Define Obligatory and Facultative water excretion?

i) Obligatory water excretion: The kidney is 'obligated' to excrete some water to rid the body of its daily load of urinary solutes; the body's excretory system is designed to maintain the necessary balance through its filtering and selective reabsorbing system in the kidneys. About 15 ml of water is required to dissolve 1g of solute materials arising out of the metabolism. The quantity of obligatory water excretion depends on the load on the metabolic products - chiefly urea and sodium chloride. The average adult obligatory water excretion is about 900 ml.

ii) Facultative water excretion: In addition to obligatory water loss, an additional 500 ml, more or less water is excreted for maintaining water balance.