I have quite a few Opuntia ficus indica here on my farm in Baja Arizona (there are dozens of
native species of Opuntia growing wild here). After closely observing fruit setting for the past 32 years
the answer is a bit convoluted. What I can tell you is that insufficient water does not aid in fruit setting. I can also tell you that over watering does not aid in fruit setting. I don't irrigate my nopal orchard at all, but I do
mulch it heavily. The mulch should not be rich
compost, nor should it be something that will make a tight mat (like old alfalfa) - plain old loose
straw works well. And the mulch needs to be kept away from the plant.
Fruit setting is a tricky thing. It depends more on the type of rain and when that rain comes, than the amount. First it's important to understand that there is a relationship that exists between the ability of nopal to store water and nutrients and its ability to make fruit. Many varieties of Opuntia set fruit based on winter rains that occur from December into March. The last half of March, April and May here is typically bone dry, which are the conditions that nopal likes to make flowers and set fruit. In other words, they're counting on winter rains to store up water and nutrients to produce flowers and fruit in the late spring. If winter rains have been dismal, flowering may be ok but fruit-set may be poor. If the winter rains have been decent but sporadic there will be a number of flowers and some fruit set. If the winter rains have been exceedingly heavy there will be problems with blossom rot, and other fungus issues as well as pest infestations. The best conditions for fruit set seem to occur after winters with slow, soaking rains that occur over an extended period of time. My speculation is that this provides the plant with the time and ability to not only soak up and store a lot of water, but nutrients as well. The latter occurs because when the soil is dry the plant derives little nutrients from it. So the longer the soil remains moist (not wet) through the winter, the more time it has to also pull nutrients out of the soil and stockpile them for fruit production later in the spring. Hence the need to understand the plants ability to store water and nutrients for later use. Also, fruit set is typically low after years of heavy production.