It takes about 80 gallons of water to grow one pound of weeds - if the weeds don't get the water, the crop has a greater chance of getting it. The method of choice today is probably some type of herbicide. Herbicides can do a good job but they don't work miracles. The ground must be in good shape, ie. Except for grasses, it is very hard to clean pre-existing conditions once the crop is up and growing.
Crop selection has a lot to do with drought management capabilities.
If the water source is unreliable - naturally we should avoid establishing long term perennial crops. Let your water source, quantity and quality dictate what you plant. Fruit and nut crops are perennial crops demanding water for the current crop, as well as crops in future years. Plan orchards where a long term reliable water source is available. Vegetables are annual crops, requiring large amounts of water to establish the crop and carry it to maturity.