The Moorish conquerors brought rice to Spain as far back as the 8th century, and it’s been a popular crop ever since it arrived. There is a small amount of rice grown in Andalucia, but a very large majority of the rice in Spain is grown in Valencia, where there are naturally occurring wetlands. In Valencia, three different rices have been granted P.D.O. (Protected Region of origin) status. “arroz Bomba” is, by far, the most prestigious variety. The other two P.D.O. rices from Valencia are “arroz Senia” and “arroz Bahía.”
All three of these rices are small, round grains that can absorb about three times their volume in water. This is A LOT more absorption than most of the typical Asian rice varieties most people are more familiar with. This allows them to soak up a lot more of the flavorful cooking liquid, and also means they grow a lot more when cooked. These rices are all very starchy, and fairly sticky after cooking.
Arroz Bomba is relatively expensive, but it can be found fairly easily in specialty markets around the USA, and certainly online. In Spain, Bahía or Senia rice may be a little bit cheaper than Bomba, but in the USA, they are difficult to find. There is one P.D.O. rice from Spain, often sold as “Arroz Valenciana” that is not labeled with a specific variety – that is typically arroz Senia. While Bomba is certainly the most prized of these 3 rices, you can use any of them interchangeably for dishes calling for Bomba rice.
If you want to make a proper paella or other similar rice dish, you need to get the real deal. Using the wrong rice will give you a very different finished product, in terms of taste, texture, aroma, and appearance. If you can’t find real arroz Valenciana, the best substitute is Arborio rice. It’s a larger grain, and doesn’t absorb as much liquid, but it’s far more similar to the Spanish rices than any of the typically available Asian or Asian-style rices.