I'm thinking you probably have sandy soil, which has trouble holding nutrients, and the water you need to provide is flushing the nitrogen out of reach of the plant roots.

Try watering them with a solution of liquid fish fertilizer and see if that helps. If it does, the yellowing was due to lack of nitrogen.

Sue

I see a tan mulch on the ground. This confirms Sue's diagnosis of a lack of nitrogen. When too much 'brown' goes on top of the soil, the bacteria that break down matter use all available nitrogen in that process and it will not become available to your plants until the breakdown is in the later stages.

Regardless of your soil, take Sue's advice and add fish emulsion (or water-in a side dressing of compost or manure) and they'll be producing like crazy again.

But...take it easy. Too much nitrogen can cause them to stop producing and you'll get a lot of lush greenery. That's not what we want from beans.