conserving a resource requires using less of a resource & water is a kind of natural resource ->  conserving water requires using less water  conserving water requires using less water & to use less of something means to decrease the use of something ->  conserving water requires decreasing the use of water  conserving water requires decreasing the use of water & as time spent taking a shower decreases , water used will decrease ->  decreasing the time spent taking a shower conserves water  decreasing the time spent taking a shower conserves water & to shorten something can mean to decrease something -> shortening the time spent taking a shower conserves water