I'm no expert, but bamboo is very hardy and can withstand a good bit of water and drought. Mine occassionally dropped leaves when it wasnt getting enough water (it was bone dry for quite some time, and it got black spots before dropping leaves), and they should be able to handle a pretty good variety of conditions. They also wont send up shoots any closer than it healthy for them, because they wont crowd themselves out (and bamboo can tollerate being very close together). Is your bamboo getting new growth too? or is it getting bare with all the leaves dropping? Have you fertilized? Any other signs of distress? Yellowing of leaves is normally (certainly not always) caused by too much water, too little sunlight, or lack of nitrogen. I sounds like too much water is not the problem, and probably not too little sun if its on a hill, which leaves nitrogen deficiency. If you havent been fertilizing, then I would suggest picking up something high nitrogen (a normal lawn fertilizer is good, or if you go organic rich compost, seaweed fertilizer, or fish emulsion are good. You can check the organic forum for more ideas.) How long has your bamboo been planted? Is it new shoots that are loosing leaves, older ones, or all of them?