Not all farmers have access to a constant water supply. Though some farms use an aquifers for water, even these can dry up after prolonged use during times of no rain.
Lack of access to fresh water means that drought can have a major impact on farming and productivity. This can be true even if irrigation is used. If the water source used in the irrigation process is affected by drought caused by climate change, the farms using that irrigation system will be negatively impacted.
As mentioned above, temperature can effect crop yield as well. Some crops grow best in a certain range of temperatures, some crops rely on a sustained temperature (both cold and warm) in order to produce, and extreme heat or cold can kill many crops.
As climate change brings more extreme weather patterns and greater fluctuations in temperature, these temperature sensitivities can come into play.
Basically, farmers depend on rain and they depend on having the right kinds of temperatures for growing their crops. Climate change can affect both of these. Climate change can lead to changing patterns of rainfall. Farmers who could once rely on getting rain at the right time might no longer be able to do so. The rains might be too late to help the plants grow. They might come during the harvest season and ruin crops that need to be dry to be harvested. Changes in patterns of temperatures can matter too. The most important of these is that too much cold at the wrong time can stunt plant growth.
Farmers rely on the weather because it is the one thing they cannot control. If the reasosn change from the usual pattern, they might be able to grow more or they might be able to grow less. Storms can also destroy the crops.