof'); %return to beginning of line
fgetl(fid); %read whole line to position file pointer to beginning of next line
end
Code is untested, there may be bugs in the formatting string, number of header lines, etc.
