However this is based on the assumption that reducing the demand for meat will reduce the amount of land or intensity of agriculture. Agriculture in California is a $35 billion dollar industry which supports $100 billion dollars of related economic activities. The dairy industry alone employed 400,000 people in 2004. Decreasing the demand for meat or dairy will not magically make these people disappear. Rather they will find an alternative use for the 27 million acres of California land currently used in agriculture. This might be growing corn to use in biofuels or growing various natural fibers for use in the textile industry or even growing GM plants to produce/extract medicines from. These alternative agricultural uses will use similar amounts of water as growing crops for animal feed so it is unlikely to have any effect on water-shortages (or carbon emissions).
Another assumption the veganism-against-drought idea makes is that agriculture is the main culprit (in particular meat production), in the water shortage. True agriculture uses the most water of any industry but usually the scary water use statistics include the water which rains onto agricultural fields. So sure tons of water are used in agriculture if most of that is just the rain that would fall there and be used by whatever plants happen to be growing there anyway then should we really be worrying about it? (Since meat production uses crops grown over large areas it also receives the most rainfall). Surely a better measure would be the amount of water diverted from rivers and lakes or pumped from aquifers would be a better measure (so called 'blue water'). Now meat doesn't look so bad now. Nuts, spices, coffee/tea and rice all use more blue water per ton than meat/dairy production.
But agriculture as a whole is not such a huge problem: of the 27 million acres of farm land typically only 10 million will need to be irrigated with roughly 30 million acre-feet of water (= 10 trillion gallons). Whereas urban areas use approximately 10 million acre-feet of water one third of which is used for "residential landscape" (aka lawns, water features, and outdoor pools). Furthermore, the old leaky water transportation infrastructure loses 3-16% of all the water passing through it before it reaches a meter.
Being a vegetarian doesn't hurt when it comes to water conservation (unless you live off peanut butter). But it isn't going to solve California's drought problems either. If you want to help and you live in California then the best thing to do is to reduce your personal water use by getting rid of your outdoor pool or other water features, replacing your lawn with drought tolerant plants, investing in water-efficient washing machine & toilets and making sure none of your pipes or faucets leak. If you do not live in California then the best you can do is vote for politicians who believe in science and investing in infrastructure; and continue to buy California produce so that farmers have the capital to implement the vast array of technologies available to increase the water efficiency of agriculture.
